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JDK12 ShenandoahGC小试牛刀

本文主要试用一下JDK12新引入的ShenandoahGC

ShenandoahGC

Shenandoah是一款concurrent及parallel的垃圾收集器

  • 跟ZGC一样也是面向low-pause-time的垃圾收集器,不过ZGC是基于colored pointers来实现,而Shenandoah GC是基于brooks pointers来实现
  • 与G1 GC相比,G1的evacuation是parallel的但不是concurrent,而Shenandoah的evacuation是concurrent,因而能更好地减少pause time
  • 与G1 GC一样,ShenandoahGC也是基于region的GC,不同的是ShenandoahGC在逻辑上没有分代,因而就没有young/old

GC cycle

ShenandoahGC主要有如下几个阶段:

Snapshot-at-the-beginning concurrent mark

这里包含Init Mark(Pause)、Concurrent Mark、Final Mark(Pause);这里使用到了White(not yet visited)、Gray(visited, but references are not scanned yet)、Black(visited, and fully scanned) Color算法进行mark

Concurrent evacuation

这个就是与G1不同的evacuation阶段,它是concurrent的;这里用到了Brooks Pointers(object version change with additional atomically changed indirection)算法进行copy,

Concurrent update references (optional)

这里包含Init update Refs(Pause)、Concurrent update Refs、Final update Refs(Pause)Final Mark或者Final update Refs之后都可能进行Concurrent cleanup,进行垃圾回收,reclaims region

相关参数

Shenandoah开头的参数

     bool ShenandoahAcmpBarrier                    = true                                   {diagnostic} {default}
     bool ShenandoahAllocFailureALot               = false                                  {diagnostic} {default}
    uintx ShenandoahAllocSpikeFactor               = 5                                    {experimental} {default}
     intx ShenandoahAllocationStallThreshold       = 10000                                  {diagnostic} {default}
    uintx ShenandoahAllocationThreshold            = 0                                    {experimental} {default}
     bool ShenandoahAllocationTrace                = false                                  {diagnostic} {default}
     bool ShenandoahAllowMixedAllocs               = true                                   {diagnostic} {default}
     bool ShenandoahAlwaysClearSoftRefs            = false                                {experimental} {default}
     bool ShenandoahAlwaysPreTouch                 = false                                  {diagnostic} {default}
     bool ShenandoahCASBarrier                     = true                                   {diagnostic} {default}
     bool ShenandoahCloneBarrier                   = true                                   {diagnostic} {default}
    uintx ShenandoahCodeRootsStyle                 = 2                                    {experimental} {default}
     bool ShenandoahCommonGCStateLoads             = false                                {experimental} {default}
     bool ShenandoahConcurrentScanCodeRoots        = true                                 {experimental} {default}
    uintx ShenandoahControlIntervalAdjustPeriod    = 1000                                 {experimental} {default}
    uintx ShenandoahControlIntervalMax             = 10                                   {experimental} {default}
    uintx ShenandoahControlIntervalMin             = 1                                    {experimental} {default}
    uintx ShenandoahCriticalFreeThreshold          = 1                                    {experimental} {default}
     bool ShenandoahDecreaseRegisterPressure       = false                                  {diagnostic} {default}
     bool ShenandoahDegeneratedGC                  = true                                   {diagnostic} {default}
     bool ShenandoahDontIncreaseWBFreq             = true                                 {experimental} {default}
     bool ShenandoahElasticTLAB                    = true                                   {diagnostic} {default}
    uintx ShenandoahEvacAssist                     = 10                                   {experimental} {default}
    uintx ShenandoahEvacReserve                    = 5                                    {experimental} {default}
     bool ShenandoahEvacReserveOverflow            = true                                 {experimental} {default}
   double ShenandoahEvacWaste                      = 1.200000                             {experimental} {default}
    uintx ShenandoahFreeThreshold                  = 10                                   {experimental} {default}
    uintx ShenandoahFullGCThreshold                = 3                                    {experimental} {default}
    ccstr ShenandoahGCHeuristics                   = adaptive                             {experimental} {default}
    uintx ShenandoahGarbageThreshold               = 60                                   {experimental} {default}
    uintx ShenandoahGuaranteedGCInterval           = 300000                               {experimental} {default}
   size_t ShenandoahHeapRegionSize                 = 0                                    {experimental} {default}
     bool ShenandoahHumongousMoves                 = true                                 {experimental} {default}
     intx ShenandoahHumongousThreshold             = 100                                  {experimental} {default}
    uintx ShenandoahImmediateThreshold             = 90                                   {experimental} {default}
     bool ShenandoahImplicitGCInvokesConcurrent    = true                                 {experimental} {default}
    uintx ShenandoahInitFreeThreshold              = 70                                   {experimental} {default}
     bool ShenandoahKeepAliveBarrier               = true                                   {diagnostic} {default}
    uintx ShenandoahLearningSteps                  = 5                                    {experimental} {default}
     bool ShenandoahLoopOptsAfterExpansion         = true                                 {experimental} {default}
    uintx ShenandoahMarkLoopStride                 = 1000                                 {experimental} {default}
     intx ShenandoahMarkScanPrefetch               = 32                                   {experimental} {default}
   size_t ShenandoahMaxRegionSize                  = 33554432                             {experimental} {default}
    uintx ShenandoahMergeUpdateRefsMaxGap          = 200                                  {experimental} {default}
    uintx ShenandoahMergeUpdateRefsMinGap          = 100                                  {experimental} {default}
    uintx ShenandoahMinFreeThreshold               = 10                                   {experimental} {default}
   size_t ShenandoahMinRegionSize                  = 262144                               {experimental} {default}
     bool ShenandoahOOMDuringEvacALot              = false                                  {diagnostic} {default}
     bool ShenandoahOptimizeInstanceFinals         = false                                {experimental} {default}
     bool ShenandoahOptimizeStableFinals           = false                                {experimental} {default}
     bool ShenandoahOptimizeStaticFinals           = true                                 {experimental} {default}
     bool ShenandoahPacing                         = true                                 {experimental} {default}
    uintx ShenandoahPacingCycleSlack               = 10                                   {experimental} {default}
    uintx ShenandoahPacingIdleSlack                = 2                                    {experimental} {default}
    uintx ShenandoahPacingMaxDelay                 = 10                                   {experimental} {default}
   double ShenandoahPacingSurcharge                = 1.100000                             {experimental} {default}
    uintx ShenandoahParallelRegionStride           = 1024                                 {experimental} {default}
     uint ShenandoahParallelSafepointThreads       = 4                                    {experimental} {default}
     bool ShenandoahPreclean                       = true                                 {experimental} {default}
     bool ShenandoahReadBarrier                    = true                                   {diagnostic} {default}
    uintx ShenandoahRefProcFrequency               = 5                                    {experimental} {default}
     bool ShenandoahRegionSampling                 = true                                 {experimental} {command line}
      int ShenandoahRegionSamplingRate             = 40                                   {experimental} {default}
     bool ShenandoahSATBBarrier                    = true                                   {diagnostic} {default}
    uintx ShenandoahSATBBufferFlushInterval        = 100                                  {experimental} {default}
   size_t ShenandoahSATBBufferSize                 = 1024                                 {experimental} {default}
     bool ShenandoahStoreCheck                     = false                                  {diagnostic} {default}
     bool ShenandoahStoreValEnqueueBarrier         = false                                  {diagnostic} {default}
     bool ShenandoahStoreValReadBarrier            = true                                   {diagnostic} {default}
     bool ShenandoahSuspendibleWorkers             = false                                {experimental} {default}
   size_t ShenandoahTargetNumRegions               = 2048                                 {experimental} {default}
     bool ShenandoahTerminationTrace               = false                                  {diagnostic} {default}
     bool ShenandoahUncommit                       = true                                 {experimental} {default}
    uintx ShenandoahUncommitDelay                  = 300000                               {experimental} {default}
    uintx ShenandoahUnloadClassesFrequency         = 0                                    {experimental} {default}
    ccstr ShenandoahUpdateRefsEarly                = adaptive                             {experimental} {default}
     bool ShenandoahVerify                         = false                                  {diagnostic} {default}
     intx ShenandoahVerifyLevel                    = 4                                      {diagnostic} {default}
     bool ShenandoahWriteBarrier                   = true                                   {diagnostic} {default}

其中有一些是diagnostic用的,比如ShenandoahAcmpBarrier、ShenandoahAllocFailureALot、ShenandoahAllocationStallThreshold等

Heuristics相关参数

    ccstr ShenandoahGCHeuristics                   = adaptive                             {experimental} {default}
    uintx ShenandoahInitFreeThreshold              = 70                                   {experimental} {default}
    uintx ShenandoahMinFreeThreshold               = 10                                   {experimental} {default}
    uintx ShenandoahAllocSpikeFactor               = 5                                    {experimental} {default}
    uintx ShenandoahGarbageThreshold               = 60                                   {experimental} {default}
    uintx ShenandoahFreeThreshold                  = 10                                   {experimental} {default}
    uintx ShenandoahAllocationThreshold            = 0                                    {experimental} {default}
    ccstr ShenandoahUpdateRefsEarly                = adaptive                             {experimental} {default}

Heuristics主要用于告诉Shenandoah何时启动一个GC cycle,其中ShenandoahGCHeuristics用于选择不同的策略,其可选值有adaptive(默认)、static、compact、passive(diagnostic用)、aggressive(diagnostic用)

  • adaptive方式主要通过ShenandoahInitFreeThreshold(Initial remaining free heap threshold for learning steps)、ShenandoahMinFreeThreshold(free space threshold at which heuristics triggers the GC unconditionally)、ShenandoahAllocSpikeFactor(How much heap to reserve for absorbing allocation spikes)、XX:ShenandoahGarbageThreshold(Sets the percentage of garbage a region need to contain before it can be marked for collection)来设置合适启动GC cycle
  • static方式主要是基于heap occupancy以及allocation pressure来决定是否启动GC cycle,相关参数有:ShenandoahFreeThreshold(Set the percentage of free heap at which a GC cycle is started)、ShenandoahAllocationThreshold(Set percentage of memory allocated since last GC cycle before a new GC cycle is started)、ShenandoahGarbageThreshold
  • compact方式是continuous方式的,只要有allocation发生,上一个GC cycle结束之后就启动新的GC cycle,相关参数有ConcGCThreads(Trim down the number of concurrent GC threads to make more room for application to run)、ShenandoahAllocationThreshold
  • passive方式是完全passive,当内存耗尽时触发STW,通常用于diagnostic
  • aggressive方式是完全active的,上一个GC cycle结束之后就启动新的GC cycle(有点类似compact方式),不过它会evacuate所有的live objects,通常用于diagnostic

Failure Modes

当allocation failure发生的时候,Shenandoah有一些优雅的degradation ladder用于处理这种情况,如下:

  • Pacing(<10 ms)

ShenandoahPacing参数默认开启,Pacer用于在gc不够快的时候去stall正在分配对象的线程,当gc速度跟上来了就解除对这些线程的stall;stall不是无期限的,有个ShenandoahPacingMaxDelay(单位毫秒)参数可以设置,一旦超过该值allocation就会产生。当allocation压力大的时候,Pacer就无能为力了,这个时候就会进入下一个step

  • Degenerated GC(<100 ms)

ShenandoahDegeneratedGC参数默认开启,在这个Degenerated cycle,Shenandoah使用的线程数取之于ParallelGCThreads而非ConcCGThreads

  • Full GC(>100 ms)

当Degenerated GC之后还没有足够的内存,则进入Full GC cycle,它会尽可能地进行compact然后释放内存以确保不发生OOM

实例

启动参数

-server -XX:+UnlockExperimentalVMOptions -XX:+UseShenandoahGC -XX:+UsePerfData -XX:+ShenandoahRegionSampling -XX:ParallelGCThreads=4 -XX:ConcGCThreads=4 -XX:+UnlockDiagnosticVMOptions -Xlog:age*,ergo*,gc*=info

gc日志

[2019-03-21T15:12:53.771-0800][8707][gc] Consider -XX:+ClassUnloadingWithConcurrentMark if large pause times are observed on class-unloading sensitive workloads
[2019-03-21T15:12:53.862-0800][8707][gc,init] Regions: 2048 x 1024K
[2019-03-21T15:12:53.862-0800][8707][gc,init] Humongous object threshold: 1024K
[2019-03-21T15:12:53.863-0800][8707][gc,init] Max TLAB size: 1024K
[2019-03-21T15:12:53.863-0800][8707][gc,init] GC threads: 4 parallel, 4 concurrent
[2019-03-21T15:12:53.863-0800][8707][gc,init] Reference processing: parallel
[2019-03-21T15:12:53.864-0800][8707][gc     ] Heuristics ergonomically sets -XX:+ExplicitGCInvokesConcurrent
[2019-03-21T15:12:53.864-0800][8707][gc     ] Heuristics ergonomically sets -XX:+ShenandoahImplicitGCInvokesConcurrent
[2019-03-21T15:12:53.864-0800][8707][gc,init] Shenandoah heuristics: adaptive
[2019-03-21T15:12:53.864-0800][8707][gc,heap] Initialize Shenandoah heap with initial size 128M
[2019-03-21T15:12:53.865-0800][8707][gc,ergo] Pacer for Idle. Initial: 40M, Alloc Tax Rate: 1.0x
[2019-03-21T15:12:53.883-0800][8707][gc,init] Safepointing mechanism: global-page poll
[2019-03-21T15:12:53.883-0800][8707][gc     ] Using Shenandoah
[2019-03-21T15:12:53.884-0800][8707][gc,heap,coops] Heap address: 0x0000000780000000, size: 2048 MB, Compressed Oops mode: Zero based, Oop shift amount: 3
[2019-03-21T15:12:59.530-0800][14083][gc           ] Trigger: Metadata GC Threshold
[2019-03-21T15:12:59.532-0800][14083][gc,ergo      ] Free: 1813M (1813 regions), Max regular: 1024K, Max humongous: 1855488K, External frag: 1%, Internal frag: 0%
[2019-03-21T15:12:59.532-0800][14083][gc,ergo      ] Evacuation Reserve: 103M (103 regions), Max regular: 1024K
[2019-03-21T15:12:59.532-0800][14083][gc,start     ] GC(0) Concurrent reset
[2019-03-21T15:12:59.533-0800][14083][gc,task      ] GC(0) Using 4 of 4 workers for concurrent reset
[2019-03-21T15:12:59.533-0800][14083][gc           ] GC(0) Concurrent reset 132M->132M(2048M) 0.441ms
[2019-03-21T15:12:59.533-0800][15619][gc,start     ] GC(0) Pause Init Mark (process weakrefs) (unload classes)
[2019-03-21T15:12:59.533-0800][15619][gc,task      ] GC(0) Using 4 of 4 workers for init marking
[2019-03-21T15:12:59.541-0800][15619][gc,ergo      ] GC(0) Pacer for Mark. Expected Live: 204M, Free: 1813M, Non-Taxable: 181M, Alloc Tax Rate: 0.4x
[2019-03-21T15:12:59.541-0800][15619][gc           ] GC(0) Pause Init Mark (process weakrefs) (unload classes) 7.568ms
[2019-03-21T15:12:59.541-0800][14083][gc,start     ] GC(0) Concurrent marking (process weakrefs) (unload classes)
[2019-03-21T15:12:59.541-0800][14083][gc,task      ] GC(0) Using 4 of 4 workers for concurrent marking
[2019-03-21T15:12:59.619-0800][14083][gc           ] GC(0) Concurrent marking (process weakrefs) (unload classes) 132M->134M(2048M) 78.373ms
[2019-03-21T15:12:59.619-0800][14083][gc,start     ] GC(0) Concurrent precleaning
[2019-03-21T15:12:59.619-0800][14083][gc,task      ] GC(0) Using 1 of 4 workers for concurrent preclean
[2019-03-21T15:12:59.622-0800][14083][gc           ] GC(0) Concurrent precleaning 134M->134M(2048M) 2.397ms
[2019-03-21T15:12:59.622-0800][15619][gc,start     ] GC(0) Pause Final Mark (process weakrefs) (unload classes)
[2019-03-21T15:12:59.622-0800][15619][gc,task      ] GC(0) Using 4 of 4 workers for final marking
[2019-03-21T15:12:59.625-0800][15619][gc,stringtable] GC(0) Cleaned string table, strings: 13692 processed, 50 removed
[2019-03-21T15:12:59.626-0800][15619][gc,ergo       ] GC(0) Adaptive CSet Selection. Target Free: 204M, Actual Free: 1914M, Max CSet: 85M, Min Garbage: 0M
[2019-03-21T15:12:59.626-0800][15619][gc,ergo       ] GC(0) Collectable Garbage: 117M (97% of total), 8M CSet, 126 CSet regions
[2019-03-21T15:12:59.626-0800][15619][gc,ergo       ] GC(0) Immediate Garbage: 0M (0% of total), 0 regions
[2019-03-21T15:12:59.626-0800][15619][gc,ergo       ] GC(0) Pacer for Evacuation. Used CSet: 126M, Free: 1811M, Non-Taxable: 181M, Alloc Tax Rate: 1.1x
[2019-03-21T15:12:59.626-0800][15619][gc            ] GC(0) Pause Final Mark (process weakrefs) (unload classes) 4.712ms
[2019-03-21T15:12:59.626-0800][14083][gc,start      ] GC(0) Concurrent cleanup
[2019-03-21T15:12:59.627-0800][14083][gc            ] GC(0) Concurrent cleanup 134M->135M(2048M) 0.132ms
[2019-03-21T15:12:59.627-0800][14083][gc,ergo       ] GC(0) Free: 1810M (1810 regions), Max regular: 1024K, Max humongous: 1852416K, External frag: 1%, Internal frag: 0%
[2019-03-21T15:12:59.627-0800][14083][gc,ergo       ] GC(0) Evacuation Reserve: 102M (103 regions), Max regular: 1024K
[2019-03-21T15:12:59.627-0800][14083][gc,start      ] GC(0) Concurrent evacuation
[2019-03-21T15:12:59.627-0800][14083][gc,task       ] GC(0) Using 4 of 4 workers for concurrent evacuation
[2019-03-21T15:12:59.643-0800][14083][gc            ] GC(0) Concurrent evacuation 135M->145M(2048M) 15.912ms
[2019-03-21T15:12:59.643-0800][15619][gc,start      ] GC(0) Pause Init Update Refs
[2019-03-21T15:12:59.643-0800][15619][gc,ergo       ] GC(0) Pacer for Update Refs. Used: 145M, Free: 1810M, Non-Taxable: 181M, Alloc Tax Rate: 1.1x
[2019-03-21T15:12:59.643-0800][15619][gc            ] GC(0) Pause Init Update Refs 0.090ms
[2019-03-21T15:12:59.643-0800][14083][gc,start      ] GC(0) Concurrent update references
[2019-03-21T15:12:59.643-0800][14083][gc,task       ] GC(0) Using 4 of 4 workers for concurrent reference update
[2019-03-21T15:12:59.652-0800][14083][gc            ] GC(0) Concurrent update references 145M->147M(2048M) 9.028ms
[2019-03-21T15:12:59.652-0800][15619][gc,start      ] GC(0) Pause Final Update Refs
[2019-03-21T15:12:59.652-0800][15619][gc,task       ] GC(0) Using 4 of 4 workers for final reference update
[2019-03-21T15:12:59.653-0800][15619][gc            ] GC(0) Pause Final Update Refs 0.489ms
[2019-03-21T15:12:59.653-0800][14083][gc,start      ] GC(0) Concurrent cleanup
[2019-03-21T15:12:59.653-0800][14083][gc            ] GC(0) Concurrent cleanup 147M->21M(2048M) 0.088ms
[2019-03-21T15:12:59.653-0800][14083][gc,ergo       ] Free: 1924M (1924 regions), Max regular: 1024K, Max humongous: 1840128K, External frag: 7%, Internal frag: 0%
[2019-03-21T15:12:59.653-0800][14083][gc,ergo       ] Evacuation Reserve: 103M (103 regions), Max regular: 1024K
[2019-03-21T15:12:59.653-0800][14083][gc,ergo       ] Pacer for Idle. Initial: 40M, Alloc Tax Rate: 1.0x
[2019-03-21T15:17:59.666-0800][14083][gc            ] Trigger: Time since last GC (300009 ms) is larger than guaranteed interval (300000 ms)

gc visualizer

有个shenandoah-visualizer工具可以用来可视化ShenandoahGC,可视化效果如下:

小结

  • Shenandoah是一款concurrent及parallel的垃圾收集器;跟ZGC一样也是面向low-pause-time的垃圾收集器,不过ZGC是基于colored pointers来实现,而Shenandoah GC是基于brooks pointers来实现;与G1 GC相比,G1的evacuation是parallel的但不是concurrent,而Shenandoah的evacuation是concurrent,因而能更好地减少pause time;与G1 GC一样,ShenandoahGC也是基于region的GC,不同的是ShenandoahGC在逻辑上没有分代,因而就没有young/old
  • Shenandoah的GC cycle主要有Snapshot-at-the-beginning concurrent mark包括Init Mark(Pause)、Concurrent Mark、Final Mark(Pause)、Concurrent evacuation、Concurrent update references (optional)包括Init update Refs(Pause)、Concurrent update Refs、Final update Refs(Pause);其中Final Mark或者Final update Refs之后都可能进行Concurrent cleanup,进行垃圾回收,reclaims region
  • Heuristics主要用于告诉Shenandoah何时启动一个GC,其中ShenandoahGCHeuristics用于选择不同的策略,其可选值有adaptive(默认)、static、compact、passive(diagnostic用)、aggressive(diagnostic用);另外当allocation failure发生的时候,Shenandoah有一些优雅的degradation ladder用于处理这种情况,包括Pacing(<10 ms)、Degenerated GC(<100 ms)、Full GC(>100 ms)

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