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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.

Categories of Java HotSpot VM Options

  
Standard options recognized by the Java HotSpot VM are described on the Java Application Launcher reference pages for WindowsSolaris 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 subject to change without notice.
Users of JDKs older than 1.3.0 who wish to port to a Java HotSpot VM, should see Java HotSpot Equivalents of Exact VM flags.

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:+ and turned off with -XX:-.Disa
  • Numeric options are set with -XX:. Numbers can include 'm' or 'M' for megabytes, 'k' or 'K' for kilobytes, and 'g' or 'G' for gigabytes (for example, 32k is the same as 32768).
  • String options are set with -XX:, are usually used to specify a file, a path, or a list of commands
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 categories.
  

Behavioral Options


Option and Default ValueDescription
-XX:-AllowUserSignalHandlersDo not complain if the application installs signal handlers. (Relevant to Solaris and Linux only.)
-XX:AltStackSize=16384Alternate signal stack size (in Kbytes). (Relevant to Solaris only, removed from 5.0.)
-XX:-DisableExplicitGCBy default calls to System.gc() are enabled (-XX:-DisableExplicitGC). Use -XX:+DisableExplicitGC to disable calls to System.gc(). Note that the JVM still performs garbage collection when necessary.
-XX:+FailOverToOldVerifierFail over to old verifier when the new type checker fails. (Introduced in 6.)
-XX:+HandlePromotionFailureThe youngest generation collection does not require a guarantee of full promotion of all live objects. (Introduced in 1.4.2 update 11) [5.0 and earlier: false.]
-XX:+MaxFDLimitBump the number of file descriptors to max. (Relevant  to Solaris only.)
-XX:PreBlockSpin=10Spin count variable for use with -XX:+UseSpinning. Controls the maximum spin iterations allowed before entering operating system thread synchronization code. (Introduced in 1.4.2.)
-XX:-RelaxAccessControlCheckRelax the access control checks in the verifier. (Introduced in 6.)
-XX:+ScavengeBeforeFullGCDo young generation GC prior to a full GC. (Introduced in 1.4.1.)
-XX:+UseAltSigsUse alternate signals instead of SIGUSR1 and SIGUSR2 for VM internal signals. (Introduced in 1.3.1 update 9, 1.4.1. Relevant to Solaris only.)
-XX:+UseBoundThreadsBind user level threads to kernel threads. (Relevant to Solaris only.)
-XX:-UseConcMarkSweepGCUse concurrent mark-sweep collection for the old generation. (Introduced in 1.4.1)
-XX:+UseGCOverheadLimitUse 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:+UseLWPSynchronizationUse LWP-based instead of thread based synchronization. (Introduced in 1.4.0. Relevant to Solaris only.)
-XX:-UseParallelGCUse parallel garbage collection for scavenges. (Introduced in 1.4.1)
-XX:-UseParallelOldGCUse parallel garbage collection for the full collections. Enabling this option automatically sets -XX:+UseParallelGC. (Introduced in 5.0 update 6.)
-XX:-UseSerialGCUse serial garbage collection. (Introduced in 5.0.)
-XX:-UseSpinningEnable naive spinning on Java monitor before entering operating system thread synchronizaton code. (Relevant to 1.4.2 and 5.0 only.) [1.4.2, multi-processor Windows platforms: true]
-XX:+UseTLABUse thread-local object allocation (Introduced in 1.4.0, known as UseTLE prior to that.) [1.4.2 and earlier, x86 or with -client: false]
-XX:+UseSplitVerifierUse the new type checker with StackMapTable attributes. (Introduced in 5.0.)[5.0: false]
-XX:+UseThreadPrioritiesUse native thread priorities.
-XX:+UseVMInterruptibleIOThread interrupt before or with EINTR for I/O operations results in OS_INTRPT. (Introduced in 6. Relevant to Solaris only.)

Back to Options 
  

Garbage First (G1) Garbage Collection Options


Option and Default ValueDescription
-XX:+UseG1GCUse the Garbage First (G1) Collector
-XX:MaxGCPauseMillis=nSets a target for the maximum GC pause time. This is a soft goal, and the JVM will make its best effort to achieve it.
-XX:InitiatingHeapOccupancyPercent=nPercentage of the (entire) heap occupancy to start a concurrent GC cycle. It is used by GCs that trigger a concurrent GC cycle based on the occupancy of the entire heap, not just one of the generations (e.g., G1). A value of 0 denotes 'do constant GC cycles'. The default value is 45.
-XX:NewRatio=nRatio of old/new generation sizes. The default value is 2.
-XX:SurvivorRatio=nRatio of eden/survivor space size. The default value is 8.
-XX:MaxTenuringThreshold=nMaximum value for tenuring threshold. The default value is 15.
-XX:ParallelGCThreads=nSets the number of threads used during parallel phases of the garbage collectors. The default value varies with the platform on which the JVM is running.
-XX:ConcGCThreads=nNumber of threads concurrent garbage collectors will use. The default value varies with the platform on which the JVM is running.
-XX:G1ReservePercent=nSets the amount of heap that is reserved as a false ceiling to reduce the possibility of promotion failure. The default value is 10.
-XX:G1HeapRegionSize=nWith G1 the Java heap is subdivided into uniformly sized regions. This sets the size of the individual sub-divisions. The default value of this parameter is determined ergonomically based upon heap size. The minimum value is 1Mb and the maximum value is 32Mb.

Back to Options 
 

Performance Options


Option and Default ValueDescription
-XX:+AggressiveOptsTurn on point performance compiler optimizations that are expected to be default in upcoming releases. (Introduced in 5.0 update 6.)
-XX:CompileThreshold=10000Number of method invocations/branches before compiling [-client: 1,500]
-XX:LargePageSizeInBytes=4mSets the large page size used for the Java heap. (Introduced in 1.4.0 update 1.) [amd64: 2m.]
-XX:MaxHeapFreeRatio=70Maximum percentage of heap free after GC to avoid shrinking.
-XX:MaxNewSize=sizeMaximum size of new generation (in bytes). Since 1.4, MaxNewSize is computed as a function of NewRatio. [1.3.1 Sparc: 32m; 1.3.1 x86: 2.5m.]
-XX:MaxPermSize=64mSize of the Permanent Generation.  [5.0 and newer: 64 bit VMs are scaled 30% larger; 1.4 amd64: 96m; 1.3.1 -client: 32m.]
-XX:MinHeapFreeRatio=40Minimum percentage of heap free after GC to avoid expansion.
-XX:NewRatio=2Ratio of old/new generation sizes. [Sparc -client: 8; x86 -server: 8; x86 -client: 12.]-client: 4 (1.3) 8 (1.3.1+), x86: 12]
-XX:NewSize=2mDefault size of new generation (in bytes) [5.0 and newer: 64 bit VMs are scaled 30% larger; x86: 1m; x86, 5.0 and older: 640k]
-XX:ReservedCodeCacheSize=32mReserved code cache size (in bytes) - maximum code cache size. [Solaris 64-bit, amd64, and -server x86: 2048m; in 1.5.0_06 and earlier, Solaris 64-bit and amd64: 1024m.]
-XX:SurvivorRatio=8Ratio of eden/survivor space size [Solaris amd64: 6; Sparc in 1.3.1: 25; other Solaris platforms in 5.0 and earlier: 32]
-XX:TargetSurvivorRatio=50Desired percentage of survivor space used after scavenge.
-XX:ThreadStackSize=512Thread 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 0 in 5.0 and earlier); all others 0.]
-XX:+UseBiasedLockingEnable biased locking. For more details, see thistuning example. (Introduced in 5.0 update 6.) [5.0: false]
-XX:+UseFastAccessorMethodsUse optimized versions of GetField.
-XX:-UseISMUse Intimate Shared Memory. [Not accepted for non-Solaris platforms.] For details, see Intimate Shared Memory.
-XX:+UseLargePagesUse large page memory. (Introduced in 5.0 update 5.) For details, see Java Support for Large Memory Pages.
-XX:+UseMPSSUse Multiple Page Size Support w/4mb pages for the heap. Do not use with ISM as this replaces the need for ISM. (Introduced in 1.4.0 update 1, Relevant to Solaris 9 and newer.) [1.4.1 and earlier: false]
-XX:+UseStringCacheEnables caching of commonly allocated strings.
 
-XX:AllocatePrefetchLines=1Number 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=1Generated 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:+UseCompressedStringsUse a byte[] for Strings which can be represented as pure ASCII. (Introduced in Java 6 Update 21 Performance Release) 
 
-XX:+OptimizeStringConcatOptimize String concatenation operations where possible. (Introduced in Java 6 Update 20) 
 

Back to Options 
  

Debugging Options


Option and Default ValueDescription
-XX:-CITimePrints time spent in JIT Compiler. (Introduced in 1.4.0.)
-XX:ErrorFile=./hs_err_pid.logIf an error occurs, save the error data to this file. (Introduced in 6.)
-XX:-ExtendedDTraceProbesEnable performance-impacting dtrace probes. (Introduced in 6. Relevant to Solaris only.)
-XX:HeapDumpPath=./java_pid.hprofPath to directory or filename for heap dump.Manageable. (Introduced in 1.4.2 update 12, 5.0 update 7.)
-XX:-HeapDumpOnOutOfMemoryErrorDump heap to file when java.lang.OutOfMemoryError is thrown. Manageable. (Introduced in 1.4.2 update 12, 5.0 update 7.)
-XX:OnError=";"Run user-defined commands on fatal error. (Introduced in 1.4.2 update 9.)
-XX:OnOutOfMemoryError="
"
Run user-defined commands when an OutOfMemoryError is first thrown. (Introduced in 1.4.2 update 12, 6)
-XX:-PrintClassHistogramPrint a histogram of class instances on Ctrl-Break.Manageable. (Introduced in 1.4.2.) The jmap -histocommand provides equivalent functionality.
-XX:-PrintConcurrentLocksPrint java.util.concurrent locks in Ctrl-Break thread dump. Manageable. (Introduced in 6.) The jstack -lcommand provides equivalent functionality.
-XX:-PrintCommandLineFlagsPrint flags that appeared on the command line. (Introduced in 5.0.)
-XX:-PrintCompilationPrint message when a method is compiled.
-XX:-PrintGCPrint messages at garbage collection. Manageable.
-XX:-PrintGCDetailsPrint more details at garbage collection.Manageable. (Introduced in 1.4.0.)
-XX:-PrintGCTimeStampsPrint timestamps at garbage collection. Manageable(Introduced in 1.4.0.)
-XX:-PrintTenuringDistributionPrint tenuring age information.
-XX:-PrintAdaptiveSizePolicyEnables printing of information about adaptive generation sizing.
-XX:-TraceClassLoadingTrace loading of classes.
-XX:-TraceClassLoadingPreorderTrace all classes loaded in order referenced (not loaded). (Introduced in 1.4.2.)
-XX:-TraceClassResolutionTrace constant pool resolutions. (Introduced in 1.4.2.)
-XX:-TraceClassUnloadingTrace unloading of classes.
-XX:-TraceLoaderConstraintsTrace recording of loader constraints. (Introduced in 6.)
-XX:+PerfSaveDataToFileSaves jvmstat binary data on exit.
-XX:ParallelGCThreads=nSets 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:+UseCompressedOopsEnables 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:+AlwaysPreTouchPre-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=nSets 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=nInline 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=35Maximum bytecode size of a method to be inlined.
-XX:FreqInlineSize=nMaximum 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=nUnroll 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=7Sets 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=nSets 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.
-Xloggc:Log GC verbose output to specified file. The verbose output is controlled by the normal verbose GC flags.
-XX:-UseGCLogFileRotationEnabled GC log rotation, requires -Xloggc.
-XX:NumberOfGClogFiles=1Set the number of files to use when rotating logs, must be >= 1. The rotated log files will use the following naming scheme, .0, .1, ..., .n-1.
-XX:GCLogFileSize=8KThe size of the log file at which point the log will be rotated, must be >= 8K.

Back to Options

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