JDK12 ShenandoahGC小試牛刀
序
本文主要試用一下JDK12新引入的ShenandoahGChtml
ShenandoahGC
Shenandoah是一款concurrent及parallel的垃圾收集器
java
- 跟ZGC同樣也是面向low-pause-time的垃圾收集器，不過ZGC是基於colored pointers來實現，而Shenandoah GC是基於brooks pointers來實現
- 與G1 GC相比，G1的evacuation是parallel的但不是concurrent，而Shenandoah的evacuation是concurrent，於是能更好地減小pause time
- 與G1 GC同樣，ShenandoahGC也是基於region的GC，不一樣的是ShenandoahGC在邏輯上沒有分代，於是就沒有young/old
GC cycle
ShenandoahGC主要有以下幾個階段：算法
Snapshot-at-the-beginning concurrent mark
這裏包含Init Mark(Pause)、Concurrent Mark、Final Mark(Pause)；這裏使用到了White(not yet visited)、Gray(visited, but references are not scanned yet)、Black(visited, and fully scanned) Color算法進行mark
Concurrent evacuation
這個就是與G1不一樣的evacuation階段，它是concurrent的；這裏用到了Brooks Pointers(
object version change with additional atomically changed indirection)算法進行copy，
Concurrent update references (optional)
這裏包含Init update Refs(Pause)、Concurrent update Refs、Final update Refs(Pause)
Final Mark或者Final update Refs以後均可能進行Concurrent cleanup，進行垃圾回收，reclaims region
相關參數
Shenandoah開頭的參數
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，好比ShenandoahAcmpBarrier、ShenandoahAllocFailureALot、ShenandoahAllocationStallThreshold等
Heuristics相關參數
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主要用於告訴Shenandoah什麼時候啓動一個GC cycle，其中ShenandoahGCHeuristics用於選擇不一樣的策略，其可選值有adaptive(默認)、static、compact、passive(diagnostic用)、aggressive(diagnostic用)
- adaptive方式主要經過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)來設置合適啓動GC cycle - static方式主要是基於heap occupancy以及allocation pressure來決定是否啓動GC cycle，相關參數有：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發生，上一個GC cycle結束以後就啓動新的GC cycle，相關參數有ConcGCThreads(
Trim down the number of concurrent GC threads to make more room for application to run)、ShenandoahAllocationThreshold - passive方式是徹底passive，當內存耗盡時觸發STW，一般用於diagnostic
- aggressive方式是徹底active的，上一個GC cycle結束以後就啓動新的GC cycle(
有點相似compact方式)，不過它會evacuate全部的live objects，一般用於diagnostic
Failure Modes
當allocation failure發生的時候，Shenandoah有一些優雅的degradation ladder用於處理這種狀況，以下：oracle
- Pacing(
<10 ms)
ShenandoahPacing參數默認開啓，Pacer用於在gc不夠快的時候去stall正在分配對象的線程，當gc速度跟上來了就解除對這些線程的stall；stall不是無期限的，有個ShenandoahPacingMaxDelay(
單位毫秒)參數能夠設置，一旦超過該值allocation就會產生。當allocation壓力大的時候，Pacer就無能爲力了，這個時候就會進入下一個step
- Degenerated GC(
<100 ms)
ShenandoahDegeneratedGC參數默認開啓，在這個Degenerated cycle，Shenandoah使用的線程數取之於ParallelGCThreads而非ConcCGThreads
- Full GC(
>100 ms)
當Degenerated GC以後尚未足夠的內存，則進入Full GC cycle，它會盡量地進行compact而後釋放內存以確保不發生OOM
實例
啓動參數
-server -XX:+UnlockExperimentalVMOptions -XX:+UseShenandoahGC -XX:+UsePerfData -XX:+ShenandoahRegionSampling -XX:ParallelGCThreads=4 -XX:ConcGCThreads=4 -XX:+UnlockDiagnosticVMOptions -Xlog:age*,ergo*,gc*=info
gc日誌
[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
有個shenandoah-visualizer工具能夠用來可視化ShenandoahGC，可視化效果以下：app
小結
- Shenandoah是一款concurrent及parallel的垃圾收集器；跟ZGC同樣也是面向low-pause-time的垃圾收集器，不過ZGC是基於colored pointers來實現，而Shenandoah GC是基於brooks pointers來實現；與G1 GC相比，G1的evacuation是parallel的但不是concurrent，而Shenandoah的evacuation是concurrent，於是能更好地減小pause time；與G1 GC同樣，ShenandoahGC也是基於region的GC，不一樣的是ShenandoahGC在邏輯上沒有分代，於是就沒有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以後均可能進行Concurrent cleanup，進行垃圾回收，reclaims region - Heuristics主要用於告訴Shenandoah什麼時候啓動一個GC，其中ShenandoahGCHeuristics用於選擇不一樣的策略，其可選值有adaptive(
默認)、static、compact、passive(diagnostic用)、aggressive(diagnostic用)；另外當allocation failure發生的時候，Shenandoah有一些優雅的degradation ladder用於處理這種狀況，包括Pacing(<10 ms)、Degenerated GC(<100 ms)、Full GC(>100 ms)
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