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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 1.3.0 who wish to port to a Java HotSpot VM, should see Java HotSpot Equivalents of Exact VM flags.
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:+<option> and turned off with -XX:-<option>.
· Numeric options are set with -XX:<option>=<number>. 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:<option>=<string>, 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 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
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Option and Default Value
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Description
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-XX:-AllowUserSignalHandlers
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Do not complain if the application installs signal handlers. (Relevant to Solaris and Linux only.)
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-XX:AltStackSize=16384
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Alternate signal stack size (in Kbytes). (Relevant to Solaris only, removed from 5.0.)
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-XX:-DisableExplicitGC
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Disable calls to System.gc(), JVM still performs garbage collection when necessary.
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-XX:+FailOverToOldVerifier
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Fail over to old verifier when the new type checker fails. (Introduced in 6.)
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-XX:+HandlePromotionFailure
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The 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.]
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-XX:+MaxFDLimit
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Bump the number of file descriptors to max. (Relevant to Solaris only.)
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-XX:PreBlockSpin=10
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Spin 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.)
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-XX:-RelaxAccessControlCheck
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Relax the access control checks in the verifier. (Introduced in 6.)
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-XX:+ScavengeBeforeFullGC
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Do young generation GC prior to a full GC. (Introduced in 1.4.1.)
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-XX:+UseAltSigs
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Use 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.)
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-XX:+UseBoundThreads
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Bind user level threads to kernel threads. (Relevant to Solaris only.)
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-XX:-UseConcMarkSweepGC
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Use concurrent mark-sweep collection for the old generation. (Introduced in 1.4.1)
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-XX:+UseGCOverheadLimit
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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.)
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-XX:+UseLWPSynchronization
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Use LWP-based instead of thread based synchronization. (Introduced in 1.4.0. Relevant to Solaris only.)
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-XX:-UseParallelGC
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Use parallel garbage collection for scavenges. (Introduced in 1.4.1)
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-XX:-UseParallelOldGC
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Use parallel garbage collection for the full collections. Enabling this option automatically sets -XX:+UseParallelGC. (Introduced in 5.0 update 6.)
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-XX:-UseSerialGC
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Use serial garbage collection. (Introduced in 5.0.)
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-XX:-UseSpinning
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Enable 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]
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-XX:+UseTLAB
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Use 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]
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-XX:+UseSplitVerifier
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Use the new type checker with StackMapTable attributes. (Introduced in 5.0.)[5.0: false]
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-XX:+UseThreadPriorities
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Use native thread priorities.
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-XX:+UseVMInterruptibleIO
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Thread interrupt before or with EINTR for I/O operations results in OS_INTRPT. (Introduced in 6. Relevant to Solaris only.)
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Back to Options
Performance Options
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Option and Default Value
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Description
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-XX:+AggressiveOpts
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Turn on point performance compiler optimizations that are expected to be default in upcoming releases. (Introduced in 5.0 update 6.)
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-XX:CompileThreshold=10000
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Number of method invocations/branches before compiling [-client: 1,500]
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-XX:LargePageSizeInBytes=4m
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Sets the large page size used for the Java heap. (Introduced in 1.4.0 update 1.) [amd64: 2m.]
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-XX:MaxHeapFreeRatio=70
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Maximum percentage of heap free after GC to avoid shrinking.
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-XX:MaxNewSize=size
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Maximum 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.]
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-XX:MaxPermSize=64m
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Size of the Permanent Generation. [5.0 and newer: 64 bit VMs are scaled 30% larger; 1.4 amd64: 96m; 1.3.1 -client: 32m.]
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-XX:MinHeapFreeRatio=40
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Minimum percentage of heap free after GC to avoid expansion.
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-XX:NewRatio=2
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Ratio of new/old generation sizes. [Sparc -client: 8; x86 -server: 8; x86 -client: 12.]-client: 4 (1.3) 8 (1.3.1+), x86: 12]
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-XX:NewSize=2.125m
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Default 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]
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-XX:ReservedCodeCacheSize=32m
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Reserved code cache size (in bytes) - maximum code cache size. [Solaris 64-bit, amd64, and -server x86: 48m; in 1.5.0_06 and earlier, Solaris 64-bit and and64: 1024m.]
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-XX:SurvivorRatio=8
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Ratio of eden/survivor space size [Solaris amd64: 6; Sparc in 1.3.1: 25; other Solaris platforms in 5.0 and earlier: 32]
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-XX:TargetSurvivorRatio=50
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Desired percentage of survivor space used after scavenge.
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-XX:ThreadStackSize=512
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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 0 in 5.0 and earlier); all others 0.]
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-XX:+UseBiasedLocking
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Enable biased locking. For more details, see this tuning example. (Introduced in 5.0 update 6.) [5.0: false]
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-XX:+UseFastAccessorMethods
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Use optimized versions of Get<Primitive>Field.
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-XX:-UseISM
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Use Intimate Shared Memory. [Not accepted for non-Solaris platforms.] For details, see Intimate Shared Memory.
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-XX:+UseLargePages
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Use large page memory. (Introduced in 5.0 update 5.) For details, see Java Support for Large Memory Pages.
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-XX:+UseMPSS
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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 1.4.0 update 1, Relevant to Solaris 9 and newer.) [1.4.1 and earlier: false]
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-XX:+UseStringCache
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Enables caching of commonly allocated strings.
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-XX:AllocatePrefetchLines=1
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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.
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-XX:AllocatePrefetchStyle=1
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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.
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-XX:+UseCompressedStrings
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Use a byte[] for Strings which can be represented as pure ASCII. (Introduced in Java 6 Update 21 Performance Release)
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-XX:+OptimizeStringConcat
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Optimize String concatenation operations where possible. (Introduced in Java 6 Update 20)
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Back to Options
Debugging Options
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Option and Default Value
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Description
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-XX:-CITime
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Prints time spent in JIT Compiler. (Introduced in 1.4.0.)
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-XX:ErrorFile=./hs_err_pid<pid>.log
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If an error occurs, save the error data to this file. (Introduced in 6.)
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-XX:-ExtendedDTraceProbes
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Enable performance-impacting dtrace probes. (Introduced in 6. Relevant to Solaris only.)
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-XX:HeapDumpPath=./java_pid<pid>.hprof
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Path to directory or filename for heap dump. Manageable. (Introduced in 1.4.2 update 12, 5.0 update 7.)
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-XX:-HeapDumpOnOutOfMemoryError
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Dump heap to file when java.lang.OutOfMemoryError is thrown. Manageable. (Introduced in 1.4.2 update 12, 5.0 update 7.)
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-XX:OnError="<cmd args>;<cmd args>"
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Run user-defined commands on fatal error. (Introduced in 1.4.2 update 9.)
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-XX:OnOutOfMemoryError="<cmd args>;
<cmd args>"
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Run user-defined commands when an OutOfMemoryError is first thrown. (Introduced in 1.4.2 update 12, 6)
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-XX:-PrintClassHistogram
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Print a histogram of class instances on Ctrl-Break. Manageable. (Introduced in 1.4.2.) The jmap -histo command provides equivalent functionality.
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-XX:-PrintConcurrentLocks
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Print java.util.concurrent locks in Ctrl-Break thread dump. Manageable. (Introduced in 6.) The jstack -l command provides equivalent functionality.
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-XX:-PrintCommandLineFlags
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Print flags that appeared on the command line. (Introduced in 5.0.)
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-XX:-PrintCompilation
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Print message when a method is compiled.
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-XX:-PrintGC
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Print messages at garbage collection. Manageable.
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-XX:-PrintGCDetails
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Print more details at garbage collection. Manageable. (Introduced in 1.4.0.)
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-XX:-PrintGCTimeStamps
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Print timestamps at garbage collection. Manageable (Introduced in 1.4.0.)
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-XX:-PrintTenuringDistribution
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Print tenuring age information.
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-XX:-TraceClassLoading
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Trace loading of classes.
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-XX:-TraceClassLoadingPreorder
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Trace all classes loaded in order referenced (not loaded). (Introduced in 1.4.2.)
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-XX:-TraceClassResolution
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Trace constant pool resolutions. (Introduced in 1.4.2.)
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-XX:-TraceClassUnloading
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Trace unloading of classes.
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-XX:-TraceLoaderConstraints
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Trace recording of loader constraints. (Introduced in 6.)
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-XX:+PerfSaveDataToFile
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Saves jvmstat binary data on exit.
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-XX:ParallelGCThreads=
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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.
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-XX:+UseCompressedOops
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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.
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-XX:+AlwaysPreTouch
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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.
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-XX:AllocatePrefetchDistance=
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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.
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-XX:InlineSmallCode=
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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.
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-XX:MaxInlineSize=35
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Maximum bytecode size of a method to be inlined.
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-XX:FreqInlineSize=
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Maximum bytecode size of a frequently executed method to be inlined. The default value varies with the platform on which the JVM is running.
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-XX:LoopUnrollLimit=
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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.
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-XX:InitialTenuringThreshold=7
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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.
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-XX:MaxTenuringThreshold=
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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.
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