故宫文物保护:Java HotSpot VM Options
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This document provides information on typical command-line options and environment variables that can affect the performance characteristics of the Java HotSpot Virtual Machine. Unless otherwise noted, all information in this document pertains to both the Java HotSpot Client VM and the Java HotSpot Server VM.
Users of JDKs older than
Categories of Java HotSpot VM Options
Standard options recognized by the Java HotSpot VM are described on the Java Application Launcher reference pages for Windows, Solaris and Linux. This document deals exclusively with non-standard options recognized by the Java HotSpot VM:
· Options that begin with -X are non-standard (not guaranteed to be supported on all VM implementations), and are subject to change without notice in subsequent releases of the JDK.
· Options that are specified with -XX are not stable and are not recommended for casual use. These options are subject to change without notice.
Some Useful -XX Options
Default values are listed for Java SE 6 for Solaris Sparc with -server. Some options may vary per architecture/OS/JVM version. Platforms with a differing default value are listed in the description.
· Boolean options are turned on with -XX:+
· Numeric options are set with -XX:
· String options are set with -XX:
Flags marked as manageable are dynamically writeable through the JDK management interface (com.sun.management.HotSpotDiagnosticMXBean API) and also through JConsole. In Monitoring and Managing Java SE 6 Platform Applications, Figure 3 shows an example. The manageable flags can also be set through jinfo -flag.
The options below are loosely grouped into three categories.
· Behavioral options change the basic behavior of the VM.
· Performance tuning options are knobs which can be used to tune VM performance.
· Debugging options generally enable tracing, printing, or output of VM information.
Behavioral Options
Option and Default Value
Description
-XX:-AllowUserSignalHandlers
Do not complain if the application installs signal handlers. (Relevant to Solaris and Linux only.)
-XX:AltStackSize=16384
Alternate signal stack size (in Kbytes). (Relevant to Solaris only, removed from 5.0.)
-XX:-DisableExplicitGC
Disable calls to System.gc(), JVM still performs garbage collection when necessary.
-XX:+FailOverToOldVerifier
Fail over to old verifier when the new type checker fails. (Introduced in 6.)
-XX:+HandlePromotionFailure
The youngest generation collection does not require a guarantee of full promotion of all live objects. (Introduced in
-XX:+MaxFDLimit
Bump the number of file descriptors to max. (Relevant to Solaris only.)
-XX:PreBlockSpin=10
Spin count variable for use with -XX:+UseSpinning. Controls the maximum spin iterations allowed before entering operating system thread synchronization code. (Introduced in
-XX:-RelaxAccessControlCheck
Relax the access control checks in the verifier. (Introduced in 6.)
-XX:+ScavengeBeforeFullGC
Do young generation GC prior to a full GC. (Introduced in
-XX:+UseAltSigs
Use alternate signals instead of SIGUSR1 and SIGUSR2 for VM internal signals. (Introduced in
-XX:+UseBoundThreads
Bind user level threads to kernel threads. (Relevant to Solaris only.)
-XX:-UseConcMarkSweepGC
Use concurrent mark-sweep collection for the old generation. (Introduced in
-XX:+UseGCOverheadLimit
Use a policy that limits the proportion of the VM's time that is spent in GC before an OutOfMemory error is thrown. (Introduced in 6.)
-XX:+UseLWPSynchronization
Use LWP-based instead of thread based synchronization. (Introduced in
-XX:-UseParallelGC
Use parallel garbage collection for scavenges. (Introduced in
-XX:-UseParallelOldGC
Use parallel garbage collection for the full collections. Enabling this option automatically sets -XX:+UseParallelGC. (Introduced in 5.0 update 6.)
-XX:-UseSerialGC
Use serial garbage collection. (Introduced in 5.0.)
-XX:-UseSpinning
Enable naive spinning on Java monitor before entering operating system thread synchronizaton code. (Relevant to
-XX:+UseTLAB
Use thread-local object allocation (Introduced in
-XX:+UseSplitVerifier
Use the new type checker with StackMapTable attributes. (Introduced in 5.0.)[5.0: false]
-XX:+UseThreadPriorities
Use native thread priorities.
-XX:+UseVMInterruptibleIO
Thread interrupt before or with EINTR for I/O operations results in OS_INTRPT. (Introduced in 6. Relevant to Solaris only.)
Back to Options
Performance Options
Option and Default Value
Description
-XX:+AggressiveOpts
Turn on point performance compiler optimizations that are expected to be default in upcoming releases. (Introduced in 5.0 update 6.)
-XX:CompileThreshold=10000
Number of method invocations/branches before compiling [-client: 1,500]
-XX:LargePageSizeInBytes=
Sets the large page size used for the Java heap. (Introduced in
-XX:MaxHeapFreeRatio=70
Maximum percentage of heap free after GC to avoid shrinking.
-XX:MaxNewSize=size
Maximum size of new generation (in bytes). Since 1.4, MaxNewSize is computed as a function of NewRatio. [
-XX:MaxPermSize=
Size of the Permanent Generation. [5.0 and newer: 64 bit VMs are scaled 30% larger; 1.4 amd64:
-XX:MinHeapFreeRatio=40
Minimum percentage of heap free after GC to avoid expansion.
-XX:NewRatio=2
Ratio of new/old generation sizes. [Sparc -client: 8; x86 -server: 8; x86 -client: 12.]-client: 4 (1.3) 8 (
-XX:NewSize=
Default size of new generation (in bytes) [5.0 and newer: 64 bit VMs are scaled 30% larger; x86:
-XX:ReservedCodeCacheSize=
Reserved code cache size (in bytes) - maximum code cache size. [Solaris 64-bit, amd64, and -server x86:
-XX:SurvivorRatio=8
Ratio of eden/survivor space size [Solaris amd64: 6; Sparc in
-XX:TargetSurvivorRatio=50
Desired percentage of survivor space used after scavenge.
-XX:ThreadStackSize=512
Thread Stack Size (in Kbytes). (0 means use default stack size) [Sparc: 512; Solaris x86: 320 (was 256 prior in 5.0 and earlier); Sparc 64 bit: 1024; Linux amd64: 1024 (was
-XX:+UseBiasedLocking
Enable biased locking. For more details, see this tuning example. (Introduced in 5.0 update 6.) [5.0: false]
-XX:+UseFastAccessorMethods
Use optimized versions of Get
-XX:-UseISM
Use Intimate Shared Memory. [Not accepted for non-Solaris platforms.] For details, see Intimate Shared Memory.
-XX:+UseLargePages
Use large page memory. (Introduced in 5.0 update 5.) For details, see Java Support for Large Memory Pages.
-XX:+UseMPSS
Use Multiple Page Size Support w/4mb pages for the heap. Do not use with ISM as this replaces the need for ISM. (Introduced in
-XX:+UseStringCache
Enables caching of commonly allocated strings.
-XX:AllocatePrefetchLines=1
Number of cache lines to load after the last object allocation using prefetch instructions generated in JIT compiled code. Default values are 1 if the last allocated object was an instance and 3 if it was an array.
-XX:AllocatePrefetchStyle=1
Generated code style for prefetch instructions.
0 - no prefetch instructions are generate*d*,
1 - execute prefetch instructions after each allocation,
2 - use TLAB allocation watermark pointer to gate when prefetch instructions are executed.
-XX:+UseCompressedStrings
Use a byte[] for Strings which can be represented as pure ASCII. (Introduced in Java 6 Update 21 Performance Release)
-XX:+OptimizeStringConcat
Optimize String concatenation operations where possible. (Introduced in Java 6 Update 20)
Back to Options
Debugging Options
Option and Default Value
Description
-XX:-CITime
Prints time spent in JIT Compiler. (Introduced in
-XX:ErrorFile=./hs_err_pid
If an error occurs, save the error data to this file. (Introduced in 6.)
-XX:-ExtendedDTraceProbes
Enable performance-impacting dtrace probes. (Introduced in 6. Relevant to Solaris only.)
-XX:HeapDumpPath=./java_pid
Path to directory or filename for heap dump. Manageable. (Introduced in
-XX:-HeapDumpOnOutOfMemoryError
Dump heap to file when java.lang.OutOfMemoryError is thrown. Manageable. (Introduced in
-XX:OnError="
Run user-defined commands on fatal error. (Introduced in
-XX:OnOutOfMemoryError="
Run user-defined commands when an OutOfMemoryError is first thrown. (Introduced in
-XX:-PrintClassHistogram
Print a histogram of class instances on Ctrl-Break. Manageable. (Introduced in
-XX:-PrintConcurrentLocks
Print java.util.concurrent locks in Ctrl-Break thread dump. Manageable. (Introduced in 6.) The jstack -l command provides equivalent functionality.
-XX:-PrintCommandLineFlags
Print flags that appeared on the command line. (Introduced in 5.0.)
-XX:-PrintCompilation
Print message when a method is compiled.
-XX:-PrintGC
Print messages at garbage collection. Manageable.
-XX:-PrintGCDetails
Print more details at garbage collection. Manageable. (Introduced in
-XX:-PrintGCTimeStamps
Print timestamps at garbage collection. Manageable (Introduced in
-XX:-PrintTenuringDistribution
Print tenuring age information.
-XX:-TraceClassLoading
Trace loading of classes.
-XX:-TraceClassLoadingPreorder
Trace all classes loaded in order referenced (not loaded). (Introduced in
-XX:-TraceClassResolution
Trace constant pool resolutions. (Introduced in
-XX:-TraceClassUnloading
Trace unloading of classes.
-XX:-TraceLoaderConstraints
Trace recording of loader constraints. (Introduced in 6.)
-XX:+PerfSaveDataToFile
Saves jvmstat binary data on exit.
-XX:ParallelGCThreads=
Sets the number of garbage collection threads in the young and old parallel garbage collectors. The default value varies with the platform on which the JVM is running.
-XX:+UseCompressedOops
Enables the use of compressed pointers (object references represented as 32 bit offsets instead of 64-bit pointers) for optimized 64-bit performance with Java heap sizes less than 32gb.
-XX:+AlwaysPreTouch
Pre-touch the Java heap during JVM initialization. Every page of the heap is thus demand-zeroed during initialization rather than incrementally during application execution.
-XX:AllocatePrefetchDistance=
Sets the prefetch distance for object allocation. Memory about to be written with the value of new objects is prefetched into cache at this distance (in bytes) beyond the address of the last allocated object. Each Java thread has its own allocation point. The default value varies with the platform on which the JVM is running.
-XX:InlineSmallCode=
Inline a previously compiled method only if its generated native code size is less than this. The default value varies with the platform on which the JVM is running.
-XX:MaxInlineSize=35
Maximum bytecode size of a method to be inlined.
-XX:FreqInlineSize=
Maximum bytecode size of a frequently executed method to be inlined. The default value varies with the platform on which the JVM is running.
-XX:LoopUnrollLimit=
Unroll loop bodies with server compiler intermediate representation node count less than this value. The limit used by the server compiler is a function of this value, not the actual value. The default value varies with the platform on which the JVM is running.
-XX:InitialTenuringThreshold=7
Sets the initial tenuring threshold for use in adaptive GC sizing in the parallel young collector. The tenuring threshold is the number of times an object survives a young collection before being promoted to the old, or tenured, generation.
-XX:MaxTenuringThreshold=
Sets the maximum tenuring threshold for use in adaptive GC sizing. The current largest value is 15. The default value is 15 for the parallel collector and is 4 for CMS.