JDK12 ShenandoahGC小试牛刀

序

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)

doc

• Shenandoah GC
• JEP 189: Shenandoah: A Low-Pause-Time Garbage Collector (Experimental)
• Changes to Garbage Collection in Java 12
• 9 Garbage-First Garbage Collector
• G1GC – Java 9 Garbage Collector explained in 5 minutes
• devoxx-Nov2017-shenandoah(部分图片来源于此pdf)