Java 命令使用介绍
$ info java
Common parameter
理论上来说，虚拟机的堆大小只会受到虚拟内存地址空间大小(比如32位最大是4G)的限制。那么，什么时候操作系统才会真的分配物理内存给虚拟机呢？答案就是，在内存第一次被使用的时候。这个使用并不是说我新建了一个对象，然后操作系统就唰的把所有物理内存分配好了，它只是按照虚拟机的需求，分配了一点点。（实际上是，只是把使用到的这部分内存按照页映射到了物理内存上）
# 操作系统在虚拟机启动的时候就直接完成物理内存的绑定
-XX:+AlwaysPreTouch
Usage And Description
File: *manpages*, Node: java, Up: (dir)
java(1) Basic Tools java(1)
NAME
java - Launches a Java application.
SYNOPSIS
java [options] classname [args]
java [options] -jar filename [args]
options
Command-line options separated by spaces. See Options.
classname
The name of the class to be launched.
filename
The name of the Java Archive (JAR) file to be called. Used only
with the -jar option.
args
The arguments passed to the main() method separated by spaces.
DESCRIPTION
The java command starts a Java application. It does this by starting
the Java Runtime Environment (JRE), loading the specified class, and
calling that class's main() method. The method must be declared public
and static, it must not return any value, and it must accept a String
array as a parameter. The method declaration has the following form:
public static void main(String[] args)
The java command can be used to launch a JavaFX application by loading
a class that either has a main() method or that extends
javafx.application.Application. In the latter case, the launcher
constructs an instance of the Application class, calls its init()
method, and then calls the start(javafx.stage.Stage) method.
By default, the first argument that is not an option of the java
command is the fully qualified name of the class to be called. If the
-jar option is specified, its argument is the name of the JAR file
containing class and resource files for the application. The startup
class must be indicated by the Main-Class manifest header in its source
code.
The JRE searches for the startup class (and other classes used by the
application) in three sets of locations: the bootstrap class path, the
installed extensions, and the user’s class path.
Arguments after the class file name or the JAR file name are passed to
the main() method.
OPTIONS
OPTIONS
The java command supports a wide range of options that can be divided
into the following categories:
· Standard Options
· Non-Standard Options
· Advanced Runtime Options
· Advanced JIT Compiler Options
· Advanced Serviceability Options
· Advanced Garbage Collection Options
Standard options are guaranteed to be supported by all implementations
of the Java Virtual Machine (JVM). They are used for common actions,
such as checking the version of the JRE, setting the class path,
enabling verbose output, and so on.
Non-standard options are general purpose options that are specific to
the Java HotSpot Virtual Machine, so they are not guaranteed to be
supported by all JVM implementations, and are subject to change. These
options start with -X.
Advanced options are not recommended for casual use. These are
developer options used for tuning specific areas of the Java HotSpot
Virtual Machine operation that often have specific system requirements
and may require privileged access to system configuration parameters.
They are also not guaranteed to be supported by all JVM
implementations, and are subject to change. Advanced options start with
-XX.
To keep track of the options that were deprecated or removed in the
latest release, there is a section named Deprecated and Removed Options
at the end of the document.
Boolean options are used to either enable a feature that is disabled by
default or disable a feature that is enabled by default. Such options
do not require a parameter. Boolean -XX options are enabled using the
plus sign (-XX:+OptionName) and disabled using the minus sign
(-XX:-OptionName).
For options that require an argument, the argument may be separated
from the option name by a space, a colon (:), or an equal sign (=), or
the argument may directly follow the option (the exact syntax differs
for each option). If you are expected to specify the size in bytes, you
can use no suffix, or use the suffix k or K for kilobytes (KB), m or M
for megabytes (MB), g or G for gigabytes (GB). For example, to set the
size to 8 GB, you can specify either 8g, 8192m, 8388608k, or 8589934592
as the argument. If you are expected to specify the percentage, use a
number from 0 to 1 (for example, specify 0.25 for 25%).
Standard Options
Standard Options
These are the most commonly used options that are supported by all
implementations of the JVM.
-agentlib:libname[=options]
Loads the specified native agent library. After the library name, a
comma-separated list of options specific to the library can be
used.
If the option -agentlib:foo is specified, then the JVM attempts to
load the library named libfoo.so in the location specified by the
LD_LIBRARY_PATH system variable (on OS X this variable is
DYLD_LIBRARY_PATH).
The following example shows how to load the heap profiling tool
(HPROF) library and get sample CPU information every 20 ms, with a
stack depth of 3:
-agentlib:hprof=cpu=samples,interval=20,depth=3
The following example shows how to load the Java Debug Wire
Protocol (JDWP) library and listen for the socket connection on
port 8000, suspending the JVM before the main class loads:
-agentlib:jdwp=transport=dt_socket,server=y,address=8000
For more information about the native agent libraries, refer to the
following:
· The java.lang.instrument package description at
http://docs.oracle.com/javase/8/docs/api/java/lang/instrument/package-summary.html
· Agent Command Line Options in the JVM Tools Interface guide at
http://docs.oracle.com/javase/8/docs/platform/jvmti/jvmti.html#starting
-agentpath:pathname[=options]
Loads the native agent library specified by the absolute path name.
This option is equivalent to -agentlib but uses the full path and
file name of the library.
-client
Selects the Java HotSpot Client VM. The 64-bit version of the Java
SE Development Kit (JDK) currently ignores this option and instead
uses the Server JVM.
For default JVM selection, see Server-Class Machine Detection at
http://docs.oracle.com/javase/8/docs/technotes/guides/vm/server-class.html
-Dproperty=value
Sets a system property value. The property variable is a string
with no spaces that represents the name of the property. The value
variable is a string that represents the value of the property. If
value is a string with spaces, then enclose it in quotation marks
(for example -Dfoo="foo bar").
-d32
Runs the application in a 32-bit environment. If a 32-bit
environment is not installed or is not supported, then an error
will be reported. By default, the application is run in a 32-bit
environment unless a 64-bit system is used.
-d64
Runs the application in a 64-bit environment. If a 64-bit
environment is not installed or is not supported, then an error
will be reported. By default, the application is run in a 32-bit
environment unless a 64-bit system is used.
Currently only the Java HotSpot Server VM supports 64-bit
operation, and the -server option is implicit with the use of -d64.
The -client option is ignored with the use of -d64. This is subject
to change in a future release.
-disableassertions[:[packagename]...|:classname]
-da[:[packagename]...|:classname]
Disables assertions. By default, assertions are disabled in all
packages and classes.
With no arguments, -disableassertions (-da) disables assertions in
all packages and classes. With the packagename argument ending in
..., the switch disables assertions in the specified package and
any subpackages. If the argument is simply ..., then the switch
disables assertions in the unnamed package in the current working
directory. With the classname argument, the switch disables
assertions in the specified class.
The -disableassertions (-da) option applies to all class loaders
and to system classes (which do not have a class loader). There is
one exception to this rule: if the option is provided with no
arguments, then it does not apply to system classes. This makes it
easy to disable assertions in all classes except for system
classes. The -disablesystemassertions option enables you to disable
assertions in all system classes.
To explicitly enable assertions in specific packages or classes,
use the -enableassertions (-ea) option. Both options can be used at
the same time. For example, to run the MyClass application with
assertions enabled in package com.wombat.fruitbat (and any
subpackages) but disabled in class com.wombat.fruitbat.Brickbat,
use the following command:
java -ea:com.wombat.fruitbat... -da:com.wombat.fruitbat.Brickbat MyClass
-disablesystemassertions
-dsa
Disables assertions in all system classes.
-enableassertions[:[packagename]...|:classname]
-ea[:[packagename]...|:classname]
Enables assertions. By default, assertions are disabled in all
packages and classes.
With no arguments, -enableassertions (-ea) enables assertions in
all packages and classes. With the packagename argument ending in
..., the switch enables assertions in the specified package and any
subpackages. If the argument is simply ..., then the switch enables
assertions in the unnamed package in the current working directory.
With the classname argument, the switch enables assertions in the
specified class.
The -enableassertions (-ea) option applies to all class loaders and
to system classes (which do not have a class loader). There is one
exception to this rule: if the option is provided with no
arguments, then it does not apply to system classes. This makes it
easy to enable assertions in all classes except for system classes.
The -enablesystemassertions option provides a separate switch to
enable assertions in all system classes.
To explicitly disable assertions in specific packages or classes,
use the -disableassertions (-da) option. If a single command
contains multiple instances of these switches, then they are
processed in order before loading any classes. For example, to run
the MyClass application with assertions enabled only in package
com.wombat.fruitbat (and any subpackages) but disabled in class
com.wombat.fruitbat.Brickbat, use the following command:
java -ea:com.wombat.fruitbat... -da:com.wombat.fruitbat.Brickbat MyClass
-enablesystemassertions
-esa
Enables assertions in all system classes.
-help
-?
Displays usage information for the java command without actually
running the JVM.
-jar filename
Executes a program encapsulated in a JAR file. The filename
argument is the name of a JAR file with a manifest that contains a
line in the form Main-Class:classname that defines the class with
the public static void main(String[] args) method that serves as
your application's starting point.
When you use the -jar option, the specified JAR file is the source
of all user classes, and other class path settings are ignored.
For more information about JAR files, see the following resources:
· jar(1)
· The Java Archive (JAR) Files guide at
http://docs.oracle.com/javase/8/docs/technotes/guides/jar/index.html
· Lesson: Packaging Programs in JAR Files at
http://docs.oracle.com/javase/tutorial/deployment/jar/index.html
-javaagent:jarpath[=options]
Loads the specified Java programming language agent. For more
information about instrumenting Java applications, see the
java.lang.instrument package description in the Java API
documentation at
http://docs.oracle.com/javase/8/docs/api/java/lang/instrument/package-summary.html
-jre-restrict-search
Includes user-private JREs in the version search.
-no-jre-restrict-search
Excludes user-private JREs from the version search.
-server
Selects the Java HotSpot Server VM. The 64-bit version of the JDK
supports only the Server VM, so in that case the option is
implicit.
For default JVM selection, see Server-Class Machine Detection at
http://docs.oracle.com/javase/8/docs/technotes/guides/vm/server-class.html
-showversion
Displays version information and continues execution of the
application. This option is equivalent to the -version option
except that the latter instructs the JVM to exit after displaying
version information.
-splash:imgname
Shows the splash screen with the image specified by imgname. For
example, to show the splash.gif file from the images directory when
starting your application, use the following option:
-splash:images/splash.gif
-verbose:class
Displays information about each loaded class.
-verbose:gc
Displays information about each garbage collection (GC) event.
-verbose:jni
Displays information about the use of native methods and other Java
Native Interface (JNI) activity.
-version
Displays version information and then exits. This option is
equivalent to the -showversion option except that the latter does
not instruct the JVM to exit after displaying version information.
-version:release
Specifies the release version to be used for running the
application. If the version of the java command called does not
meet this specification and an appropriate implementation is found
on the system, then the appropriate implementation will be used.
The release argument specifies either the exact version string, or
a list of version strings and ranges separated by spaces. A version
string is the developer designation of the version number in the
following form: 1.x.0_u (where x is the major version number, and u
is the update version number). A version range is made up of a
version string followed by a plus sign (+) to designate this
version or later, or a part of a version string followed by an
asterisk (*) to designate any version string with a matching
prefix. Version strings and ranges can be combined using a space
for a logical OR combination, or an ampersand (&) for a logical AND
combination of two version strings/ranges. For example, if running
the class or JAR file requires either JRE 6u13 (1.6.0_13), or any
JRE 6 starting from 6u10 (1.6.0_10), specify the following:
-version:"1.6.0_13 1.6* & 1.6.0_10+"
Quotation marks are necessary only if there are spaces in the
release parameter.
For JAR files, the preference is to specify version requirements in
the JAR file manifest rather than on the command line.
Non-Standard Options
Non-Standard Options
These options are general purpose options that are specific to the Java
HotSpot Virtual Machine.
-X
Displays help for all available -X options.
-Xbatch
Disables background compilation. By default, the JVM compiles the
method as a background task, running the method in interpreter mode
until the background compilation is finished. The -Xbatch flag
disables background compilation so that compilation of all methods
proceeds as a foreground task until completed.
This option is equivalent to -XX:-BackgroundCompilation.
-Xbootclasspath:path
Specifies a list of directories, JAR files, and ZIP archives
separated by colons (:) to search for boot class files. These are
used in place of the boot class files included in the JDK.
Do not deploy applications that use this option to override a class
in rt.jar, because this violates the JRE binary code license.
-Xbootclasspath/a:path
Specifies a list of directories, JAR files, and ZIP archives
separated by colons (:) to append to the end of the default
bootstrap class path.
Do not deploy applications that use this option to override a class
in rt.jar, because this violates the JRE binary code license.
-Xbootclasspath/p:path
Specifies a list of directories, JAR files, and ZIP archives
separated by colons (:) to prepend to the front of the default
bootstrap class path.
Do not deploy applications that use this option to override a class
in rt.jar, because this violates the JRE binary code license.
-Xcheck:jni
Performs additional checks for Java Native Interface (JNI)
functions. Specifically, it validates the parameters passed to the
JNI function and the runtime environment data before processing the
JNI request. Any invalid data encountered indicates a problem in
the native code, and the JVM will terminate with an irrecoverable
error in such cases. Expect a performance degradation when this
option is used.
-Xcomp
Forces compilation of methods on first invocation. By default, the
Client VM (-client) performs 1,000 interpreted method invocations
and the Server VM (-server) performs 10,000 interpreted method
invocations to gather information for efficient compilation.
Specifying the -Xcomp option disables interpreted method
invocations to increase compilation performance at the expense of
efficiency.
You can also change the number of interpreted method invocations
before compilation using the -XX:CompileThreshold option.
-Xdebug
Does nothing. Provided for backward compatibility.
-Xdiag
Shows additional diagnostic messages.
-Xfuture
Enables strict class-file format checks that enforce close
conformance to the class-file format specification. Developers are
encouraged to use this flag when developing new code because the
stricter checks will become the default in future releases.
-Xint
Runs the application in interpreted-only mode. Compilation to
native code is disabled, and all bytecode is executed by the
interpreter. The performance benefits offered by the just in time
(JIT) compiler are not present in this mode.
-Xinternalversion
Displays more detailed JVM version information than the -version
option, and then exits.
-Xloggc:filename
Sets the file to which verbose GC events information should be
redirected for logging. The information written to this file is
similar to the output of -verbose:gc with the time elapsed since
the first GC event preceding each logged event. The -Xloggc option
overrides -verbose:gc if both are given with the same java command.
Example:
-Xloggc:garbage-collection.log
-Xmaxjitcodesize=size
Specifies the maximum code cache size (in bytes) for JIT-compiled
code. Append the letter k or K to indicate kilobytes, m or M to
indicate megabytes, g or G to indicate gigabytes. The default
maximum code cache size is 240 MB; if you disable tiered
compilation with the option -XX:-TieredCompilation, then the
default size is 48 MB:
-Xmaxjitcodesize=240m
This option is equivalent to -XX:ReservedCodeCacheSize.
-Xmixed
Executes all bytecode by the interpreter except for hot methods,
which are compiled to native code.
-Xmnsize
Sets the initial and maximum size (in bytes) of the heap for the
young generation (nursery). Append the letter k or K to indicate
kilobytes, m or M to indicate megabytes, g or G to indicate
gigabytes.
The young generation region of the heap is used for new objects. GC
is performed in this region more often than in other regions. If
the size for the young generation is too small, then a lot of minor
garbage collections will be performed. If the size is too large,
then only full garbage collections will be performed, which can
take a long time to complete. Oracle recommends that you keep the
size for the young generation between a half and a quarter of the
overall heap size.
The following examples show how to set the initial and maximum size
of young generation to 256 MB using various units:
-Xmn256m
-Xmn262144k
-Xmn268435456
Instead of the -Xmn option to set both the initial and maximum size
of the heap for the young generation, you can use -XX:NewSize to
set the initial size and -XX:MaxNewSize to set the maximum size.
-Xmssize
Sets the initial size (in bytes) of the heap. This value must be a
multiple of 1024 and greater than 1 MB. Append the letter k or K to
indicate kilobytes, m or M to indicate megabytes, g or G to
indicate gigabytes.
The following examples show how to set the size of allocated memory
to 6 MB using various units:
-Xms6291456
-Xms6144k
-Xms6m
If you do not set this option, then the initial size will be set as
the sum of the sizes allocated for the old generation and the young
generation. The initial size of the heap for the young generation
can be set using the -Xmn option or the -XX:NewSize option.
-Xmxsize
Specifies the maximum size (in bytes) of the memory allocation pool
in bytes. This value must be a multiple of 1024 and greater than 2
MB. Append the letter k or K to indicate kilobytes, m or M to
indicate megabytes, g or G to indicate gigabytes. The default value
is chosen at runtime based on system configuration. For server
deployments, -Xms and -Xmx are often set to the same value. See the
section "Ergonomics" in Java SE HotSpot Virtual Machine Garbage
Collection Tuning Guide at
http://docs.oracle.com/javase/8/docs/technotes/guides/vm/gctuning/index.html.
The following examples show how to set the maximum allowed size of
allocated memory to 80 MB using various units:
-Xmx83886080
-Xmx81920k
-Xmx80m
The -Xmx option is equivalent to -XX:MaxHeapSize.
-Xnoclassgc
Disables garbage collection (GC) of classes. This can save some GC
time, which shortens interruptions during the application run.
When you specify -Xnoclassgc at startup, the class objects in the
application will be left untouched during GC and will always be
considered live. This can result in more memory being permanently
occupied which, if not used carefully, will throw an out of memory
exception.
-Xprof
Profiles the running program and sends profiling data to standard
output. This option is provided as a utility that is useful in
program development and is not intended to be used in production
systems.
-Xrs
Reduces the use of operating system signals by the JVM.
Shutdown hooks enable orderly shutdown of a Java application by
running user cleanup code (such as closing database connections) at
shutdown, even if the JVM terminates abruptly.
The JVM catches signals to implement shutdown hooks for unexpected
termination. The JVM uses SIGHUP, SIGINT, and SIGTERM to initiate
the running of shutdown hooks.
The JVM uses a similar mechanism to implement the feature of
dumping thread stacks for debugging purposes. The JVM uses SIGQUIT
to perform thread dumps.
Applications embedding the JVM frequently need to trap signals such
as SIGINT or SIGTERM, which can lead to interference with the JVM
signal handlers. The -Xrs option is available to address this
issue. When -Xrs is used, the signal masks for SIGINT, SIGTERM,
SIGHUP, and SIGQUIT are not changed by the JVM, and signal handlers
for these signals are not installed.
There are two consequences of specifying -Xrs:
· SIGQUIT thread dumps are not available.
· User code is responsible for causing shutdown hooks to run, for
example, by calling System.exit() when the JVM is to be
terminated.
-Xshare:mode
Sets the class data sharing (CDS) mode. Possible mode arguments for
this option include the following:
auto
Use CDS if possible. This is the default value for Java HotSpot
32-Bit Client VM.
on
Require the use of CDS. Print an error message and exit if
class data sharing cannot be used.
off
Do not use CDS. This is the default value for Java HotSpot
32-Bit Server VM, Java HotSpot 64-Bit Client VM, and Java
HotSpot 64-Bit Server VM.
dump
Manually generate the CDS archive. Specify the application
class path as described in "Setting the Class Path ".
You should regenerate the CDS archive with each new JDK
release.
-XshowSettings:category
Shows settings and continues. Possible category arguments for this
option include the following:
all
Shows all categories of settings. This is the default value.
locale
Shows settings related to locale.
properties
Shows settings related to system properties.
vm
Shows the settings of the JVM.
-Xsssize
Sets the thread stack size (in bytes). Append the letter k or K to
indicate KB, m or M to indicate MB, g or G to indicate GB. The
default value depends on the platform:
· Linux/ARM (32-bit): 320 KB
· Linux/i386 (32-bit): 320 KB
· Linux/x64 (64-bit): 1024 KB
· OS X (64-bit): 1024 KB
· Oracle Solaris/i386 (32-bit): 320 KB
· Oracle Solaris/x64 (64-bit): 1024 KB
The following examples set the thread stack size to 1024 KB in
different units:
-Xss1m
-Xss1024k
-Xss1048576
This option is equivalent to -XX:ThreadStackSize.
-Xusealtsigs
Use alternative signals instead of SIGUSR1 and SIGUSR2 for JVM
internal signals. This option is equivalent to -XX:+UseAltSigs.
-Xverify:mode
Sets the mode of the bytecode verifier. Bytecode verification
ensures that class files are properly formed and satisfy the
constraints listed in section 4.10, "Verification of class Files"
in the The Java Virtual Machine Specification:
https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-4.html#jvms-4.10
Do not turn off verification as this reduces the protection
provided by Java and could cause problems due to ill-formed class
files.
Possible mode arguments for this option include the following:
remote
Verifies all bytecodes not loaded by the bootstrap class
loader. This is the default behavior if you do not specify the
-Xverify option.
all
Enables verification of all bytecodes.
none
Disables verification of all bytecodes. Use of -Xverify:none is
unsupported.
Advanced Runtime Options
Advanced Runtime Options
These options control the runtime behavior of the Java HotSpot VM.
-XX:+CheckEndorsedAndExtDirs
Enables the option to prevent the java command from running a Java
application if it uses the endorsed-standards override mechanism or
the extension mechanism. This option checks if an application is
using one of these mechanisms by checking the following:
· The java.ext.dirs or java.endorsed.dirs system property is set.
· The lib/endorsed directory exists and is not empty.
· The lib/ext directory contains any JAR files other than those
of the JDK.
· The system-wide platform-specific extension directory contains
any JAR files.
-XX:+DisableAttachMechanism
Enables the option that disables the mechanism that lets tools
attach to the JVM. By default, this option is disabled, meaning
that the attach mechanism is enabled and you can use tools such as
jcmd, jstack, jmap, and jinfo.
-XX:ErrorFile=filename
Specifies the path and file name to which error data is written
when an irrecoverable error occurs. By default, this file is
created in the current working directory and named
hs_err_pidpid.log where pid is the identifier of the process that
caused the error. The following example shows how to set the
default log file (note that the identifier of the process is
specified as %p):
-XX:ErrorFile=./hs_err_pid%p.log
The following example shows how to set the error log to
/var/log/java/java_error.log:
-XX:ErrorFile=/var/log/java/java_error.log
If the file cannot be created in the specified directory (due to
insufficient space, permission problem, or another issue), then the
file is created in the temporary directory for the operating
system. The temporary directory is /tmp.
-XX:+FailOverToOldVerifier
Enables automatic failover to the old verifier when the new type
checker fails. By default, this option is disabled and it is
ignored (that is, treated as disabled) for classes with a recent
bytecode version. You can enable it for classes with older versions
of the bytecode.
-XX:+FlightRecorder
Enables the use of the Java Flight Recorder (JFR) during the
runtime of the application. This is a commercial feature that works
in conjunction with the -XX:+UnlockCommercialFeatures option as
follows:
java -XX:+UnlockCommercialFeatures -XX:+FlightRecorder
If this option is not provided, Java Flight Recorder can still be
enabled in a running JVM by providing the appropriate jcmd
diagnostic commands.
-XX:-FlightRecorder
Disables the use of the Java Flight Recorder (JFR) during the
runtime of the application. This is a commercial feature that works
in conjunction with the -XX:+UnlockCommercialFeatures option as
follows:
java -XX:+UnlockCommercialFeatures -XX:-FlightRecorder
If this option is provided, Java Flight Recorder cannot be enabled
in a running JVM.
-XX:FlightRecorderOptions=parameter=value
Sets the parameters that control the behavior of JFR. This is a
commercial feature that works in conjunction with the
-XX:+UnlockCommercialFeatures option. This option can be used only
when JFR is enabled (that is, the -XX:+FlightRecorder option is
specified).
The following list contains all available JFR parameters:
defaultrecording={true|false}
Specifies whether the recording is a continuous background
recording or if it runs for a limited time. By default, this
parameter is set to false (recording runs for a limited time).
To make the recording run continuously, set the parameter to
true.
disk={true|false}
Specifies whether JFR should write a continuous recording to
disk. By default, this parameter is set to false (continuous
recording to disk is disabled). To enable it, set the parameter
to true, and also set defaultrecording=true.
dumponexit={true|false}
Specifies whether a dump file of JFR data should be generated
when the JVM terminates in a controlled manner. By default,
this parameter is set to false (dump file on exit is not
generated). To enable it, set the parameter to true, and also
set defaultrecording=true.
The dump file is written to the location defined by the
dumponexitpath parameter.
dumponexitpath=path
Specifies the path and name of the dump file with JFR data that
is created when the JVM exits in a controlled manner if you set
the dumponexit=true parameter. Setting the path makes sense
only if you also set defaultrecording=true.
If the specified path is a directory, the JVM assigns a file
name that shows the creation date and time. If the specified
path includes a file name and if that file already exists, the
JVM creates a new file by appending the date and time stamp to
the specified file name.
globalbuffersize=size
Specifies the total amount of primary memory (in bytes) used
for data retention. Append k or K, to specify the size in KB, m
or M to specify the size in MB, g or G to specify the size in
GB. By default, the size is set to 462848 bytes.
loglevel={quiet|error|warning|info|debug|trace}
Specify the amount of data written to the log file by JFR. By
default, it is set to info.
maxage=time
Specifies the maximum age of disk data to keep for the default
recording. Append s to specify the time in seconds, m for
minutes, h for hours, or d for days (for example, specifying
30s means 30 seconds). By default, the maximum age is set to 15
minutes (15m).
This parameter is valid only if you set the disk=true
parameter.
maxchunksize=size
Specifies the maximum size (in bytes) of the data chunks in a
recording. Append k or K, to specify the size in KB, m or M to
specify the size in MB, g or G to specify the size in GB. By
default, the maximum size of data chunks is set to 12 MB.
maxsize=size
Specifies the maximum size (in bytes) of disk data to keep for
the default recording. Append k or K, to specify the size in
KB, m or M to specify the size in MB, g or G to specify the
size in GB. By default, the maximum size of disk data is not
limited, and this parameter is set to 0.
This parameter is valid only if you set the disk=true
parameter.
repository=path
Specifies the repository (a directory) for temporary disk
storage. By default, the system’s temporary directory is used.
samplethreads={true|false}
Specifies whether thread sampling is enabled. Thread sampling
occurs only if the sampling event is enabled along with this
parameter. By default, this parameter is enabled.
settings=path
Specifies the path and name of the event settings file (of type
JFC). By default, the default.jfc file is used, which is
located in JAVA_HOME/jre/lib/jfr.
stackdepth=depth
Stack depth for stack traces by JFR. By default, the depth is
set to 64 method calls. The maximum is 2048, minimum is 1.
threadbuffersize=size
Specifies the per-thread local buffer size (in bytes). Append k
or K, to specify the size in KB, m or M to specify the size in
MB, g or G to specify the size in GB. Higher values for this
parameter allow more data gathering without contention to flush
it to the global storage. It can increase application footprint
in a thread-rich environment. By default, the local buffer size
is set to 5 KB.
You can specify values for multiple parameters by separating them
with a comma. For example, to instruct JFR to write a continuous
recording to disk, and set the maximum size of data chunks to 10
MB, specify the following:
-XX:FlightRecorderOptions=defaultrecording=true,disk=true,maxchunksize=10M
-XX:LargePageSizeInBytes=size
On Solaris, sets the maximum size (in bytes) for large pages used
for Java heap. The size argument must be a power of 2 (2, 4, 8, 16,
...). Append the letter k or K to indicate kilobytes, m or M to
indicate megabytes, g or G to indicate gigabytes. By default, the
size is set to 0, meaning that the JVM chooses the size for large
pages automatically.
The following example illustrates how to set the large page size to
4 megabytes (MB):
-XX:LargePageSizeInBytes=4m
-XX:MaxDirectMemorySize=size
Sets the maximum total size (in bytes) of the New I/O (the java.nio
package) direct-buffer allocations. Append the letter k or K to
indicate kilobytes, m or M to indicate megabytes, g or G to
indicate gigabytes. By default, the size is set to 0, meaning that
the JVM chooses the size for NIO direct-buffer allocations
automatically.
The following examples illustrate how to set the NIO size to 1024
KB in different units:
-XX:MaxDirectMemorySize=1m
-XX:MaxDirectMemorySize=1024k
-XX:MaxDirectMemorySize=1048576
-XX:NativeMemoryTracking=mode
Specifies the mode for tracking JVM native memory usage. Possible
mode arguments for this option include the following:
off
Do not track JVM native memory usage. This is the default
behavior if you do not specify the -XX:NativeMemoryTracking
option.
summary
Only track memory usage by JVM subsystems, such as Java heap,
class, code, and thread.
detail
In addition to tracking memory usage by JVM subsystems, track
memory usage by individual CallSite, individual virtual memory
region and its committed regions.
-XX:ObjectAlignmentInBytes=alignment
Sets the memory alignment of Java objects (in bytes). By default,
the value is set to 8 bytes. The specified value should be a power
of two, and must be within the range of 8 and 256 (inclusive). This
option makes it possible to use compressed pointers with large Java
heap sizes.
The heap size limit in bytes is calculated as:
4GB * ObjectAlignmentInBytes
Note: As the alignment value increases, the unused space between
objects will also increase. As a result, you may not realize any
benefits from using compressed pointers with large Java heap sizes.
-XX:OnError=string
Sets a custom command or a series of semicolon-separated commands
to run when an irrecoverable error occurs. If the string contains
spaces, then it must be enclosed in quotation marks.
The following example shows how the -XX:OnError option can be used
to run the gcore command to create the core image, and the debugger
is started to attach to the process in case of an irrecoverable
error (the %p designates the current process):
-XX:OnError="gcore %p;dbx - %p"
-XX:OnOutOfMemoryError=string
Sets a custom command or a series of semicolon-separated commands
to run when an OutOfMemoryError exception is first thrown. If the
string contains spaces, then it must be enclosed in quotation
marks. For an example of a command string, see the description of
the -XX:OnError option.
-XX:+PerfDataSaveToFile
If enabled, saves jstat(1) binary data when the Java application
exits. This binary data is saved in a file named hsperfdata_<pid>,
where <pid> is the process identifier of the Java application you
ran. Use jstat to display the performance data contained in this
file as follows:
jstat -class file:///<path>/hsperfdata_<pid>
jstat -gc file:///<path>/hsperfdata_<pid>
-XX:+PrintCommandLineFlags
Enables printing of ergonomically selected JVM flags that appeared
on the command line. It can be useful to know the ergonomic values
set by the JVM, such as the heap space size and the selected
garbage collector. By default, this option is disabled and flags
are not printed.
-XX:+PrintNMTStatistics
Enables printing of collected native memory tracking data at JVM
exit when native memory tracking is enabled (see
-XX:NativeMemoryTracking). By default, this option is disabled and
native memory tracking data is not printed.
-XX:+RelaxAccessControlCheck
Decreases the amount of access control checks in the verifier. By
default, this option is disabled, and it is ignored (that is,
treated as disabled) for classes with a recent bytecode version.
You can enable it for classes with older versions of the bytecode.
-XX:+ResourceManagement
Enables the use of Resource Management during the runtime of the
application.
This is a commercial feature that requires you to also specify the
-XX:+UnlockCommercialFeatures option as follows:
java -XX:+UnlockCommercialFeatures -XX:+ResourceManagement
-XX:ResourceManagementSampleInterval=value (milliseconds)
Sets the parameter that controls the sampling interval for Resource
Management measurements, in milliseconds.
This option can be used only when Resource Management is enabled
(that is, the -XX:+ResourceManagement option is specified).
-XX:SharedArchiveFile=path
Specifies the path and name of the class data sharing (CDS) archive
file
-XX:SharedClassListFile=file_name
Specifies the text file that contains the names of the class files
to store in the class data sharing (CDS) archive. This file
contains the full name of one class file per line, except slashes
(/) replace dots (.). For example, to specify the classes
java.lang.Object and hello.Main, create a text file that contains
the following two lines:
java/lang/Object
hello/Main
The class files that you specify in this text file should include
the classes that are commonly used by the application. They may
include any classes from the application, extension, or bootstrap
class paths.
-XX:+ShowMessageBoxOnError
Enables displaying of a dialog box when the JVM experiences an
irrecoverable error. This prevents the JVM from exiting and keeps
the process active so that you can attach a debugger to it to
investigate the cause of the error. By default, this option is
disabled.
-XX:StartFlightRecording=parameter=value
Starts a JFR recording for the Java application. This is a
commercial feature that works in conjunction with the
-XX:+UnlockCommercialFeatures option. This option is equivalent to
the JFR.start diagnostic command that starts a recording during
runtime. You can set the following parameters when starting a JFR
recording:
compress={true|false}
Specifies whether to compress the JFR recording log file (of
type JFR) on the disk using the gzip file compression utility.
This parameter is valid only if the filename parameter is
specified. By default it is set to false (recording is not
compressed). To enable compression, set the parameter to true.
defaultrecording={true|false}
Specifies whether the recording is a continuous background
recording or if it runs for a limited time. By default, this
parameter is set to false (recording runs for a limited time).
To make the recording run continuously, set the parameter to
true.
delay=time
Specifies the delay between the Java application launch time
and the start of the recording. Append s to specify the time in
seconds, m for minutes, h for hours, or d for days (for
example, specifying 10m means 10 minutes). By default, there is
no delay, and this parameter is set to 0.
dumponexit={true|false}
Specifies whether a dump file of JFR data should be generated
when the JVM terminates in a controlled manner. By default,
this parameter is set to false (dump file on exit is not
generated). To enable it, set the parameter to true.
The dump file is written to the location defined by the
filename parameter.
Example:
-XX:StartFlightRecording=name=test,filename=D:\test.jfr,dumponexit=true
duration=time
Specifies the duration of the recording. Append s to specify
the time in seconds, m for minutes, h for hours, or d for days
(for example, specifying 5h means 5 hours). By default, the
duration is not limited, and this parameter is set to 0.
filename=path
Specifies the path and name of the JFR recording log file.
name=identifier
Specifies the identifier for the JFR recording. By default, it
is set to Recording x.
maxage=time
Specifies the maximum age of disk data to keep for the default
recording. Append s to specify the time in seconds, m for
minutes, h for hours, or d for days (for example, specifying
30s means 30 seconds). By default, the maximum age is set to 15
minutes (15m).
maxsize=size
Specifies the maximum size (in bytes) of disk data to keep for
the default recording. Append k or K, to specify the size in
KB, m or M to specify the size in MB, g or G to specify the
size in GB. By default, the maximum size of disk data is not
limited, and this parameter is set to 0.
settings=path
Specifies the path and name of the event settings file (of type
JFC). By default, the default.jfc file is used, which is
located in JAVA_HOME/jre/lib/jfr.
You can specify values for multiple parameters by separating them
with a comma. For example, to save the recording to test.jfr in the
current working directory, and instruct JFR to compress the log
file, specify the following:
-XX:StartFlightRecording=filename=test.jfr,compress=true
-XX:ThreadStackSize=size
Sets the thread stack size (in bytes). Append the letter k or K to
indicate kilobytes, m or M to indicate megabytes, g or G to
indicate gigabytes. The default value depends on the platform:
· Linux/ARM (32-bit): 320 KB
· Linux/i386 (32-bit): 320 KB
· Linux/x64 (64-bit): 1024 KB
· OS X (64-bit): 1024 KB
· Oracle Solaris/i386 (32-bit): 320 KB
· Oracle Solaris/x64 (64-bit): 1024 KB
The following examples show how to set the thread stack size to
1024 KB in different units:
-XX:ThreadStackSize=1m
-XX:ThreadStackSize=1024k
-XX:ThreadStackSize=1048576
This option is equivalent to -Xss.
-XX:+TraceClassLoading
Enables tracing of classes as they are loaded. By default, this
option is disabled and classes are not traced.
-XX:+TraceClassLoadingPreorder
Enables tracing of all loaded classes in the order in which they
are referenced. By default, this option is disabled and classes are
not traced.
-XX:+TraceClassResolution
Enables tracing of constant pool resolutions. By default, this
option is disabled and constant pool resolutions are not traced.
-XX:+TraceClassUnloading
Enables tracing of classes as they are unloaded. By default, this
option is disabled and classes are not traced.
-XX:+TraceLoaderConstraints
Enables tracing of the loader constraints recording. By default,
this option is disabled and loader constraints recording is not
traced.
-XX:+UnlockCommercialFeatures
Enables the use of commercial features. Commercial features are
included with Oracle Java SE Advanced or Oracle Java SE Suite
packages, as defined on the Java SE Products page at
http://www.oracle.com/technetwork/java/javase/terms/products/index.html
By default, this option is disabled and the JVM runs without the
commercial features. Once they were enabled for a JVM process, it
is not possible to disable their use for that process.
If this option is not provided, commercial features can still be
unlocked in a running JVM by using the appropriate jcmd diagnostic
commands.
-XX:+UseAltSigs
Enables the use of alternative signals instead of SIGUSR1 and
SIGUSR2 for JVM internal signals. By default, this option is
disabled and alternative signals are not used. This option is
equivalent to -Xusealtsigs.
-XX:+UseAppCDS
Enables application class data sharing (AppCDS). To use AppCDS, you
must also specify values for the options -XX:SharedClassListFile
and -XX:SharedArchiveFile during both CDS dump time (see the option
-Xshare:dump) and application run time.
This is a commercial feature that requires you to also specify the
-XX:+UnlockCommercialFeatures option. This is also an experimental
feature; it may change in future releases.
See "Application Class Data Sharing".
-XX:-UseBiasedLocking
Disables the use of biased locking. Some applications with
significant amounts of uncontended synchronization may attain
significant speedups with this flag enabled, whereas applications
with certain patterns of locking may see slowdowns. For more
information about the biased locking technique, see the example in
Java Tuning White Paper at
http://www.oracle.com/technetwork/java/tuning-139912.html#section4.2.5
By default, this option is enabled.
-XX:-UseCompressedOops
Disables the use of compressed pointers. By default, this option is
enabled, and compressed pointers are used when Java heap sizes are
less than 32 GB. When this option is enabled, object references are
represented as 32-bit offsets instead of 64-bit pointers, which
typically increases performance when running the application with
Java heap sizes less than 32 GB. This option works only for 64-bit
JVMs.
It is also possible to use compressed pointers when Java heap sizes
are greater than 32GB. See the -XX:ObjectAlignmentInBytes option.
-XX:+UseHugeTLBFS
This option for Linux is the equivalent of specifying
-XX:+UseLargePages. This option is disabled by default. This option
pre-allocates all large pages up-front, when memory is reserved;
consequently the JVM cannot dynamically grow or shrink large pages
memory areas; see -XX:UseTransparentHugePages if you want this
behavior.
For more information, see "Large Pages".
-XX:+UseLargePages
Enables the use of large page memory. By default, this option is
disabled and large page memory is not used.
For more information, see "Large Pages".
-XX:+UseMembar
Enables issuing of membars on thread state transitions. This option
is disabled by default on all platforms except ARM servers, where
it is enabled. (It is recommended that you do not disable this
option on ARM servers.)
-XX:+UsePerfData
Enables the perfdata feature. This option is enabled by default to
allow JVM monitoring and performance testing. Disabling it
suppresses the creation of the hsperfdata_userid directories. To
disable the perfdata feature, specify -XX:-UsePerfData.
-XX:+UseTransparentHugePages
On Linux, enables the use of large pages that can dynamically grow
or shrink. This option is disabled by default. You may encounter
performance problems with transparent huge pages as the OS moves
other pages around to create huge pages; this option is made
available for experimentation.
For more information, see "Large Pages".
-XX:+AllowUserSignalHandlers
Enables installation of signal handlers by the application. By
default, this option is disabled and the application is not allowed
to install signal handlers.
Advanced JIT Compiler Options
Advanced JIT Compiler Options
These options control the dynamic just-in-time (JIT) compilation
performed by the Java HotSpot VM.
-XX:+AggressiveOpts
Enables the use of aggressive performance optimization features,
which are expected to become default in upcoming releases. By
default, this option is disabled and experimental performance
features are not used.
-XX:AllocateInstancePrefetchLines=lines
Sets the number of lines to prefetch ahead of the instance
allocation pointer. By default, the number of lines to prefetch is
set to 1:
-XX:AllocateInstancePrefetchLines=1
Only the Java HotSpot Server VM supports this option.
-XX:AllocatePrefetchDistance=size
Sets the size (in bytes) of the prefetch distance for object
allocation. Memory about to be written with the value of new
objects is prefetched up to this distance starting from the address
of the last allocated object. Each Java thread has its own
allocation point.
Negative values denote that prefetch distance is chosen based on
the platform. Positive values are bytes to prefetch. Append the
letter k or K to indicate kilobytes, m or M to indicate megabytes,
g or G to indicate gigabytes. The default value is set to -1.
The following example shows how to set the prefetch distance to
1024 bytes:
-XX:AllocatePrefetchDistance=1024
Only the Java HotSpot Server VM supports this option.
-XX:AllocatePrefetchInstr=instruction
Sets the prefetch instruction to prefetch ahead of the allocation
pointer. Only the Java HotSpot Server VM supports this option.
Possible values are from 0 to 3. The actual instructions behind the
values depend on the platform. By default, the prefetch instruction
is set to 0:
-XX:AllocatePrefetchInstr=0
Only the Java HotSpot Server VM supports this option.
-XX:AllocatePrefetchLines=lines
Sets the number of cache lines to load after the last object
allocation by using the prefetch instructions generated in compiled
code. The default value is 1 if the last allocated object was an
instance, and 3 if it was an array.
The following example shows how to set the number of loaded cache
lines to 5:
-XX:AllocatePrefetchLines=5
Only the Java HotSpot Server VM supports this option.
-XX:AllocatePrefetchStepSize=size
Sets the step size (in bytes) for sequential prefetch instructions.
Append the letter k or K to indicate kilobytes, m or M to indicate
megabytes, g or G to indicate gigabytes. By default, the step size
is set to 16 bytes:
-XX:AllocatePrefetchStepSize=16
Only the Java HotSpot Server VM supports this option.
-XX:AllocatePrefetchStyle=style
Sets the generated code style for prefetch instructions. The style
argument is an integer from 0 to 3:
0
Do not generate prefetch instructions.
1
Execute prefetch instructions after each allocation. This is
the default parameter.
2
Use the thread-local allocation block (TLAB) watermark pointer
to determine when prefetch instructions are executed.
3
Use BIS instruction on SPARC for allocation prefetch.
Only the Java HotSpot Server VM supports this option.
-XX:+BackgroundCompilation
Enables background compilation. This option is enabled by default.
To disable background compilation, specify
-XX:-BackgroundCompilation (this is equivalent to specifying
-Xbatch).
-XX:CICompilerCount=threads
Sets the number of compiler threads to use for compilation. By
default, the number of threads is set to 2 for the server JVM, to 1
for the client JVM, and it scales to the number of cores if tiered
compilation is used. The following example shows how to set the
number of threads to 2:
-XX:CICompilerCount=2
-XX:CodeCacheMinimumFreeSpace=size
Sets the minimum free space (in bytes) required for compilation.
Append the letter k or K to indicate kilobytes, m or M to indicate
megabytes, g or G to indicate gigabytes. When less than the minimum
free space remains, compiling stops. By default, this option is set
to 500 KB. The following example shows how to set the minimum free
space to 1024 MB:
-XX:CodeCacheMinimumFreeSpace=1024m
-XX:CompileCommand=command,method[,option]
Specifies a command to perform on a method. For example, to exclude
the indexOf() method of the String class from being compiled, use
the following:
-XX:CompileCommand=exclude,java/lang/String.indexOf
Note that the full class name is specified, including all packages
and subpackages separated by a slash (/). For easier cut and paste
operations, it is also possible to use the method name format
produced by the -XX:+PrintCompilation and -XX:+LogCompilation
options:
-XX:CompileCommand=exclude,java.lang.String::indexOf
If the method is specified without the signature, the command will
be applied to all methods with the specified name. However, you can
also specify the signature of the method in the class file format.
In this case, you should enclose the arguments in quotation marks,
because otherwise the shell treats the semicolon as command end.
For example, if you want to exclude only the indexOf(String) method
of the String class from being compiled, use the following:
-XX:CompileCommand="exclude,java/lang/String.indexOf,(Ljava/lang/String;)I"
You can also use the asterisk (*) as a wildcard for class and
method names. For example, to exclude all indexOf() methods in all
classes from being compiled, use the following:
-XX:CompileCommand=exclude,*.indexOf
The commas and periods are aliases for spaces, making it easier to
pass compiler commands through a shell. You can pass arguments to
-XX:CompileCommand using spaces as separators by enclosing the
argument in quotation marks:
-XX:CompileCommand="exclude java/lang/String indexOf"
Note that after parsing the commands passed on the command line
using the -XX:CompileCommand options, the JIT compiler then reads
commands from the .hotspot_compiler file. You can add commands to
this file or specify a different file using the
-XX:CompileCommandFile option.
To add several commands, either specify the -XX:CompileCommand
option multiple times, or separate each argument with the newline
separator (\n). The following commands are available:
break
Set a breakpoint when debugging the JVM to stop at the
beginning of compilation of the specified method.
compileonly
Exclude all methods from compilation except for the specified
method. As an alternative, you can use the -XX:CompileOnly
option, which allows to specify several methods.
dontinline
Prevent inlining of the specified method.
exclude
Exclude the specified method from compilation.
help
Print a help message for the -XX:CompileCommand option.
inline
Attempt to inline the specified method.
log
Exclude compilation logging (with the -XX:+LogCompilation
option) for all methods except for the specified method. By
default, logging is performed for all compiled methods.
option
This command can be used to pass a JIT compilation option to
the specified method in place of the last argument (option).
The compilation option is set at the end, after the method
name. For example, to enable the BlockLayoutByFrequency option
for the append() method of the StringBuffer class, use the
following:
-XX:CompileCommand=option,java/lang/StringBuffer.append,BlockLayoutByFrequency
You can specify multiple compilation options, separated by
commas or spaces.
print
Print generated assembler code after compilation of the
specified method.
quiet
Do not print the compile commands. By default, the commands
that you specify with the -XX:CompileCommand option are
printed; for example, if you exclude from compilation the
indexOf() method of the String class, then the following will
be printed to standard output:
CompilerOracle: exclude java/lang/String.indexOf
You can suppress this by specifying the
-XX:CompileCommand=quiet option before other -XX:CompileCommand
options.
-XX:CompileCommandFile=filename
Sets the file from which JIT compiler commands are read. By
default, the .hotspot_compiler file is used to store commands
performed by the JIT compiler.
Each line in the command file represents a command, a class name,
and a method name for which the command is used. For example, this
line prints assembly code for the toString() method of the String
class:
print java/lang/String toString
For more information about specifying the commands for the JIT
compiler to perform on methods, see the -XX:CompileCommand option.
-XX:CompileOnly=methods
Sets the list of methods (separated by commas) to which compilation
should be restricted. Only the specified methods will be compiled.
Specify each method with the full class name (including the
packages and subpackages). For example, to compile only the
length() method of the String class and the size() method of the
List class, use the following:
-XX:CompileOnly=java/lang/String.length,java/util/List.size
Note that the full class name is specified, including all packages
and subpackages separated by a slash (/). For easier cut and paste
operations, it is also possible to use the method name format
produced by the -XX:+PrintCompilation and -XX:+LogCompilation
options:
-XX:CompileOnly=java.lang.String::length,java.util.List::size
Although wildcards are not supported, you can specify only the
class or package name to compile all methods in that class or
package, as well as specify just the method to compile methods with
this name in any class:
-XX:CompileOnly=java/lang/String
-XX:CompileOnly=java/lang
-XX:CompileOnly=.length
-XX:CompileThreshold=invocations
Sets the number of interpreted method invocations before
compilation. By default, in the server JVM, the JIT compiler
performs 10,000 interpreted method invocations to gather
information for efficient compilation. For the client JVM, the
default setting is 1,500 invocations. This option is ignored when
tiered compilation is enabled; see the option
-XX:+TieredCompilation. The following example shows how to set the
number of interpreted method invocations to 5,000:
-XX:CompileThreshold=5000
You can completely disable interpretation of Java methods before
compilation by specifying the -Xcomp option.
-XX:+DoEscapeAnalysis
Enables the use of escape analysis. This option is enabled by
default. To disable the use of escape analysis, specify
-XX:-DoEscapeAnalysis. Only the Java HotSpot Server VM supports
this option.
-XX:InitialCodeCacheSize=size
Sets the initial code cache size (in bytes). Append the letter k or
K to indicate kilobytes, m or M to indicate megabytes, g or G to
indicate gigabytes. The default value is set to 500 KB. The initial
code cache size should be not less than the system's minimal memory
page size. The following example shows how to set the initial code
cache size to 32 KB:
-XX:InitialCodeCacheSize=32k
-XX:+Inline
Enables method inlining. This option is enabled by default to
increase performance. To disable method inlining, specify
-XX:-Inline.
-XX:InlineSmallCode=size
Sets the maximum code size (in bytes) for compiled methods that
should be inlined. Append the letter k or K to indicate kilobytes,
m or M to indicate megabytes, g or G to indicate gigabytes. Only
compiled methods with the size smaller than the specified size will
be inlined. By default, the maximum code size is set to 1000 bytes:
-XX:InlineSmallCode=1000
-XX:+LogCompilation
Enables logging of compilation activity to a file named hotspot.log
in the current working directory. You can specify a different log
file path and name using the -XX:LogFile option.
By default, this option is disabled and compilation activity is not
logged. The -XX:+LogCompilation option has to be used together with
the -XX:UnlockDiagnosticVMOptions option that unlocks diagnostic
JVM options.
You can enable verbose diagnostic output with a message printed to
the console every time a method is compiled by using the
-XX:+PrintCompilation option.
-XX:MaxInlineSize=size
Sets the maximum bytecode size (in bytes) of a method to be
inlined. Append the letter k or K to indicate kilobytes, m or M to
indicate megabytes, g or G to indicate gigabytes. By default, the
maximum bytecode size is set to 35 bytes:
-XX:MaxInlineSize=35
-XX:MaxNodeLimit=nodes
Sets the maximum number of nodes to be used during single method
compilation. By default, the maximum number of nodes is set to
65,000:
-XX:MaxNodeLimit=65000
-XX:MaxTrivialSize=size
Sets the maximum bytecode size (in bytes) of a trivial method to be
inlined. Append the letter k or K to indicate kilobytes, m or M to
indicate megabytes, g or G to indicate gigabytes. By default, the
maximum bytecode size of a trivial method is set to 6 bytes:
-XX:MaxTrivialSize=6
-XX:+OptimizeStringConcat
Enables the optimization of String concatenation operations. This
option is enabled by default. To disable the optimization of String
concatenation operations, specify -XX:-OptimizeStringConcat. Only
the Java HotSpot Server VM supports this option.
-XX:+PrintAssembly
Enables printing of assembly code for bytecoded and native methods
by using the external disassembler.so library. This enables you to
see the generated code, which may help you to diagnose performance
issues.
By default, this option is disabled and assembly code is not
printed. The -XX:+PrintAssembly option has to be used together with
the -XX:UnlockDiagnosticVMOptions option that unlocks diagnostic
JVM options.
-XX:+PrintCompilation
Enables verbose diagnostic output from the JVM by printing a
message to the console every time a method is compiled. This
enables you to see which methods actually get compiled. By default,
this option is disabled and diagnostic output is not printed.
You can also log compilation activity to a file by using the
-XX:+LogCompilation option.
-XX:+PrintInlining
Enables printing of inlining decisions. This enables you to see
which methods are getting inlined.
By default, this option is disabled and inlining information is not
printed. The -XX:+PrintInlining option has to be used together with
the -XX:+UnlockDiagnosticVMOptions option that unlocks diagnostic
JVM options.
-XX:ReservedCodeCacheSize=size
Sets the maximum code cache size (in bytes) for JIT-compiled code.
Append the letter k or K to indicate kilobytes, m or M to indicate
megabytes, g or G to indicate gigabytes. The default maximum code
cache size is 240 MB; if you disable tiered compilation with the
option -XX:-TieredCompilation, then the default size is 48 MB. This
option has a limit of 2 GB; otherwise, an error is generated. The
maximum code cache size should not be less than the initial code
cache size; see the option -XX:InitialCodeCacheSize. This option is
equivalent to -Xmaxjitcodesize.
-XX:RTMAbortRatio=abort_ratio
The RTM abort ratio is specified as a percentage (%) of all
executed RTM transactions. If a number of aborted transactions
becomes greater than this ratio, then the compiled code will be
deoptimized. This ratio is used when the -XX:+UseRTMDeopt option is
enabled. The default value of this option is 50. This means that
the compiled code will be deoptimized if 50% of all transactions
are aborted.
-XX:RTMRetryCount=number_of_retries
RTM locking code will be retried, when it is aborted or busy, the
number of times specified by this option before falling back to the
normal locking mechanism. The default value for this option is 5.
The -XX:UseRTMLocking option must be enabled.
-XX:-TieredCompilation
Disables the use of tiered compilation. By default, this option is
enabled. Only the Java HotSpot Server VM supports this option.
-XX:+UseAES
Enables hardware-based AES intrinsics for Intel, AMD, and SPARC
hardware. Intel Westmere (2010 and newer), AMD Bulldozer (2011 and
newer), and SPARC (T4 and newer) are the supported hardware. UseAES
is used in conjunction with UseAESIntrinsics.
-XX:+UseAESIntrinsics
UseAES and UseAESIntrinsics flags are enabled by default and are
supported only for Java HotSpot Server VM 32-bit and 64-bit. To
disable hardware-based AES intrinsics, specify -XX:-UseAES
-XX:-UseAESIntrinsics. For example, to enable hardware AES, use the
following flags:
-XX:+UseAES -XX:+UseAESIntrinsics
To support UseAES and UseAESIntrinsics flags for 32-bit and 64-bit
use -server option to choose Java HotSpot Server VM. These flags
are not supported on Client VM.
-XX:+UseCodeCacheFlushing
Enables flushing of the code cache before shutting down the
compiler. This option is enabled by default. To disable flushing of
the code cache before shutting down the compiler, specify
-XX:-UseCodeCacheFlushing.
-XX:+UseCondCardMark
Enables checking of whether the card is already marked before
updating the card table. This option is disabled by default and
should only be used on machines with multiple sockets, where it
will increase performance of Java applications that rely heavily on
concurrent operations. Only the Java HotSpot Server VM supports
this option.
-XX:+UseRTMDeopt
Auto-tunes RTM locking depending on the abort ratio. This ratio is
specified by -XX:RTMAbortRatio option. If the number of aborted
transactions exceeds the abort ratio, then the method containing
the lock will be deoptimized and recompiled with all locks as
normal locks. This option is disabled by default. The
-XX:+UseRTMLocking option must be enabled.
-XX:+UseRTMLocking
Generate Restricted Transactional Memory (RTM) locking code for all
inflated locks, with the normal locking mechanism as the fallback
handler. This option is disabled by default. Options related to RTM
are only available for the Java HotSpot Server VM on x86 CPUs that
support Transactional Synchronization Extensions (TSX).
RTM is part of Intel's TSX, which is an x86 instruction set
extension and facilitates the creation of multithreaded
applications. RTM introduces the new instructions XBEGIN, XABORT,
XEND, and XTEST. The XBEGIN and XEND instructions enclose a set of
instructions to run as a transaction. If no conflict is found when
running the transaction, the memory and register modifications are
committed together at the XEND instruction. The XABORT instruction
can be used to explicitly abort a transaction and the XEND
instruction to check if a set of instructions are being run in a
transaction.
A lock on a transaction is inflated when another thread tries to
access the same transaction, thereby blocking the thread that did
not originally request access to the transaction. RTM requires that
a fallback set of operations be specified in case a transaction
aborts or fails. An RTM lock is a lock that has been delegated to
the TSX's system.
RTM improves performance for highly contended locks with low
conflict in a critical region (which is code that must not be
accessed by more than one thread concurrently). RTM also improves
the performance of coarse-grain locking, which typically does not
perform well in multithreaded applications. (Coarse-grain locking
is the strategy of holding locks for long periods to minimize the
overhead of taking and releasing locks, while fine-grained locking
is the strategy of trying to achieve maximum parallelism by locking
only when necessary and unlocking as soon as possible.) Also, for
lightly contended locks that are used by different threads, RTM can
reduce false cache line sharing, also known as cache line
ping-pong. This occurs when multiple threads from different
processors are accessing different resources, but the resources
share the same cache line. As a result, the processors repeatedly
invalidate the cache lines of other processors, which forces them
to read from main memory instead of their cache.
-XX:+UseSHA
Enables hardware-based intrinsics for SHA crypto hash functions for
SPARC hardware. UseSHA is used in conjunction with the
UseSHA1Intrinsics, UseSHA256Intrinsics, and UseSHA512Intrinsics
options.
The UseSHA and UseSHA*Intrinsics flags are enabled by default, and
are supported only for Java HotSpot Server VM 64-bit on SPARC T4
and newer.
This feature is only applicable when using the
sun.security.provider.Sun provider for SHA operations.
To disable all hardware-based SHA intrinsics, specify -XX:-UseSHA.
To disable only a particular SHA intrinsic, use the appropriate
corresponding option. For example: -XX:-UseSHA256Intrinsics.
-XX:+UseSHA1Intrinsics
Enables intrinsics for SHA-1 crypto hash function.
-XX:+UseSHA256Intrinsics
Enables intrinsics for SHA-224 and SHA-256 crypto hash functions.
-XX:+UseSHA512Intrinsics
Enables intrinsics for SHA-384 and SHA-512 crypto hash functions.
-XX:+UseSuperWord
Enables the transformation of scalar operations into superword
operations. This option is enabled by default. To disable the
transformation of scalar operations into superword operations,
specify -XX:-UseSuperWord. Only the Java HotSpot Server VM supports
this option.
Advanced Serviceability Options
Advanced Serviceability Options
These options provide the ability to gather system information and
perform extensive debugging.
-XX:+ExtendedDTraceProbes
Enables additional dtrace tool probes that impact the performance.
By default, this option is disabled and dtrace performs only
standard probes.
-XX:+HeapDumpOnOutOfMemory
Enables the dumping of the Java heap to a file in the current
directory by using the heap profiler (HPROF) when a
java.lang.OutOfMemoryError exception is thrown. You can explicitly
set the heap dump file path and name using the -XX:HeapDumpPath
option. By default, this option is disabled and the heap is not
dumped when an OutOfMemoryError exception is thrown.
-XX:HeapDumpPath=path
Sets the path and file name for writing the heap dump provided by
the heap profiler (HPROF) when the -XX:+HeapDumpOnOutOfMemoryError
option is set. By default, the file is created in the current
working directory, and it is named java_pidpid.hprof where pid is
the identifier of the process that caused the error. The following
example shows how to set the default file explicitly (%p represents
the current process identificator):
-XX:HeapDumpPath=./java_pid%p.hprof
The following example shows how to set the heap dump file to
/var/log/java/java_heapdump.hprof:
-XX:HeapDumpPath=/var/log/java/java_heapdump.hprof
-XX:LogFile=path
Sets the path and file name where log data is written. By default,
the file is created in the current working directory, and it is
named hotspot.log.
The following example shows how to set the log file to
/var/log/java/hotspot.log:
-XX:LogFile=/var/log/java/hotspot.log
-XX:+PrintClassHistogram
Enables printing of a class instance histogram after a Control+C
event (SIGTERM). By default, this option is disabled.
Setting this option is equivalent to running the jmap -histo
command, or the jcmd pid GC.class_histogram command, where pid is
the current Java process identifier.
-XX:+PrintConcurrentLocks
Enables printing of locks after a event. By default, this option is
disabled.
Enables printing of java.util.concurrent locks after a Control+C
event (SIGTERM). By default, this option is disabled.
Setting this option is equivalent to running the jstack -l command
or the jcmd pid Thread.print -l command, where pid is the current
Java process identifier.
-XX:+UnlockDiagnosticVMOptions
Unlocks the options intended for diagnosing the JVM. By default,
this option is disabled and diagnostic options are not available.
Advanced Garbage Collection Options
Advanced Garbage Collection Options
These options control how garbage collection (GC) is performed by the
Java HotSpot VM.
-XX:+AggressiveHeap
Enables Java heap optimization. This sets various parameters to be
optimal for long-running jobs with intensive memory allocation,
based on the configuration of the computer (RAM and CPU). By
default, the option is disabled and the heap is not optimized.
-XX:+AlwaysPreTouch
Enables touching of every page on the Java heap during JVM
initialization. This gets all pages into the memory before entering
the main() method. The option can be used in testing to simulate a
long-running system with all virtual memory mapped to physical
memory. By default, this option is disabled and all pages are
committed as JVM heap space fills.
-XX:+CMSClassUnloadingEnabled
Enables class unloading when using the concurrent mark-sweep (CMS)
garbage collector. This option is enabled by default. To disable
class unloading for the CMS garbage collector, specify
-XX:-CMSClassUnloadingEnabled.
-XX:CMSExpAvgFactor=percent
Sets the percentage of time (0 to 100) used to weight the current
sample when computing exponential averages for the concurrent
collection statistics. By default, the exponential averages factor
is set to 25%. The following example shows how to set the factor to
15%:
-XX:CMSExpAvgFactor=15
-XX:CMSInitiatingOccupancyFraction=percent
Sets the percentage of the old generation occupancy (0 to 100) at
which to start a CMS collection cycle. The default value is set to
-1. Any negative value (including the default) implies that
-XX:CMSTriggerRatio is used to define the value of the initiating
occupancy fraction.
The following example shows how to set the occupancy fraction to
20%:
-XX:CMSInitiatingOccupancyFraction=20
-XX:+CMSScavengeBeforeRemark
Enables scavenging attempts before the CMS remark step. By default,
this option is disabled.
-XX:CMSTriggerRatio=percent
Sets the percentage (0 to 100) of the value specified by
-XX:MinHeapFreeRatio that is allocated before a CMS collection
cycle commences. The default value is set to 80%.
The following example shows how to set the occupancy fraction to
75%:
-XX:CMSTriggerRatio=75
-XX:ConcGCThreads=threads
Sets the number of threads used for concurrent GC. The default
value depends on the number of CPUs available to the JVM.
For example, to set the number of threads for concurrent GC to 2,
specify the following option:
-XX:ConcGCThreads=2
-XX:+DisableExplicitGC
Enables the option that disables processing of calls to
System.gc(). This option is disabled by default, meaning that calls
to System.gc() are processed. If processing of calls to System.gc()
is disabled, the JVM still performs GC when necessary.
-XX:+ExplicitGCInvokesConcurrent
Enables invoking of concurrent GC by using the System.gc() request.
This option is disabled by default and can be enabled only together
with the -XX:+UseConcMarkSweepGC option.
-XX:+ExplicitGCInvokesConcurrentAndUnloadsClasses
Enables invoking of concurrent GC by using the System.gc() request
and unloading of classes during the concurrent GC cycle. This
option is disabled by default and can be enabled only together with
the -XX:+UseConcMarkSweepGC option.
-XX:G1HeapRegionSize=size
Sets the size of the regions into which the Java heap is subdivided
when using the garbage-first (G1) collector. The value can be
between 1 MB and 32 MB. The default region size is determined
ergonomically based on the heap size.
The following example shows how to set the size of the subdivisions
to 16 MB:
-XX:G1HeapRegionSize=16m
-XX:+G1PrintHeapRegions
Enables the printing of information about which regions are
allocated and which are reclaimed by the G1 collector. By default,
this option is disabled.
-XX:G1ReservePercent=percent
Sets the percentage of the heap (0 to 50) that is reserved as a
false ceiling to reduce the possibility of promotion failure for
the G1 collector. By default, this option is set to 10%.
The following example shows how to set the reserved heap to 20%:
-XX:G1ReservePercent=20
-XX:InitialHeapSize=size
Sets the initial size (in bytes) of the memory allocation pool.
This value must be either 0, or a multiple of 1024 and greater than
1 MB. Append the letter k or K to indicate kilobytes, m or M to
indicate megabytes, g or G to indicate gigabytes. The default value
is chosen at runtime based on system configuration. See the section
"Ergonomics" in Java SE HotSpot Virtual Machine Garbage Collection
Tuning Guide at
http://docs.oracle.com/javase/8/docs/technotes/guides/vm/gctuning/index.html.
The following examples show how to set the size of allocated memory
to 6 MB using various units:
-XX:InitialHeapSize=6291456
-XX:InitialHeapSize=6144k
-XX:InitialHeapSize=6m
If you set this option to 0, then the initial size will be set as
the sum of the sizes allocated for the old generation and the young
generation. The size of the heap for the young generation can be
set using the -XX:NewSize option.
-XX:InitialSurvivorRatio=ratio
Sets the initial survivor space ratio used by the throughput
garbage collector (which is enabled by the -XX:+UseParallelGC
and/or -XX:+UseParallelOldGC options). Adaptive sizing is enabled
by default with the throughput garbage collector by using the
-XX:+UseParallelGC and -XX:+UseParallelOldGC options, and survivor
space is resized according to the application behavior, starting
with the initial value. If adaptive sizing is disabled (using the
-XX:-UseAdaptiveSizePolicy option), then the -XX:SurvivorRatio
option should be used to set the size of the survivor space for the
entire execution of the application.
The following formula can be used to calculate the initial size of
survivor space (S) based on the size of the young generation (Y),
and the initial survivor space ratio (R):
S=Y/(R+2)
The 2 in the equation denotes two survivor spaces. The larger the
value specified as the initial survivor space ratio, the smaller
the initial survivor space size.
By default, the initial survivor space ratio is set to 8. If the
default value for the young generation space size is used (2 MB),
the initial size of the survivor space will be 0.2 MB.
The following example shows how to set the initial survivor space
ratio to 4:
-XX:InitialSurvivorRatio=4
-XX:InitiatingHeapOccupancyPercent=percent
Sets the percentage of the heap occupancy (0 to 100) at which to
start a concurrent GC cycle. It is used by garbage collectors that
trigger a concurrent GC cycle based on the occupancy of the entire
heap, not just one of the generations (for example, the G1 garbage
collector).
By default, the initiating value is set to 45%. A value of 0
implies nonstop GC cycles. The following example shows how to set
the initiating heap occupancy to 75%:
-XX:InitiatingHeapOccupancyPercent=75
-XX:MaxGCPauseMillis=time
Sets a target for the maximum GC pause time (in milliseconds). This
is a soft goal, and the JVM will make its best effort to achieve
it. By default, there is no maximum pause time value.
The following example shows how to set the maximum target pause
time to 500 ms:
-XX:MaxGCPauseMillis=500
-XX:MaxHeapSize=size
Sets the maximum size (in byes) of the memory allocation pool. This
value must be a multiple of 1024 and greater than 2 MB. Append the
letter k or K to indicate kilobytes, m or M to indicate megabytes,
g or G to indicate gigabytes. The default value is chosen at
runtime based on system configuration. For server deployments,
-XX:InitialHeapSize and -XX:MaxHeapSize are often set to the same
value. See the section "Ergonomics" in Java SE HotSpot Virtual
Machine Garbage Collection Tuning Guide at
http://docs.oracle.com/javase/8/docs/technotes/guides/vm/gctuning/index.html.
The following examples show how to set the maximum allowed size of
allocated memory to 80 MB using various units:
-XX:MaxHeapSize=83886080
-XX:MaxHeapSize=81920k
-XX:MaxHeapSize=80m
On Oracle Solaris 7 and Oracle Solaris 8 SPARC platforms, the upper
limit for this value is approximately 4,000 MB minus overhead
amounts. On Oracle Solaris 2.6 and x86 platforms, the upper limit
is approximately 2,000 MB minus overhead amounts. On Linux
platforms, the upper limit is approximately 2,000 MB minus overhead
amounts.
The -XX:MaxHeapSize option is equivalent to -Xmx.
-XX:MaxHeapFreeRatio=percent
Sets the maximum allowed percentage of free heap space (0 to 100)
after a GC event. If free heap space expands above this value, then
the heap will be shrunk. By default, this value is set to 70%.
The following example shows how to set the maximum free heap ratio
to 75%:
-XX:MaxHeapFreeRatio=75
-XX:MaxMetaspaceSize=size
Sets the maximum amount of native memory that can be allocated for
class metadata. By default, the size is not limited. The amount of
metadata for an application depends on the application itself,
other running applications, and the amount of memory available on
the system.
The following example shows how to set the maximum class metadata
size to 256 MB:
-XX:MaxMetaspaceSize=256m
-XX:MaxNewSize=size
Sets the maximum size (in bytes) of the heap for the young
generation (nursery). The default value is set ergonomically.
-XX:MaxTenuringThreshold=threshold
Sets the maximum tenuring threshold for use in adaptive GC sizing.
The largest value is 15. The default value is 15 for the parallel
(throughput) collector, and 6 for the CMS collector.
The following example shows how to set the maximum tenuring
threshold to 10:
-XX:MaxTenuringThreshold=10
-XX:MetaspaceSize=size
Sets the size of the allocated class metadata space that will
trigger a garbage collection the first time it is exceeded. This
threshold for a garbage collection is increased or decreased
depending on the amount of metadata used. The default size depends
on the platform.
-XX:MinHeapFreeRatio=percent
Sets the minimum allowed percentage of free heap space (0 to 100)
after a GC event. If free heap space falls below this value, then
the heap will be expanded. By default, this value is set to 40%.
The following example shows how to set the minimum free heap ratio
to 25%:
-XX:MinHeapFreeRatio=25
-XX:NewRatio=ratio
Sets the ratio between young and old generation sizes. By default,
this option is set to 2. The following example shows how to set the
young/old ratio to 1:
-XX:NewRatio=1
-XX:NewSize=size
Sets the initial size (in bytes) of the heap for the young
generation (nursery). Append the letter k or K to indicate
kilobytes, m or M to indicate megabytes, g or G to indicate
gigabytes.
The young generation region of the heap is used for new objects. GC
is performed in this region more often than in other regions. If
the size for the young generation is too low, then a large number
of minor GCs will be performed. If the size is too high, then only
full GCs will be performed, which can take a long time to complete.
Oracle recommends that you keep the size for the young generation
between a half and a quarter of the overall heap size.
The following examples show how to set the initial size of young
generation to 256 MB using various units:
-XX:NewSize=256m
-XX:NewSize=262144k
-XX:NewSize=268435456
The -XX:NewSize option is equivalent to -Xmn.
-XX:ParallelGCThreads=threads
Sets the number of threads used for parallel garbage collection in
the young and old generations. The default value depends on the
number of CPUs available to the JVM.
For example, to set the number of threads for parallel GC to 2,
specify the following option:
-XX:ParallelGCThreads=2
-XX:+ParallelRefProcEnabled
Enables parallel reference processing. By default, this option is
disabled.
-XX:+PrintAdaptiveSizePolicy
Enables printing of information about adaptive generation sizing.
By default, this option is disabled.
-XX:+PrintGC
Enables printing of messages at every GC. By default, this option
is disabled.
-XX:+PrintGCApplicationConcurrentTime
Enables printing of how much time elapsed since the last pause (for
example, a GC pause). By default, this option is disabled.
-XX:+PrintGCApplicationStoppedTime
Enables printing of how much time the pause (for example, a GC
pause) lasted. By default, this option is disabled.
-XX:+PrintGCDateStamps
Enables printing of a date stamp at every GC. By default, this
option is disabled.
-XX:+PrintGCDetails
Enables printing of detailed messages at every GC. By default, this
option is disabled.
-XX:+PrintGCTaskTimeStamps
Enables printing of time stamps for every individual GC worker
thread task. By default, this option is disabled.
-XX:+PrintGCTimeStamps
Enables printing of time stamps at every GC. By default, this
option is disabled.
-XX:+PrintStringDeduplicationStatistics
Prints detailed deduplication statistics. By default, this option
is disabled. See the -XX:+UseStringDeduplication option.
-XX:+PrintTenuringDistribution
Enables printing of tenuring age information. The following is an
example of the output:
Desired survivor size 48286924 bytes, new threshold 10 (max 10)
- age 1: 28992024 bytes, 28992024 total
- age 2: 1366864 bytes, 30358888 total
- age 3: 1425912 bytes, 31784800 total
...
Age 1 objects are the youngest survivors (they were created after
the previous scavenge, survived the latest scavenge, and moved from
eden to survivor space). Age 2 objects have survived two scavenges
(during the second scavenge they were copied from one survivor
space to the next). And so on.
In the preceding example, 28 992 024 bytes survived one scavenge
and were copied from eden to survivor space, 1 366 864 bytes are
occupied by age 2 objects, etc. The third value in each row is the
cumulative size of objects of age n or less.
By default, this option is disabled.
-XX:+ScavengeBeforeFullGC
Enables GC of the young generation before each full GC. This option
is enabled by default. Oracle recommends that you do not disable
it, because scavenging the young generation before a full GC can
reduce the number of objects reachable from the old generation
space into the young generation space. To disable GC of the young
generation before each full GC, specify -XX:-ScavengeBeforeFullGC.
-XX:SoftRefLRUPolicyMSPerMB=time
Sets the amount of time (in milliseconds) a softly reachable object
is kept active on the heap after the last time it was referenced.
The default value is one second of lifetime per free megabyte in
the heap. The -XX:SoftRefLRUPolicyMSPerMB option accepts integer
values representing milliseconds per one megabyte of the current
heap size (for Java HotSpot Client VM) or the maximum possible heap
size (for Java HotSpot Server VM). This difference means that the
Client VM tends to flush soft references rather than grow the heap,
whereas the Server VM tends to grow the heap rather than flush soft
references. In the latter case, the value of the -Xmx option has a
significant effect on how quickly soft references are garbage
collected.
The following example shows how to set the value to 2.5 seconds:
-XX:SoftRefLRUPolicyMSPerMB=2500
-XX:StringDeduplicationAgeThreshold=threshold
String objects reaching the specified age are considered candidates
for deduplication. An object's age is a measure of how many times
it has survived garbage collection. This is sometimes referred to
as tenuring; see the -XX:+PrintTenuringDistribution option. Note
that String objects that are promoted to an old heap region before
this age has been reached are always considered candidates for
deduplication. The default value for this option is 3. See the
-XX:+UseStringDeduplication option.
-XX:SurvivorRatio=ratio
Sets the ratio between eden space size and survivor space size. By
default, this option is set to 8. The following example shows how
to set the eden/survivor space ratio to 4:
-XX:SurvivorRatio=4
-XX:TargetSurvivorRatio=percent
Sets the desired percentage of survivor space (0 to 100) used after
young garbage collection. By default, this option is set to 50%.
The following example shows how to set the target survivor space
ratio to 30%:
-XX:TargetSurvivorRatio=30
-XX:TLABSize=size
Sets the initial size (in bytes) of a thread-local allocation
buffer (TLAB). Append the letter k or K to indicate kilobytes, m or
M to indicate megabytes, g or G to indicate gigabytes. If this
option is set to 0, then the JVM chooses the initial size
automatically.
The following example shows how to set the initial TLAB size to 512
KB:
-XX:TLABSize=512k
-XX:+UseAdaptiveSizePolicy
Enables the use of adaptive generation sizing. This option is
enabled by default. To disable adaptive generation sizing, specify
-XX:-UseAdaptiveSizePolicy and set the size of the memory
allocation pool explicitly (see the -XX:SurvivorRatio option).
-XX:+UseCMSInitiatingOccupancyOnly
Enables the use of the occupancy value as the only criterion for
initiating the CMS collector. By default, this option is disabled
and other criteria may be used.
-XX:+UseConcMarkSweepGC
Enables the use of the CMS garbage collector for the old
generation. Oracle recommends that you use the CMS garbage
collector when application latency requirements cannot be met by
the throughput (-XX:+UseParallelGC) garbage collector. The G1
garbage collector (-XX:+UseG1GC) is another alternative.
By default, this option is disabled and the collector is chosen
automatically based on the configuration of the machine and type of
the JVM. When this option is enabled, the -XX:+UseParNewGC option
is automatically set and you should not disable it, because the
following combination of options has been deprecated in JDK 8:
-XX:+UseConcMarkSweepGC -XX:-UseParNewGC.
-XX:+UseG1GC
Enables the use of the garbage-first (G1) garbage collector. It is
a server-style garbage collector, targeted for multiprocessor
machines with a large amount of RAM. It meets GC pause time goals
with high probability, while maintaining good throughput. The G1
collector is recommended for applications requiring large heaps
(sizes of around 6 GB or larger) with limited GC latency
requirements (stable and predictable pause time below 0.5 seconds).
By default, this option is disabled and the collector is chosen
automatically based on the configuration of the machine and type of
the JVM.
-XX:+UseGCOverheadLimit
Enables the use of a policy that limits the proportion of time
spent by the JVM on GC before an OutOfMemoryError exception is
thrown. This option is enabled, by default and the parallel GC will
throw an OutOfMemoryError if more than 98% of the total time is
spent on garbage collection and less than 2% of the heap is
recovered. When the heap is small, this feature can be used to
prevent applications from running for long periods of time with
little or no progress. To disable this option, specify
-XX:-UseGCOverheadLimit.
-XX:+UseNUMA
Enables performance optimization of an application on a machine
with nonuniform memory architecture (NUMA) by increasing the
application's use of lower latency memory. By default, this option
is disabled and no optimization for NUMA is made. The option is
only available when the parallel garbage collector is used
(-XX:+UseParallelGC).
-XX:+UseParallelGC
Enables the use of the parallel scavenge garbage collector (also
known as the throughput collector) to improve the performance of
your application by leveraging multiple processors.
By default, this option is disabled and the collector is chosen
automatically based on the configuration of the machine and type of
the JVM. If it is enabled, then the -XX:+UseParallelOldGC option is
automatically enabled, unless you explicitly disable it.
-XX:+UseParallelOldGC
Enables the use of the parallel garbage collector for full GCs. By
default, this option is disabled. Enabling it automatically enables
the -XX:+UseParallelGC option.
-XX:+UseParNewGC
Enables the use of parallel threads for collection in the young
generation. By default, this option is disabled. It is
automatically enabled when you set the -XX:+UseConcMarkSweepGC
option. Using the -XX:+UseParNewGC option without the
-XX:+UseConcMarkSweepGC option was deprecated in JDK 8.
-XX:+UseSerialGC
Enables the use of the serial garbage collector. This is generally
the best choice for small and simple applications that do not
require any special functionality from garbage collection. By
default, this option is disabled and the collector is chosen
automatically based on the configuration of the machine and type of
the JVM.
-XX:+UseSHM
On Linux, enables the JVM to use shared memory to setup large
pages.
For more information, see "Large Pages".
-XX:+UseStringDeduplication
Enables string deduplication. By default, this option is disabled.
To use this option, you must enable the garbage-first (G1) garbage
collector. See the -XX:+UseG1GC option.
String deduplication reduces the memory footprint of String objects
on the Java heap by taking advantage of the fact that many String
objects are identical. Instead of each String object pointing to
its own character array, identical String objects can point to and
share the same character array.
-XX:+UseTLAB
Enables the use of thread-local allocation blocks (TLABs) in the
young generation space. This option is enabled by default. To
disable the use of TLABs, specify -XX:-UseTLAB.
Deprecated and Removed Options
Deprecated and Removed Options
These options were included in the previous release, but have since
been considered unnecessary.
-Xincgc
Enables incremental garbage collection. This option was deprecated
in JDK 8 with no replacement.
-Xrunlibname
Loads the specified debugging/profiling library. This option was
superseded by the -agentlib option.
-XX:CMSIncrementalDutyCycle=percent
Sets the percentage of time (0 to 100) between minor collections
that the concurrent collector is allowed to run. This option was
deprecated in JDK 8 with no replacement, following the deprecation
of the -XX:+CMSIncrementalMode option.
-XX:CMSIncrementalDutyCycleMin=percent
Sets the percentage of time (0 to 100) between minor collections
that is the lower bound for the duty cycle when
-XX:+CMSIncrementalPacing is enabled. This option was deprecated in
JDK 8 with no replacement, following the deprecation of the
-XX:+CMSIncrementalMode option.
-XX:+CMSIncrementalMode
Enables the incremental mode for the CMS collector. This option was
deprecated in JDK 8 with no replacement, along with other options
that start with CMSIncremental.
-XX:CMSIncrementalOffset=percent
Sets the percentage of time (0 to 100) by which the incremental
mode duty cycle is shifted to the right within the period between
minor collections. This option was deprecated in JDK 8 with no
replacement, following the deprecation of the
-XX:+CMSIncrementalMode option.
-XX:+CMSIncrementalPacing
Enables automatic adjustment of the incremental mode duty cycle
based on statistics collected while the JVM is running. This option
was deprecated in JDK 8 with no replacement, following the
deprecation of the -XX:+CMSIncrementalMode option.
-XX:CMSIncrementalSafetyFactor=percent
Sets the percentage of time (0 to 100) used to add conservatism
when computing the duty cycle. This option was deprecated in JDK 8
with no replacement, following the deprecation of the
-XX:+CMSIncrementalMode option.
-XX:CMSInitiatingPermOccupancyFraction=percent
Sets the percentage of the permanent generation occupancy (0 to
100) at which to start a GC. This option was deprecated in JDK 8
with no replacement.
-XX:MaxPermSize=size
Sets the maximum permanent generation space size (in bytes). This
option was deprecated in JDK 8, and superseded by the
-XX:MaxMetaspaceSize option.
-XX:PermSize=size
Sets the space (in bytes) allocated to the permanent generation
that triggers a garbage collection if it is exceeded. This option
was deprecated un JDK 8, and superseded by the -XX:MetaspaceSize
option.
-XX:+UseSplitVerifier
Enables splitting of the verification process. By default, this
option was enabled in the previous releases, and verification was
split into two phases: type referencing (performed by the compiler)
and type checking (performed by the JVM runtime). This option was
deprecated in JDK 8, and verification is now split by default
without a way to disable it.
-XX:+UseStringCache
Enables caching of commonly allocated strings. This option was
removed from JDK 8 with no replacement.
PERFORMANCE TUNING EXAMPLES
PERFORMANCE TUNING EXAMPLES
The following examples show how to use experimental tuning flags to
either optimize throughput or to provide lower response time.
Example 1 Tuning for Higher Throughput
java -d64 -server -XX:+AggressiveOpts -XX:+UseLargePages -Xmn10g -Xms26g -Xmx26g
Example 2 Tuning for Lower Response Time
java -d64 -XX:+UseG1GC -Xms26g Xmx26g -XX:MaxGCPauseMillis=500 -XX:+PrintGCTimeStamp
LARGE PAGES
LARGE PAGES
Also known as huge pages, large pages are memory pages that are
significantly larger than the standard memory page size (which varies
depending on the processor and operating system). Large pages optimize
processor Translation-Lookaside Buffers.
A Translation-Lookaside Buffer (TLB) is a page translation cache that
holds the most-recently used virtual-to-physical address translations.
TLB is a scarce system resource. A TLB miss can be costly as the
processor must then read from the hierarchical page table, which may
require multiple memory accesses. By using a larger memory page size, a
single TLB entry can represent a larger memory range. There will be
less pressure on TLB, and memory-intensive applications may have better
performance.
However, large pages page memory can negatively affect system
performance. For example, when a large mount of memory is pinned by an
application, it may create a shortage of regular memory and cause
excessive paging in other applications and slow down the entire system.
Also, a system that has been up for a long time could produce excessive
fragmentation, which could make it impossible to reserve enough large
page memory. When this happens, either the OS or JVM reverts to using
regular pages.
Large Pages Support
Large Pages Support
Solaris and Linux support large pages.
Solaris
Solaris 9 and later include Multiple Page Size Support (MPSS); no
additional configuration is necessary. See
http://www.oracle.com/technetwork/server-storage/solaris10/overview/solaris9-features-scalability-135663.html.
Linux
The 2.6 kernel supports large pages. Some vendors have backported
the code to their 2.4-based releases. To check if your system can
support large page memory, try the following:
# cat /proc/meminfo | grep Huge
HugePages_Total: 0
HugePages_Free: 0
Hugepagesize: 2048 kB
If the output shows the three "Huge" variables, then your system
can support large page memory but it needs to be configured. If the
command prints nothing, then your system does not support large
pages. To configure the system to use large page memory, login as
root, and then follow these steps:
1. If you are using the option -XX:+UseSHM (instead of
-XX:+UseHugeTLBFS), then increase the SHMMAX value. It must be
larger than the Java heap size. On a system with 4 GB of
physical RAM (or less), the following will make all the memory
sharable:
# echo 4294967295 > /proc/sys/kernel/shmmax
2. If you are using the option -XX:+UseSHM or -XX:+UseHugeTLBFS,
then specify the number of large pages. In the following
example, 3 GB of a 4 GB system are reserved for large pages
(assuming a large page size of 2048kB, then 3 GB = 3 * 1024 MB
= 3072 MB = 3072 * 1024 kB = 3145728 kB and 3145728 kB / 2048
kB = 1536):
# echo 1536 > /proc/sys/vm/nr_hugepages
Note
┌──────────────────────────────────────────────┐
│ │
│ · Note that the values │
│ contained in /proc │
│ will reset after you │
│ reboot your system, │
│ so may want to set │
│ them in an │
│ initialization │
│ script (for example, │
│ rc.local or │
│ sysctl.conf). │
│ │
│ · If you configure (or │
│ resize) the OS │
│ kernel parameters │
│ /proc/sys/kernel/shmmax │
│ or │
│ /proc/sys/vm/nr_hugepages, │
│ Java processes may │
│ allocate large pages │
│ for areas in │
│ addition to the Java │
│ heap. These steps │
│ can allocate large │
│ pages for the │
│ following areas: │
│ │
│ · Java heap │
│ │
│ · Code cache │
│ │
│ · The marking │
│ bitmap data │
│ structure for │
│ the parallel GC │
│ │
│ Consequently, if you │
│ configure the │
│ nr_hugepages │
│ parameter to the │
│ size of the Java │
│ heap, then the JVM │
│ can fail in │
│ allocating the code │
│ cache areas on large │
│ pages because these │
│ areas are quite │
│ large in size. │
└──────────────────────────────────────────────┘
APPLICATION CLASS DATA SHARING
APPLICATION CLASS DATA SHARING
Application Class Data Sharing (AppCDS) extends CDS (see
https://docs.oracle.com/javase/8/docs/technotes/guides/vm/class-data-sharing.html)
to enable classes from the standard extensions directories (specified
by the system property java.ext.dirs; see
https://docs.oracle.com/javase/8/docs/technotes/guides/extensions/spec.html)
and the application class path (see "Setting the Class Path ") to be
placed in the shared archive. AppCDS reduces the footprint and
decreases start-up time of your applications provided that a
substantial number of classes are loaded from the application class
path.
This is a commercial feature that requires you to also specify the
-XX:+UnlockCommercialFeatures option. This is also an experimental
feature; it may change in future releases.
Creating a Shared Archive File, and Running an Application with It
The following steps create a shared archive file that contains all the
classes used by the test.Hello application. The last step runs the
application with the shared archive file.
1. Create a list of all classes used by the test.Hello application.
The following command creates a file named hello.classlist that
contains a list of all classes used by this application:
java -Xshare:off -XX:+UnlockCommercialFeatures
-XX:DumpLoadedClassList=hello.classlist -XX:+UseAppCDS -cp
hello.jar test.Hello
Note that the -cp parameter must contain only JAR files; the
-XX:+UseAppCDS option does not support class paths that contain
directory names.
2. Create a shared archive, named hello.jsa, that contains all the
classes in hello.classlist:
java -XX:+UnlockCommercialFeatures -Xshare:dump -XX:+UseAppCDS
-XX:SharedArchiveFile=hello.jsa
-XX:SharedClassListFile=hello.classlist -cp hello.jar
Note that the -cp parameter used at archive creation time must be
the same as (or a prefix of) the -cp used at run time.
3. Run the application test.Hello with the shared archive hello.jsa:
java -XX:+UnlockCommercialFeatures -Xshare:on -XX:+UseAppCDS
-XX:SharedArchiveFile=hello.jsa -cp hello.jar test.Hello
Ensure that you have specified the option -Xshare:on or
-Xshare:auto.
4. Verify that the test.Hello application is using the class contained
in the hello.jsa shared archive:
java -XX:+UnlockCommercialFeatures -Xshare:on -XX:+UseAppCDS
-XX:SharedArchiveFile=hello.jsa -cp hello.jar -verbose:class
test.Hello
The output of this command should contain the following text:
Loaded test.Hello from shared objects file by
sun/misc/Launcher$AppClassLoader
Sharing a Shared Archive across Multiple Application Processes
You can share the same archive file across multiple applications
processes that have the exact same class path or share a common class
path prefix. This reduces memory usage as the archive is memory-mapped
into the address space of the processes. The operating system
automatically shares the read-only pages across these processes.
The following steps create a shared archive that both applications
Hello and Hi can use.
1. Create a list of all classes used by the Hello application and
another list for the Hi application:
java -XX:+UnlockCommercialFeatures
-XX:DumpLoadedClassList=hello.classlist -XX:+UseAppCDS -cp
common.jar:hello.jar Hello
java -XX:+UnlockCommercialFeatures
-XX:DumpLoadedClassList=hi.classlist -XX:+UseAppCDS -cp
common.jar:hi.jar Hi
Note that because the Hello and Hi applications share a common
class path prefix (both of their class paths start with
common.jar), these two applications can share a shared archive
file.
2. Create a single list of classes used by all the applications that
will share the shared archive file.
The following commands combine the files hello.classlist and
hi.classlist to one file, common.classlist:
cat hello.classlist hi.classlist > common.classlist
3. Create a shared archive, named common.jsa, that contains all the
classes in common.classlist:
java -XX:+UnlockCommercialFeatures -Xshare:dump
-XX:SharedArchiveFile=common.jsa -XX:+UseAppCDS
-XX:SharedClassListFile=common.classlist -cp common.jar
The value of the -cp parameter is the common class path prefix
shared by the Hello and Hi applications.
4. Run the Hello and Hi applications with the same shared archive:
java -XX:+UnlockCommercialFeatures -Xshare:on
-XX:SharedArchiveFile=common.jsa -XX:+UseAppCDS -cp
common.jar:hello.jar Hello
java -XX:+UnlockCommercialFeatures -Xshare:on
-XX:SharedArchiveFile=common.jsa -XX:+UseAppCDS -cp
common.jar:hi.jar Hi
EXIT STATUS
The following exit values are typically returned by the launcher when
the launcher is called with the wrong arguments, serious errors, or
exceptions thrown by the JVM. However, a Java application may choose to
return any value by using the API call System.exit(exitValue). The
values are:
· 0: Successful completion
· >0: An error occurred
SEE ALSO
· javac(1)
· jdb(1)
· javah(1)
· jar(1)
· jstat(1)
JDK 8 03 March 2015 java(1)
