This post is part of series on garbage collection. I'm not remotely close to an expert on the subject, but I'm learning. If you've got comments leave feedback at the bottom!

The JVM is voodoo. Garbage collection is double voodoo. How is one to make sense of all of this shit? I’m not sure I know all the answers, but I can help you get on your way.

JVM Options

Whether it’s good ol’ fashioned Java or newfangled Scala or Clojure code, you are still running on top of the JVM.

You can send an option to the JVM to get a list of all the flags and their default values using java -XX:+PrintFlagsFinal. Note that you can also supply options to see their effect on final flags by including them a la java -Xmx4G -XX:+PrintFlagsFinal.

Here’s Oracle’s documentation on JVM options. Note that this is for Java 7, but Java 8 has links right at the top.

Defaults

Soooo this is gonna be a giant box of shit, but we’re gonna refer back to it in other post so here goes!

Here’s the output of java -XX:+PrintFlagsFinal from one of my production machines:

java -XX:+PrintFlagsFinal
[Global flags]
    uintx AdaptivePermSizeWeight                    = 20              {product}
    uintx AdaptiveSizeDecrementScaleFactor          = 4               {product}
    uintx AdaptiveSizeMajorGCDecayTimeScale         = 10              {product}
    uintx AdaptiveSizePausePolicy                   = 0               {product}
    uintx AdaptiveSizePolicyCollectionCostMargin    = 50              {product}
    uintx AdaptiveSizePolicyInitializingSteps       = 20              {product}
    uintx AdaptiveSizePolicyOutputInterval          = 0               {product}
    uintx AdaptiveSizePolicyWeight                  = 10              {product}
    uintx AdaptiveSizeThroughPutPolicy              = 0               {product}
    uintx AdaptiveTimeWeight                        = 25              {product}
     bool AdjustConcurrency                         = false           {product}
     bool AggressiveOpts                            = false           {product}
     intx AliasLevel                                = 3               {C2 product}
     bool AlignVector                               = true            {C2 product}
     intx AllocateInstancePrefetchLines             = 1               {product}
     intx AllocatePrefetchDistance                  = -1              {product}
     intx AllocatePrefetchInstr                     = 0               {product}
     intx AllocatePrefetchLines                     = 3               {product}
     intx AllocatePrefetchStepSize                  = 16              {product}
     intx AllocatePrefetchStyle                     = 1               {product}
     bool AllowJNIEnvProxy                          = false           {product}
     bool AllowNonVirtualCalls                      = false           {product}
     bool AllowParallelDefineClass                  = false           {product}
     bool AllowUserSignalHandlers                   = false           {product}
     bool AlwaysActAsServerClassMachine             = false           {product}
     bool AlwaysCompileLoopMethods                  = false           {product}
     bool AlwaysLockClassLoader                     = false           {product}
     bool AlwaysPreTouch                            = false           {product}
     bool AlwaysRestoreFPU                          = false           {product}
     bool AlwaysTenure                              = false           {product}
     bool AssertOnSuspendWaitFailure                = false           {product}
     intx Atomics                                   = 0               {product}
     intx AutoBoxCacheMax                           = 128             {C2 product}
    uintx AutoGCSelectPauseMillis                   = 5000            {product}
     intx BCEATraceLevel                            = 0               {product}
     intx BackEdgeThreshold                         = 100000          {pd product}
     bool BackgroundCompilation                     = true            {pd product}
    uintx BaseFootPrintEstimate                     = 268435456       {product}
     intx BiasedLockingBulkRebiasThreshold          = 20              {product}
     intx BiasedLockingBulkRevokeThreshold          = 40              {product}
     intx BiasedLockingDecayTime                    = 25000           {product}
     intx BiasedLockingStartupDelay                 = 4000            {product}
     bool BindGCTaskThreadsToCPUs                   = false           {product}
     bool BlockLayoutByFrequency                    = true            {C2 product}
     intx BlockLayoutMinDiamondPercentage           = 20              {C2 product}
     bool BlockLayoutRotateLoops                    = true            {C2 product}
     bool BranchOnRegister                          = false           {C2 product}
     bool BytecodeVerificationLocal                 = false           {product}
     bool BytecodeVerificationRemote                = true            {product}
     bool C1OptimizeVirtualCallProfiling            = true            {C1 product}
     bool C1ProfileBranches                         = true            {C1 product}
     bool C1ProfileCalls                            = true            {C1 product}
     bool C1ProfileCheckcasts                       = true            {C1 product}
     bool C1ProfileInlinedCalls                     = true            {C1 product}
     bool C1ProfileVirtualCalls                     = true            {C1 product}
     bool C1UpdateMethodData                        = true            {C1 product}
     intx CICompilerCount                           = 2               {product}
     bool CICompilerCountPerCPU                     = false           {product}
     bool CITime                                    = false           {product}
     bool CMSAbortSemantics                         = false           {product}
    uintx CMSAbortablePrecleanMinWorkPerIteration   = 100             {product}
     intx CMSAbortablePrecleanWaitMillis            = 100             {manageable}
    uintx CMSBitMapYieldQuantum                     = 10485760        {product}
    uintx CMSBootstrapOccupancy                     = 50              {product}
     bool CMSClassUnloadingEnabled                  = false           {product}
    uintx CMSClassUnloadingMaxInterval              = 0               {product}
     bool CMSCleanOnEnter                           = true            {product}
     bool CMSCompactWhenClearAllSoftRefs            = true            {product}
    uintx CMSConcMarkMultiple                       = 32              {product}
     bool CMSConcurrentMTEnabled                    = true            {product}
    uintx CMSCoordinatorYieldSleepCount             = 10              {product}
     bool CMSDumpAtPromotionFailure                 = false           {product}
     bool CMSEdenChunksRecordAlways                 = false           {product}
    uintx CMSExpAvgFactor                           = 50              {product}
     bool CMSExtrapolateSweep                       = false           {product}
    uintx CMSFullGCsBeforeCompaction                = 0               {product}
    uintx CMSIncrementalDutyCycle                   = 10              {product}
    uintx CMSIncrementalDutyCycleMin                = 0               {product}
     bool CMSIncrementalMode                        = false           {product}
    uintx CMSIncrementalOffset                      = 0               {product}
     bool CMSIncrementalPacing                      = true            {product}
    uintx CMSIncrementalSafetyFactor                = 10              {product}
    uintx CMSIndexedFreeListReplenish               = 4               {product}
     intx CMSInitiatingOccupancyFraction            = -1              {product}
     intx CMSInitiatingPermOccupancyFraction        = -1              {product}
     intx CMSIsTooFullPercentage                    = 98              {product}
   double CMSLargeCoalSurplusPercent                = 0.950000        {product}
   double CMSLargeSplitSurplusPercent               = 1.000000        {product}
     bool CMSLoopWarn                               = false           {product}
    uintx CMSMaxAbortablePrecleanLoops              = 0               {product}
     intx CMSMaxAbortablePrecleanTime               = 5000            {product}
    uintx CMSOldPLABMax                             = 1024            {product}
    uintx CMSOldPLABMin                             = 16              {product}
    uintx CMSOldPLABNumRefills                      = 4               {product}
    uintx CMSOldPLABReactivityFactor                = 2               {product}
     bool CMSOldPLABResizeQuicker                   = false           {product}
    uintx CMSOldPLABToleranceFactor                 = 4               {product}
     bool CMSPLABRecordAlways                       = true            {product}
    uintx CMSParPromoteBlocksToClaim                = 16              {product}
     bool CMSParallelInitialMarkEnabled             = false           {product}
     bool CMSParallelRemarkEnabled                  = true            {product}
     bool CMSParallelSurvivorRemarkEnabled          = true            {product}
     bool CMSPermGenPrecleaningEnabled              = true            {product}
    uintx CMSPrecleanDenominator                    = 3               {product}
    uintx CMSPrecleanIter                           = 3               {product}
    uintx CMSPrecleanNumerator                      = 2               {product}
     bool CMSPrecleanRefLists1                      = true            {product}
     bool CMSPrecleanRefLists2                      = false           {product}
     bool CMSPrecleanSurvivors1                     = false           {product}
     bool CMSPrecleanSurvivors2                     = true            {product}
    uintx CMSPrecleanThreshold                      = 1000            {product}
     bool CMSPrecleaningEnabled                     = true            {product}
     bool CMSPrintChunksInDump                      = false           {product}
     bool CMSPrintEdenSurvivorChunks                = false           {product}
     bool CMSPrintObjectsInDump                     = false           {product}
    uintx CMSRemarkVerifyVariant                    = 1               {product}
     bool CMSReplenishIntermediate                  = true            {product}
    uintx CMSRescanMultiple                         = 32              {product}
    uintx CMSRevisitStackSize                       = 1048576         {product}
    uintx CMSSamplingGrain                          = 16384           {product}
     bool CMSScavengeBeforeRemark                   = false           {product}
    uintx CMSScheduleRemarkEdenPenetration          = 50              {product}
    uintx CMSScheduleRemarkEdenSizeThreshold        = 2097152         {product}
    uintx CMSScheduleRemarkSamplingRatio            = 5               {product}
   double CMSSmallCoalSurplusPercent                = 1.050000        {product}
   double CMSSmallSplitSurplusPercent               = 1.100000        {product}
     bool CMSSplitIndexedFreeListBlocks             = true            {product}
     intx CMSTriggerPermRatio                       = 80              {product}
     intx CMSTriggerRatio                           = 80              {product}
     intx CMSWaitDuration                           = 2000            {manageable}
    uintx CMSWorkQueueDrainThreshold                = 10              {product}
     bool CMSYield                                  = true            {product}
    uintx CMSYieldSleepCount                        = 0               {product}
     intx CMSYoungGenPerWorker                      = 67108864        {pd product}
    uintx CMS_FLSPadding                            = 1               {product}
    uintx CMS_FLSWeight                             = 75              {product}
    uintx CMS_SweepPadding                          = 1               {product}
    uintx CMS_SweepTimerThresholdMillis             = 10              {product}
    uintx CMS_SweepWeight                           = 75              {product}
     bool CheckJNICalls                             = false           {product}
     bool ClassUnloading                            = true            {product}
     intx ClearFPUAtPark                            = 0               {product}
     bool ClipInlining                              = true            {product}
    uintx CodeCacheExpansionSize                    = 65536           {pd product}
    uintx CodeCacheFlushingMinimumFreeSpace         = 1536000         {product}
    uintx CodeCacheMinimumFreeSpace                 = 512000          {product}
     bool CollectGen0First                          = false           {product}
     bool CompactFields                             = true            {product}
     intx CompilationPolicyChoice                   = 0               {product}
     intx CompilationRepeat                         = 0               {C1 product}
ccstrlist CompileCommand                            =                 {product}
    ccstr CompileCommandFile                        =                 {product}
ccstrlist CompileOnly                               =                 {product}
     intx CompileThreshold                          = 10000           {pd product}
     bool CompilerThreadHintNoPreempt               = true            {product}
     intx CompilerThreadPriority                    = -1              {product}
     intx CompilerThreadStackSize                   = 0               {pd product}
    uintx ConcGCThreads                             = 0               {product}
     intx ConditionalMoveLimit                      = 3               {C2 pd product}
     bool ConvertSleepToYield                       = true            {pd product}
     bool ConvertYieldToSleep                       = false           {product}
     bool CreateMinidumpOnCrash                     = false           {product}
     bool CriticalJNINatives                        = true            {product}
     bool DTraceAllocProbes                         = false           {product}
     bool DTraceMethodProbes                        = false           {product}
     bool DTraceMonitorProbes                       = false           {product}
     bool Debugging                                 = false           {product}
    uintx DefaultMaxRAMFraction                     = 4               {product}
     intx DefaultThreadPriority                     = -1              {product}
     intx DeferPollingPageLoopCount                 = -1              {product}
     intx DeferThrSuspendLoopCount                  = 4000            {product}
     bool DeoptimizeRandom                          = false           {product}
     bool DisableAttachMechanism                    = false           {product}
     bool DisableExplicitGC                         = false           {product}
     bool DisplayVMOutputToStderr                   = false           {product}
     bool DisplayVMOutputToStdout                   = false           {product}
     bool DoEscapeAnalysis                          = true            {C2 product}
     bool DontCompileHugeMethods                    = true            {product}
     bool DontYieldALot                             = false           {pd product}
     bool DumpSharedSpaces                          = false           {product}
     bool EagerXrunInit                             = false           {product}
     intx EliminateAllocationArraySizeLimit         = 64              {C2 product}
     bool EliminateAllocations                      = true            {C2 product}
     bool EliminateLocks                            = true            {C2 product}
     bool EliminateNestedLocks                      = true            {C2 product}
     intx EmitSync                                  = 0               {product}
     bool EnableTracing                             = false           {product}
    uintx ErgoHeapSizeLimit                         = 0               {product}
    ccstr ErrorFile                                 =                 {product}
    ccstr ErrorReportServer                         =                 {product}
     bool EstimateArgEscape                         = true            {product}
     bool ExplicitGCInvokesConcurrent               = false           {product}
     bool ExplicitGCInvokesConcurrentAndUnloadsClasses  = false           {product}
     bool ExtendedDTraceProbes                      = false           {product}
     bool FLSAlwaysCoalesceLarge                    = false           {product}
    uintx FLSCoalescePolicy                         = 2               {product}
   double FLSLargestBlockCoalesceProximity          = 0.990000        {product}
     bool FailOverToOldVerifier                     = true            {product}
     bool FastTLABRefill                            = true            {product}
     intx FenceInstruction                          = 0               {ARCH product}
     intx FieldsAllocationStyle                     = 1               {product}
     bool FilterSpuriousWakeups                     = true            {product}
     bool ForceNUMA                                 = false           {product}
     bool ForceTimeHighResolution                   = false           {product}
     intx FreqInlineSize                            = 325             {pd product}
   double G1ConcMarkStepDurationMillis              = 10.000000       {product}
    uintx G1ConcRSHotCardLimit                      = 4               {product}
    uintx G1ConcRSLogCacheSize                      = 10              {product}
     intx G1ConcRefinementGreenZone                 = 0               {product}
     intx G1ConcRefinementRedZone                   = 0               {product}
     intx G1ConcRefinementServiceIntervalMillis     = 300             {product}
    uintx G1ConcRefinementThreads                   = 0               {product}
     intx G1ConcRefinementThresholdStep             = 0               {product}
     intx G1ConcRefinementYellowZone                = 0               {product}
    uintx G1ConfidencePercent                       = 50              {product}
    uintx G1HeapRegionSize                          = 0               {product}
    uintx G1HeapWastePercent                        = 10              {product}
    uintx G1MixedGCCountTarget                      = 8               {product}
     intx G1RSetRegionEntries                       = 0               {product}
    uintx G1RSetScanBlockSize                       = 64              {product}
     intx G1RSetSparseRegionEntries                 = 0               {product}
     intx G1RSetUpdatingPauseTimePercent            = 10              {product}
     intx G1RefProcDrainInterval                    = 10              {product}
    uintx G1ReservePercent                          = 10              {product}
    uintx G1SATBBufferEnqueueingThresholdPercent    = 60              {product}
     intx G1SATBBufferSize                          = 1024            {product}
     intx G1UpdateBufferSize                        = 256             {product}
     bool G1UseAdaptiveConcRefinement               = true            {product}
    uintx GCDrainStackTargetSize                    = 64              {product}
    uintx GCHeapFreeLimit                           = 2               {product}
    uintx GCLockerEdenExpansionPercent              = 5               {product}
     bool GCLockerInvokesConcurrent                 = false           {product}
    uintx GCLogFileSize                             = 0               {product}
    uintx GCPauseIntervalMillis                     = 0               {product}
    uintx GCTaskTimeStampEntries                    = 200             {product}
    uintx GCTimeLimit                               = 98              {product}
    uintx GCTimeRatio                               = 99              {product}
    uintx HeapBaseMinAddress                        = 2147483648      {pd product}
     bool HeapDumpAfterFullGC                       = false           {manageable}
     bool HeapDumpBeforeFullGC                      = false           {manageable}
     bool HeapDumpOnOutOfMemoryError                = false           {manageable}
    ccstr HeapDumpPath                              =                 {manageable}
    uintx HeapFirstMaximumCompactionCount           = 3               {product}
    uintx HeapMaximumCompactionInterval             = 20              {product}
    uintx HeapSizePerGCThread                       = 87241520        {product}
     bool IgnoreUnrecognizedVMOptions               = false           {product}
     bool IncrementalInline                         = true            {C2 product}
    uintx InitialCodeCacheSize                      = 2555904         {pd product}
    uintx InitialHeapSize                           = 0               {product}
    uintx InitialRAMFraction                        = 64              {product}
    uintx InitialSurvivorRatio                      = 8               {product}
     intx InitialTenuringThreshold                  = 7               {product}
    uintx InitiatingHeapOccupancyPercent            = 45              {product}
     bool Inline                                    = true            {product}
     intx InlineSmallCode                           = 1000            {pd product}
     bool InsertMemBarAfterArraycopy                = true            {C2 product}
     intx InteriorEntryAlignment                    = 16              {C2 pd product}
     intx InterpreterProfilePercentage              = 33              {product}
     bool JNIDetachReleasesMonitors                 = true            {product}
     bool JavaMonitorsInStackTrace                  = true            {product}
     intx JavaPriority10_To_OSPriority              = -1              {product}
     intx JavaPriority1_To_OSPriority               = -1              {product}
     intx JavaPriority2_To_OSPriority               = -1              {product}
     intx JavaPriority3_To_OSPriority               = -1              {product}
     intx JavaPriority4_To_OSPriority               = -1              {product}
     intx JavaPriority5_To_OSPriority               = -1              {product}
     intx JavaPriority6_To_OSPriority               = -1              {product}
     intx JavaPriority7_To_OSPriority               = -1              {product}
     intx JavaPriority8_To_OSPriority               = -1              {product}
     intx JavaPriority9_To_OSPriority               = -1              {product}
     bool LIRFillDelaySlots                         = false           {C1 pd product}
    uintx LargePageHeapSizeThreshold                = 134217728       {product}
    uintx LargePageSizeInBytes                      = 0               {product}
     bool LazyBootClassLoader                       = true            {product}
     intx LiveNodeCountInliningCutoff               = 20000           {C2 product}
     bool LoadExecStackDllInVMThread                = true            {product}
     intx LoopMaxUnroll                             = 16              {C2 product}
     intx LoopOptsCount                             = 43              {C2 product}
     intx LoopUnrollLimit                           = 60              {C2 pd product}
     intx LoopUnrollMin                             = 4               {C2 product}
     bool LoopUnswitching                           = true            {C2 product}
     bool ManagementServer                          = false           {product}
    uintx MarkStackSize                             = 4194304         {product}
    uintx MarkStackSizeMax                          = 536870912       {product}
     intx MarkSweepAlwaysCompactCount               = 4               {product}
    uintx MarkSweepDeadRatio                        = 5               {product}
     intx MaxBCEAEstimateLevel                      = 5               {product}
     intx MaxBCEAEstimateSize                       = 150             {product}
    uintx MaxDirectMemorySize                       = 0               {product}
     bool MaxFDLimit                                = true            {product}
    uintx MaxGCMinorPauseMillis                     = 18446744073709551615{product}
    uintx MaxGCPauseMillis                          = 18446744073709551615{product}
    uintx MaxHeapFreeRatio                          = 70              {manageable}
    uintx MaxHeapSize                               = 130862280       {product}
     intx MaxInlineLevel                            = 9               {product}
     intx MaxInlineSize                             = 35              {product}
     intx MaxJavaStackTraceDepth                    = 1024            {product}
     intx MaxJumpTableSize                          = 65000           {C2 product}
     intx MaxJumpTableSparseness                    = 5               {C2 product}
     intx MaxLabelRootDepth                         = 1100            {C2 product}
     intx MaxLoopPad                                = 15              {C2 product}
    uintx MaxNewSize                                = 18446744073709551615{product}
     intx MaxNodeLimit                              = 65000           {C2 product}
    uintx MaxPermHeapExpansion                      = 5452592         {product}
    uintx MaxPermSize                               = 87241520        {pd product}
 uint64_t MaxRAM                                    = 137438953472    {pd product}
    uintx MaxRAMFraction                            = 4               {product}
     intx MaxRecursiveInlineLevel                   = 1               {product}
     intx MaxTenuringThreshold                      = 15              {product}
     intx MaxTrivialSize                            = 6               {product}
     intx MaxVectorSize                             = 32              {C2 product}
     bool MethodFlushing                            = true            {product}
     intx MinCodeCacheFlushingInterval              = 30              {product}
    uintx MinHeapDeltaBytes                         = 170392          {product}
    uintx MinHeapFreeRatio                          = 40              {manageable}
     intx MinInliningThreshold                      = 250             {product}
     intx MinJumpTableSize                          = 18              {C2 product}
    uintx MinPermHeapExpansion                      = 340784          {product}
    uintx MinRAMFraction                            = 2               {product}
    uintx MinSurvivorRatio                          = 3               {product}
    uintx MinTLABSize                               = 2048            {product}
     intx MonitorBound                              = 0               {product}
     bool MonitorInUseLists                         = false           {product}
     intx MultiArrayExpandLimit                     = 6               {C2 product}
     bool MustCallLoadClassInternal                 = false           {product}
     intx NUMAChunkResizeWeight                     = 20              {product}
    uintx NUMAInterleaveGranularity                 = 2097152         {product}
     intx NUMAPageScanRate                          = 256             {product}
     intx NUMASpaceResizeRate                       = 1073741824      {product}
     bool NUMAStats                                 = false           {product}
    ccstr NativeMemoryTracking                      = off             {product}
     intx NativeMonitorFlags                        = 0               {product}
     intx NativeMonitorSpinLimit                    = 20              {product}
     intx NativeMonitorTimeout                      = -1              {product}
     bool NeedsDeoptSuspend                         = false           {pd product}
     bool NeverActAsServerClassMachine              = false           {pd product}
     bool NeverTenure                               = false           {product}
     intx NewRatio                                  = 2               {product}
    uintx NewSize                                   = 1363144         {product}
    uintx NewSizeThreadIncrease                     = 5320            {pd product}
     intx NmethodSweepCheckInterval                 = 5               {product}
     intx NmethodSweepFraction                      = 16              {product}
     intx NodeLimitFudgeFactor                      = 1000            {C2 product}
    uintx NumberOfGCLogFiles                        = 0               {product}
     intx NumberOfLoopInstrToAlign                  = 4               {C2 product}
     intx ObjectAlignmentInBytes                    = 8               {lp64_product}
    uintx OldPLABSize                               = 1024            {product}
    uintx OldPLABWeight                             = 50              {product}
    uintx OldSize                                   = 5452592         {product}
     bool OmitStackTraceInFastThrow                 = true            {product}
ccstrlist OnError                                   =                 {product}
ccstrlist OnOutOfMemoryError                        =                 {product}
     intx OnStackReplacePercentage                  = 140             {pd product}
     bool OptimizeFill                              = true            {C2 product}
     bool OptimizePtrCompare                        = true            {C2 product}
     bool OptimizeStringConcat                      = true            {C2 product}
     bool OptoBundling                              = false           {C2 pd product}
     intx OptoLoopAlignment                         = 16              {pd product}
     bool OptoScheduling                            = false           {C2 pd product}
    uintx PLABWeight                                = 75              {product}
     bool PSChunkLargeArrays                        = true            {product}
     intx ParGCArrayScanChunk                       = 50              {product}
    uintx ParGCDesiredObjsFromOverflowList          = 20              {product}
     bool ParGCTrimOverflow                         = true            {product}
     bool ParGCUseLocalOverflow                     = false           {product}
     intx ParallelGCBufferWastePct                  = 10              {product}
    uintx ParallelGCThreads                         = 0               {product}
     bool ParallelGCVerbose                         = false           {product}
    uintx ParallelOldDeadWoodLimiterMean            = 50              {product}
    uintx ParallelOldDeadWoodLimiterStdDev          = 80              {product}
     bool ParallelRefProcBalancingEnabled           = true            {product}
     bool ParallelRefProcEnabled                    = false           {product}
     bool PartialPeelAtUnsignedTests                = true            {C2 product}
     bool PartialPeelLoop                           = true            {C2 product}
     intx PartialPeelNewPhiDelta                    = 0               {C2 product}
    uintx PausePadding                              = 1               {product}
     intx PerBytecodeRecompilationCutoff            = 200             {product}
     intx PerBytecodeTrapLimit                      = 4               {product}
     intx PerMethodRecompilationCutoff              = 400             {product}
     intx PerMethodTrapLimit                        = 100             {product}
     bool PerfAllowAtExitRegistration               = false           {product}
     bool PerfBypassFileSystemCheck                 = false           {product}
     intx PerfDataMemorySize                        = 32768           {product}
     intx PerfDataSamplingInterval                  = 50              {product}
    ccstr PerfDataSaveFile                          =                 {product}
     bool PerfDataSaveToFile                        = false           {product}
     bool PerfDisableSharedMem                      = false           {product}
     intx PerfMaxStringConstLength                  = 1024            {product}
    uintx PermGenPadding                            = 3               {product}
    uintx PermMarkSweepDeadRatio                    = 20              {product}
    uintx PermSize                                  = 21810376        {pd product}
     intx PreInflateSpin                            = 10              {pd product}
     bool PreferInterpreterNativeStubs              = false           {pd product}
     intx PrefetchCopyIntervalInBytes               = -1              {product}
     intx PrefetchFieldsAhead                       = -1              {product}
     intx PrefetchScanIntervalInBytes               = -1              {product}
     bool PreserveAllAnnotations                    = false           {product}
    uintx PretenureSizeThreshold                    = 0               {product}
     bool PrintAdaptiveSizePolicy                   = false           {product}
     bool PrintCMSInitiationStatistics              = false           {product}
     intx PrintCMSStatistics                        = 0               {product}
     bool PrintClassHistogram                       = false           {manageable}
     bool PrintClassHistogramAfterFullGC            = false           {manageable}
     bool PrintClassHistogramBeforeFullGC           = false           {manageable}
     bool PrintCommandLineFlags                     = false           {product}
     bool PrintCompilation                          = false           {product}
     bool PrintConcurrentLocks                      = false           {manageable}
     intx PrintFLSCensus                            = 0               {product}
     intx PrintFLSStatistics                        = 0               {product}
     bool PrintFlagsFinal                           = false           {product}
     bool PrintFlagsInitial                         = false           {product}
     bool PrintGC                                   = false           {manageable}
     bool PrintGCApplicationConcurrentTime          = false           {product}
     bool PrintGCApplicationStoppedTime             = false           {product}
     bool PrintGCCause                              = true            {product}
     bool PrintGCDateStamps                         = false           {manageable}
     bool PrintGCDetails                            = false           {manageable}
     bool PrintGCTaskTimeStamps                     = false           {product}
     bool PrintGCTimeStamps                         = false           {manageable}
     bool PrintHeapAtGC                             = false           {product rw}
     bool PrintHeapAtGCExtended                     = false           {product rw}
     bool PrintHeapAtSIGBREAK                       = true            {product}
     bool PrintJNIGCStalls                          = false           {product}
     bool PrintJNIResolving                         = false           {product}
     bool PrintOldPLAB                              = false           {product}
     bool PrintOopAddress                           = false           {product}
     bool PrintPLAB                                 = false           {product}
     bool PrintParallelOldGCPhaseTimes              = false           {product}
     bool PrintPromotionFailure                     = false           {product}
     bool PrintReferenceGC                          = false           {product}
     bool PrintRevisitStats                         = false           {product}
     bool PrintSafepointStatistics                  = false           {product}
     intx PrintSafepointStatisticsCount             = 300             {product}
     intx PrintSafepointStatisticsTimeout           = -1              {product}
     bool PrintSharedSpaces                         = false           {product}
     bool PrintStringTableStatistics                = false           {product}
     bool PrintTLAB                                 = false           {product}
     bool PrintTenuringDistribution                 = false           {product}
     bool PrintTieredEvents                         = false           {product}
     bool PrintVMOptions                            = false           {product}
     bool PrintVMQWaitTime                          = false           {product}
     bool PrintWarnings                             = true            {product}
    uintx ProcessDistributionStride                 = 4               {product}
     bool ProfileInterpreter                        = true            {pd product}
     bool ProfileIntervals                          = false           {product}
     intx ProfileIntervalsTicks                     = 100             {product}
     intx ProfileMaturityPercentage                 = 20              {product}
     bool ProfileVM                                 = false           {product}
     bool ProfilerPrintByteCodeStatistics           = false           {product}
     bool ProfilerRecordPC                          = false           {product}
    uintx PromotedPadding                           = 3               {product}
     intx QueuedAllocationWarningCount              = 0               {product}
     bool RangeCheckElimination                     = true            {product}
     intx ReadPrefetchInstr                         = 0               {ARCH product}
     bool ReassociateInvariants                     = true            {C2 product}
     bool ReduceBulkZeroing                         = true            {C2 product}
     bool ReduceFieldZeroing                        = true            {C2 product}
     bool ReduceInitialCardMarks                    = true            {C2 product}
     bool ReduceSignalUsage                         = false           {product}
     intx RefDiscoveryPolicy                        = 0               {product}
     bool ReflectionWrapResolutionErrors            = true            {product}
     bool RegisterFinalizersAtInit                  = true            {product}
     bool RelaxAccessControlCheck                   = false           {product}
     bool RequireSharedSpaces                       = false           {product}
    uintx ReservedCodeCacheSize                     = 50331648        {pd product}
     bool ResizeOldPLAB                             = true            {product}
     bool ResizePLAB                                = true            {product}
     bool ResizeTLAB                                = true            {pd product}
     bool RestoreMXCSROnJNICalls                    = false           {product}
     bool RewriteBytecodes                          = true            {pd product}
     bool RewriteFrequentPairs                      = true            {pd product}
     intx SafepointPollOffset                       = 256             {C1 pd product}
     intx SafepointSpinBeforeYield                  = 2000            {product}
     bool SafepointTimeout                          = false           {product}
     intx SafepointTimeoutDelay                     = 10000           {product}
     bool ScavengeBeforeFullGC                      = true            {product}
     intx SelfDestructTimer                         = 0               {product}
    uintx SharedDummyBlockSize                      = 536870912       {product}
    uintx SharedMiscCodeSize                        = 4194304         {product}
    uintx SharedMiscDataSize                        = 5242880         {product}
    uintx SharedReadOnlySize                        = 10485760        {product}
    uintx SharedReadWriteSize                       = 13631488        {product}
     bool ShowMessageBoxOnError                     = false           {product}
     intx SoftRefLRUPolicyMSPerMB                   = 1000            {product}
     bool SplitIfBlocks                             = true            {C2 product}
     intx StackRedPages                             = 1               {pd product}
     intx StackShadowPages                          = 20              {pd product}
     bool StackTraceInThrowable                     = true            {product}
     intx StackYellowPages                          = 2               {pd product}
     bool StartAttachListener                       = false           {product}
     intx StarvationMonitorInterval                 = 200             {product}
     bool StressLdcRewrite                          = false           {product}
    uintx StringTableSize                           = 60013           {product}
     bool SuppressFatalErrorMessage                 = false           {product}
    uintx SurvivorPadding                           = 3               {product}
     intx SurvivorRatio                             = 8               {product}
     intx SuspendRetryCount                         = 50              {product}
     intx SuspendRetryDelay                         = 5               {product}
     intx SyncFlags                                 = 0               {product}
    ccstr SyncKnobs                                 =                 {product}
     intx SyncVerbose                               = 0               {product}
    uintx TLABAllocationWeight                      = 35              {product}
    uintx TLABRefillWasteFraction                   = 64              {product}
    uintx TLABSize                                  = 0               {product}
     bool TLABStats                                 = true            {product}
    uintx TLABWasteIncrement                        = 4               {product}
    uintx TLABWasteTargetPercent                    = 1               {product}
     intx TargetPLABWastePct                        = 10              {product}
     intx TargetSurvivorRatio                       = 50              {product}
    uintx TenuredGenerationSizeIncrement            = 20              {product}
    uintx TenuredGenerationSizeSupplement           = 80              {product}
    uintx TenuredGenerationSizeSupplementDecay      = 2               {product}
     intx ThreadPriorityPolicy                      = 0               {product}
     bool ThreadPriorityVerbose                     = false           {product}
    uintx ThreadSafetyMargin                        = 52428800        {product}
     intx ThreadStackSize                           = 1024            {pd product}
    uintx ThresholdTolerance                        = 10              {product}
     intx Tier0BackedgeNotifyFreqLog                = 10              {product}
     intx Tier0InvokeNotifyFreqLog                  = 7               {product}
     intx Tier0ProfilingStartPercentage             = 200             {product}
     intx Tier23InlineeNotifyFreqLog                = 20              {product}
     intx Tier2BackEdgeThreshold                    = 0               {product}
     intx Tier2BackedgeNotifyFreqLog                = 14              {product}
     intx Tier2CompileThreshold                     = 0               {product}
     intx Tier2InvokeNotifyFreqLog                  = 11              {product}
     intx Tier3BackEdgeThreshold                    = 60000           {product}
     intx Tier3BackedgeNotifyFreqLog                = 13              {product}
     intx Tier3CompileThreshold                     = 2000            {product}
     intx Tier3DelayOff                             = 2               {product}
     intx Tier3DelayOn                              = 5               {product}
     intx Tier3InvocationThreshold                  = 200             {product}
     intx Tier3InvokeNotifyFreqLog                  = 10              {product}
     intx Tier3LoadFeedback                         = 5               {product}
     intx Tier3MinInvocationThreshold               = 100             {product}
     intx Tier4BackEdgeThreshold                    = 40000           {product}
     intx Tier4CompileThreshold                     = 15000           {product}
     intx Tier4InvocationThreshold                  = 5000            {product}
     intx Tier4LoadFeedback                         = 3               {product}
     intx Tier4MinInvocationThreshold               = 600             {product}
     bool TieredCompilation                         = false           {pd product}
     intx TieredCompileTaskTimeout                  = 50              {product}
     intx TieredRateUpdateMaxTime                   = 25              {product}
     intx TieredRateUpdateMinTime                   = 1               {product}
     intx TieredStopAtLevel                         = 4               {product}
     bool TimeLinearScan                            = false           {C1 product}
     bool TraceBiasedLocking                        = false           {product}
     bool TraceClassLoading                         = false           {product rw}
     bool TraceClassLoadingPreorder                 = false           {product}
     bool TraceClassResolution                      = false           {product}
     bool TraceClassUnloading                       = false           {product rw}
     bool TraceDynamicGCThreads                     = false           {product}
     bool TraceGen0Time                             = false           {product}
     bool TraceGen1Time                             = false           {product}
    ccstr TraceJVMTI                                =                 {product}
     bool TraceLoaderConstraints                    = false           {product rw}
     bool TraceMonitorInflation                     = false           {product}
     bool TraceParallelOldGCTasks                   = false           {product}
     intx TraceRedefineClasses                      = 0               {product}
     bool TraceSafepointCleanupTime                 = false           {product}
     bool TraceSuspendWaitFailures                  = false           {product}
     intx TrackedInitializationLimit                = 50              {C2 product}
     bool TransmitErrorReport                       = false           {product}
     intx TypeProfileMajorReceiverPercent           = 90              {C2 product}
     intx TypeProfileWidth                          = 2               {product}
     intx UnguardOnExecutionViolation               = 0               {product}
     bool UnlinkSymbolsALot                         = false           {product}
     bool Use486InstrsOnly                          = false           {ARCH product}
     bool UseAES                                    = false           {product}
     bool UseAESIntrinsics                          = false           {product}
     intx UseAVX                                    = 99              {ARCH product}
     bool UseAdaptiveGCBoundary                     = false           {product}
     bool UseAdaptiveGenerationSizePolicyAtMajorCollection  = true            {product}
     bool UseAdaptiveGenerationSizePolicyAtMinorCollection  = true            {product}
     bool UseAdaptiveNUMAChunkSizing                = true            {product}
     bool UseAdaptiveSizeDecayMajorGCCost           = true            {product}
     bool UseAdaptiveSizePolicy                     = true            {product}
     bool UseAdaptiveSizePolicyFootprintGoal        = true            {product}
     bool UseAdaptiveSizePolicyWithSystemGC         = false           {product}
     bool UseAddressNop                             = false           {ARCH product}
     bool UseAltSigs                                = false           {product}
     bool UseAutoGCSelectPolicy                     = false           {product}
     bool UseBiasedLocking                          = true            {product}
     bool UseBimorphicInlining                      = true            {C2 product}
     bool UseBoundThreads                           = true            {product}
     bool UseCMSBestFit                             = true            {product}
     bool UseCMSCollectionPassing                   = true            {product}
     bool UseCMSCompactAtFullCollection             = true            {product}
     bool UseCMSInitiatingOccupancyOnly             = false           {product}
     bool UseCodeCacheFlushing                      = true            {product}
     bool UseCompiler                               = true            {product}
     bool UseCompilerSafepoints                     = true            {product}
     bool UseCompressedOops                         = false           {lp64_product}
     bool UseConcMarkSweepGC                        = false           {product}
     bool UseCondCardMark                           = false           {C2 product}
     bool UseCountLeadingZerosInstruction           = false           {ARCH product}
     bool UseCounterDecay                           = true            {product}
     bool UseDivMod                                 = true            {C2 product}
     bool UseDynamicNumberOfGCThreads               = false           {product}
     bool UseFPUForSpilling                         = false           {C2 product}
     bool UseFastAccessorMethods                    = true            {product}
     bool UseFastEmptyMethods                       = true            {product}
     bool UseFastJNIAccessors                       = true            {product}
     bool UseFastStosb                              = false           {ARCH product}
     bool UseG1GC                                   = false           {product}
     bool UseGCLogFileRotation                      = false           {product}
     bool UseGCOverheadLimit                        = true            {product}
     bool UseGCTaskAffinity                         = false           {product}
     bool UseHeavyMonitors                          = false           {product}
     bool UseHugeTLBFS                              = false           {product}
     bool UseInlineCaches                           = true            {product}
     bool UseInterpreter                            = true            {product}
     bool UseJumpTables                             = true            {C2 product}
     bool UseLWPSynchronization                     = true            {product}
     bool UseLargePages                             = false           {pd product}
     bool UseLargePagesIndividualAllocation         = false           {pd product}
     bool UseLinuxPosixThreadCPUClocks              = true            {product}
     bool UseLockedTracing                          = false           {product}
     bool UseLoopCounter                            = true            {product}
     bool UseLoopPredicate                          = true            {C2 product}
     bool UseMaximumCompactionOnSystemGC            = true            {product}
     bool UseMembar                                 = false           {pd product}
     bool UseNUMA                                   = false           {product}
     bool UseNUMAInterleaving                       = false           {product}
     bool UseNewLongLShift                          = false           {ARCH product}
     bool UseOSErrorReporting                       = false           {pd product}
     bool UseOldInlining                            = true            {C2 product}
     bool UseOnStackReplacement                     = true            {pd product}
     bool UseOnlyInlinedBimorphic                   = true            {C2 product}
     bool UseOprofile                               = false           {product}
     bool UseOptoBiasInlining                       = true            {C2 product}
     bool UsePPCLWSYNC                              = true            {product}
     bool UsePSAdaptiveSurvivorSizePolicy           = true            {product}
     bool UseParNewGC                               = false           {product}
     bool UseParallelGC                             = false           {product}
     bool UseParallelOldGC                          = false           {product}
     bool UsePerfData                               = true            {product}
     bool UsePopCountInstruction                    = false           {product}
     bool UseRDPCForConstantTableBase               = false           {C2 product}
     bool UseSHM                                    = false           {product}
     intx UseSSE                                    = 99              {product}
     bool UseSSE42Intrinsics                        = false           {product}
     bool UseSerialGC                               = false           {product}
     bool UseSharedSpaces                           = true            {product}
     bool UseSignalChaining                         = true            {product}
     bool UseSplitVerifier                          = true            {product}
     bool UseStoreImmI16                            = true            {ARCH product}
     bool UseStringCache                            = false           {product}
     bool UseSuperWord                              = true            {C2 product}
     bool UseTLAB                                   = true            {pd product}
     bool UseThreadPriorities                       = true            {pd product}
     bool UseTransparentHugePages                   = false           {product}
     bool UseTypeProfile                            = true            {product}
     bool UseUnalignedLoadStores                    = false           {ARCH product}
     bool UseVMInterruptibleIO                      = false           {product}
     bool UseVectoredExceptions                     = false           {pd product}
     bool UseXMMForArrayCopy                        = false           {product}
     bool UseXmmI2D                                 = false           {ARCH product}
     bool UseXmmI2F                                 = false           {ARCH product}
     bool UseXmmLoadAndClearUpper                   = true            {ARCH product}
     bool UseXmmRegToRegMoveAll                     = false           {ARCH product}
     bool VMThreadHintNoPreempt                     = false           {product}
     intx VMThreadPriority                          = -1              {product}
     intx VMThreadStackSize                         = 1024            {pd product}
     intx ValueMapInitialSize                       = 11              {C1 product}
     intx ValueMapMaxLoopSize                       = 8               {C1 product}
     intx ValueSearchLimit                          = 1000            {C2 product}
     bool VerifyMergedCPBytecodes                   = true            {product}
     intx WorkAroundNPTLTimedWaitHang               = 1               {product}
    uintx YoungGenerationSizeIncrement              = 20              {product}
    uintx YoungGenerationSizeSupplement             = 80              {product}
    uintx YoungGenerationSizeSupplementDecay        = 8               {product}
    uintx YoungPLABSize                             = 4096            {product}
     bool ZeroTLAB                                  = false           {product}
     intx hashCode                                  = 0               {product}

That’s a lot of shit!

Format

If you read the docs then this will be a bit of review for you. If you haven’t then I’ll quickly cover the important bits.

  • Boolean options are turned on with -XX:+<OPTION_NAME> and off with -XX:-<OPTION_NAME>. Note the + and -.
  • String options are set with -XX:<OPTION_NAME>=<STRING_VALUE>.
  • Numeric options just like strings, but have the special feature of allowing m or M for megabytes, k or K for kilobytes and g or G for gigabytes.

Precedence

The JVM will use the last instance of a command line flag passed to it. This can be tested with the following:

Let’s specify the maximum heap size twice. The last and higher priority is the Xmx2G.

$ java -XX:+PrintFlagsFinal -Xmx1G -Xmx2G
…
uintx MaxHeapSize                              := 2147483648      {product}
…

And again, reversed to verify:

$ java -XX:+PrintFlagsFinal -Xmx2G -Xmx1G
uintx MaxHeapSize                              := 1073741824      {product}

Default Options

There are a few options that are good to turn on pretty much all the time. Let’s talk about them shits!

GC Logging

It’s a good idea to enable GC logging for your program. This will vomit all manner of moonspeak in to a log file for analysis later when we learn more and can figure it out.

GC logging is effectively free and definitely worth what trivial amount of time it costs. I can’t find any docs on this, but I know from past experience and conversations with JVM engineers that this is the case.

-XX:+PrintGCDetails -XX:+PrintGCDateStamps -XX:+PrintHeapAtGC -XX:+PrintTenuringDistribution -XX:+PrintGCApplicationStoppedTime -XX:+PrintPromotionFailure -XX:PrintFLSStatistics=1 -Xloggc:$PUT_PATH_HERE -XX:+UseGCLogFileRotation -XX:NumberOfGCLogFiles=10 -XX:GCLogFileSize=10M

This gives the JVM instructions to log every smidge of GC info it has and put it in $PUT_PATH_HERE, rotate it 10 times at 10M for each rotation.

JMX

Enabling JMX is also a great idea because you can hook up tools like JVisualVM or metric collection agents.

-Dcom.sun.management.jmxremote -Dcom.sun.management.jmxremote.ssl=false -Dcom.sun.management.jmxremote.authenticate=false -Dcom.sun.management.jmxremote.port=$PUT_PORT_HERE -Djava.rmi.server.hostname=$PUT_HOSTNAME_HERE
No Authentication?
If this is a box available on the public internet be sure and swap that authenticate=false to true above!

HeapDump

-XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=$PUT_PATH_HERE

Last we’ll bring the HeapDump instructions up and have it drop the dump file into a directory so we can analyze any OOM failures.

Closing

This leaves us with a final set of options that we should include in all our long-running JVM programs of:

-XX:+PrintGCDetails -XX:+PrintGCDateStamps -XX:+PrintHeapAtGC -XX:+PrintTenuringDistribution -XX:+PrintGCApplicationStoppedTime -XX:+PrintPromotionFailure -XX:PrintFLSStatistics=1 -Xloggc:$PUT_PATH_HERE -XX:+UseGCLogFileRotation -XX:NumberOfGCLogFiles=10 -XX:GCLogFileSize=10M -Dcom.sun.management.jmxremote -Dcom.sun.management.jmxremote.ssl=false -Dcom.sun.management.jmxremote.authenticate=false -Dcom.sun.management.jmxremote.port=$PUT_PORT_HERE -Djava.rmi.server.hostname=$PUT_HOSTNAME_HERE -XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=$PUT_PATH_HERE

You can leave these off but in my experience this just means you’ll need to roll all your instances to get these options turned on. Also, any JVM-based services you run such as Cassandra or Zookeeper could benefit from these options. For some reason these options are not always enabled by default.