Summary
-------
Add command line options to enable and configure the Shenandoah GC.
Problem
-------
I would like to add the Shenandoah Garbage Collector. Similar to any other GC, this would have to be enabled by -XX:+UseShenandoahGC on the command line. Also, similar to other GCs, Shenandoah comes with a number of knobs that can change the behaviour of the GC.
Solution
--------
The primary command line flag to be added is -XX:+UseShenandoahGC. This option will be added as an experimental flag to gc_globals.hpp.
Additional command line options would be added to configure Shenandoah's behaviour. They are all prefixed by 'Shenandoah' and reside in shenandoah_globals.hpp.
Specification
-------------
From gc_globals.hpp:
+ experimental(bool, UseShenandoahGC, false, \
+ "Use the Shenandoah garbage collector") \
+ \
From shenandoah_globals:
+ experimental(size_t, ShenandoahHeapRegionSize, 0, \
+ "Size of the Shenandoah regions. Set to zero to detect " \
+ "automatically.") \
+ \
+ experimental(size_t, ShenandoahTargetNumRegions, 2048, \
+ "Target number of regions. We try to get around that many " \
+ "regions, based on Shenandoah{Min,Max}RegionSize.") \
+ \
+ experimental(size_t, ShenandoahMinRegionSize, 256 * K, \
+ "Minimum Shenandoah heap region size.") \
+ \
+ experimental(size_t, ShenandoahMaxRegionSize, 32 * M, \
+ "Maximum Shenandoah heap region size.") \
+ \
+ experimental(intx, ShenandoahHumongousThreshold, 100, \
+ "How large should the object be to get allocated in humongous " \
+ "region, in percents of heap region size. This also caps the " \
+ "maximum TLAB size.") \
+ range(1, 100) \
+ \
+ experimental(ccstr, ShenandoahGCHeuristics, "adaptive", \
+ "The heuristics to use in Shenandoah GC. Possible values:" \
+ " *) adaptive - adapt to maintain the given amount of free heap;" \
+ " *) static - start concurrent GC when static free heap " \
+ " threshold and static allocation threshold are " \
+ " tripped;" \
+ " *) passive - do not start concurrent GC, wait for Full GC; " \
+ " *) aggressive - run concurrent GC continuously, evacuate " \
+ " everything;" \
+ " *) compact - run GC with lower footprint target, may end up " \
+ " doing continuous GC, evacuate lots of live " \
+ " objects, uncommit heap aggressively;") \
+ \
+ experimental(ccstr, ShenandoahUpdateRefsEarly, "adaptive", \
+ "Run a separate concurrent reference updating phase after" \
+ "concurrent evacuation. Possible values: 'on', 'off', 'adaptive'")\
+ \
+ experimental(uintx, ShenandoahRefProcFrequency, 5, \
+ "How often should (weak, soft, etc) references be processed. " \
+ "References get processed at every Nth GC cycle. Set to zero " \
+ "to disable reference processing.") \
+ \
+ experimental(uintx, ShenandoahUnloadClassesFrequency, 5, \
+ "How often should classes get unloaded. " \
+ "Class unloading is performed at every Nth GC cycle. " \
+ "Set to zero to disable class unloading during concurrent GC.") \
+ \
+ experimental(uintx, ShenandoahGarbageThreshold, 60, \
+ "Sets the percentage of garbage a region need to contain before " \
+ "it can be marked for collection. Does not apply to all " \
+ "heuristics.") \
+ range(0,100) \
+ \
+ experimental(uintx, ShenandoahFreeThreshold, 10, \
+ "Set the percentage of free heap at which a GC cycle is started. "\
+ "Does not apply to all heuristics.") \
+ range(0,100) \
+ \
+ experimental(uintx, ShenandoahInitFreeThreshold, 70, \
+ "Initial remaining free heap threshold for learning steps in " \
+ "heuristics. In percents of total heap size. Does not apply to " \
+ "all heuristics.") \
+ range(0,100) \
+ \
+ experimental(uintx, ShenandoahMinFreeThreshold, 10, \
+ "Minimum remaining free space threshold, after which collection " \
+ "definitely triggers. Does not apply to all heuristics.") \
+ range(0,100) \
+ \
+ experimental(uintx, ShenandoahAllocationThreshold, 0, \
+ "Set percentage of memory allocated since last GC cycle before " \
+ "a new GC cycle can be started. Set to zero to effectively " \
+ "disable.") \
+ range(0,100) \
+ \
+ experimental(uintx, ShenandoahLearningSteps, 5, \
+ "Number of GC cycles to run in order to learn application " \
+ "and GC performance for adaptive heuristics.") \
+ range(0,100) \
+ \
+ experimental(uintx, ShenandoahImmediateThreshold, 90, \
+ "If mark identifies more than this much immediate garbage " \
+ "regions, it shall recycle them, and shall not continue the " \
+ "rest of the GC cycle. The value is in percents of total " \
+ "number of candidate regions for collection set. Setting this " \
+ "threshold to 100% effectively disables this shortcut.") \
+ range(0,100) \
+ \
+ experimental(uintx, ShenandoahMergeUpdateRefsMinGap, 100, \
+ "If GC is currently running in separate update-refs mode " \
+ "this numbers gives the threshold when to switch to " \
+ "merged update-refs mode. Number is percentage relative to" \
+ "duration(marking)+duration(update-refs).") \
+ \
+ experimental(uintx, ShenandoahMergeUpdateRefsMaxGap, 200, \
+ "If GC is currently running in merged update-refs mode " \
+ "this numbers gives the threshold when to switch to " \
+ "separate update-refs mode. Number is percentage relative " \
+ "to duration(marking)+duration(update-refs).") \
+ \
+ experimental(uintx, ShenandoahGuaranteedGCInterval, 5*60*1000, \
+ "Adaptive and dynamic heuristics would guarantee a GC cycle " \
+ "at least with this interval. This is useful when large idle" \
+ " intervals are present, where GC can run without stealing " \
+ "time from active application. Time is in milliseconds.") \
+ \
+ experimental(bool, ShenandoahAlwaysClearSoftRefs, false, \
+ "Clear soft references always, instead of using any smart " \
+ "cleanup policy. This minimizes footprint at expense of more " \
+ "softref churn in applications.") \
+ \
+ experimental(bool, ShenandoahUncommit, true, \
+ "Allow Shenandoah to uncommit unused memory.") \
+ \
+ experimental(uintx, ShenandoahUncommitDelay, 5*60*1000, \
+ "Shenandoah would start to uncommit memory for regions that were" \
+ " not used for more than this time. First use after that would " \
+ "incur allocation stalls. Actively used regions would never be " \
+ "uncommitted, because they never decay. Time is in milliseconds." \
+ "Setting this delay to 0 effectively makes Shenandoah to " \
+ "uncommit the regions almost immediately.") \
+ \
+ experimental(bool, ShenandoahRegionSampling, false, \
+ "Turns on heap region sampling via JVMStat") \
+ \
+ experimental(int, ShenandoahRegionSamplingRate, 40, \
+ "Sampling rate for heap region sampling. " \
+ "Number of milliseconds between samples") \
+ \
+ experimental(uintx, ShenandoahControlIntervalMin, 1, \
+ "The minumum sleep interval for control loop that drives " \
+ "the cycles. Lower values would increase GC responsiveness " \
+ "to changing heap conditions, at the expense of higher perf " \
+ "overhead. Time is in milliseconds.") \
+ \
+ experimental(uintx, ShenandoahControlIntervalMax, 10, \
+ "The maximum sleep interval for control loop that drives " \
+ "the cycles. Lower values would increase GC responsiveness " \
+ "to changing heap conditions, at the expense of higher perf " \
+ "overhead. Time is in milliseconds.") \
+ \
+ experimental(uintx, ShenandoahControlIntervalAdjustPeriod, 1000, \
+ "The time period for one step in control loop interval " \
+ "adjustment. Lower values make adjustments faster, at the " \
+ "expense of higher perf overhead. Time is in milliseconds.") \
+ \
+ diagnostic(bool, ShenandoahVerify, false, \
+ "Verify the Shenandoah garbage collector") \
+ \
+ diagnostic(intx, ShenandoahVerifyLevel, 4, \
+ "Shenandoah verification level: " \
+ "0 = basic heap checks; " \
+ "1 = previous level, plus basic region checks; " \
+ "2 = previous level, plus all roots; " \
+ "3 = previous level, plus all reachable objects; " \
+ "4 = previous level, plus all marked objects") \
+ \
+ diagnostic(bool, ShenandoahElasticTLAB, true, \
+ "Use Elastic TLABs with Shenandoah") \
+ \
+ diagnostic(bool, ShenandoahAllowMixedAllocs, true, \
+ "Allow mixing mutator and collector allocations in a single " \
+ "region") \
+ \
+ experimental(uintx, ShenandoahAllocSpikeFactor, 5, \
+ "The amount of heap space to reserve for absorbing the " \
+ "allocation spikes. Larger value wastes more memory in " \
+ "non-emergency cases, but provides more safety in emergency " \
+ "cases. In percents of total heap size.") \
+ range(0,100) \
+ \
+ experimental(uintx, ShenandoahEvacReserve, 5, \
+ "Maximum amount of free space to reserve for evacuation. " \
+ "Larger values make GC more aggressive, while leaving less " \
+ "headroom for application to allocate in. " \
+ "In percents of total heap size.") \
+ range(1,100) \
+ \
+ experimental(double, ShenandoahEvacWaste, 1.2, \
+ "How much waste evacuations produce within the reserved " \
+ "space. Larger values make evacuations more resilient " \
+ "against allocation failures, at expense of smaller csets " \
+ "on each cycle.") \
+ range(1.0,100.0) \
+ \
+ experimental(bool, ShenandoahEvacReserveOverflow, true, \
+ "Allow evacuations to overflow the reserved space. " \
+ "Enabling it will make evacuations more resilient when " \
+ "evacuation reserve/waste is incorrect, at the risk that " \
+ "application allocations run out of memory too early.") \
+ \
+ diagnostic(bool, ShenandoahAllocationTrace, false, \
+ "Trace allocation latencies and stalls. Can be expensive when " \
+ "lots of allocations happen, and may introduce scalability " \
+ "bottlenecks.") \
+ \
+ diagnostic(intx, ShenandoahAllocationStallThreshold, 10000, \
+ "When allocation tracing is enabled, the allocation stalls " \
+ "larger than this threshold would be reported as warnings. " \
+ "Time is in microseconds.") \
+ \
+ experimental(uintx, ShenandoahEvacAssist, 10, \
+ "How many objects to evacuate on WB assist path. " \
+ "Use zero to disable.") \
+ \
+ experimental(bool, ShenandoahPacing, true, \
+ "Pace application allocations to give GC chance to start " \
+ "and complete before allocation failure is reached.") \
+ \
+ experimental(uintx, ShenandoahPacingMaxDelay, 10, \
+ "Max delay for pacing application allocations. " \
+ "Time is in milliseconds.") \
+ \
+ experimental(uintx, ShenandoahPacingIdleSlack, 2, \
+ "Percent of heap counted as non-taxable allocations during idle. "\
+ "Larger value makes the pacing milder during idle phases, " \
+ "requiring less rendezvous with control thread. Lower value " \
+ "makes the pacing control less responsive to out-of-cycle allocs.")\
+ range(0, 100) \
+ \
+ experimental(uintx, ShenandoahPacingCycleSlack, 10, \
+ "Percent of free space taken as non-taxable allocations during " \
+ "the GC cycle. Larger value makes the pacing milder at the " \
+ "beginning of the GC cycle. Lower value makes the pacing less " \
+ "uniform during the cycle.") \
+ range(0, 100) \
+ \
+ experimental(double, ShenandoahPacingSurcharge, 1.1, \
+ "Additional pacing tax surcharge to help unclutter the heap. " \
+ "Larger values makes the pacing more aggressive. Lower values " \
+ "risk GC cycles finish with less memory than were available at " \
+ "the beginning of it.") \
+ range(1.0, 100.0) \
+ \
+ experimental(uintx, ShenandoahCriticalFreeThreshold, 1, \
+ "Percent of heap that needs to be free after recovery cycles, " \
+ "either Degenerated or Full GC. If this much space is not " \
+ "available, next recovery step would triggered.") \
+ range(0, 100) \
+ \
+ diagnostic(bool, ShenandoahDegeneratedGC, true, \
+ "Use Degenerated GC as the graceful degradation step. Disabling " \
+ "this leads to degradation to Full GC") \
+ \
+ experimental(uintx, ShenandoahFullGCThreshold, 3, \
+ "How many back-to-back Degenerated GCs to do before triggering " \
+ "a Full GC.") \
+ \
+ experimental(bool, ShenandoahImplicitGCInvokesConcurrent, false, \
+ "Should internally-caused GCs invoke concurrent cycles, or go to" \
+ "stop-the-world (degenerated/full)?") \
+ \
+ experimental(bool, ShenandoahHumongousMoves, true, \
+ "Allow moving humongous regions. This makes GC more resistant " \
+ "to external fragmentation that may otherwise fail other " \
+ "humongous allocations, at the expense of higher GC copying " \
+ "costs.") \
+ \
+ diagnostic(bool, ShenandoahOOMDuringEvacALot, false, \
+ "Simulate OOM during evacuation frequently.") \
+ \
+ diagnostic(bool, ShenandoahAllocFailureALot, false, \
+ "Make lots of artificial allocation failures.") \
+ \
+ diagnostic(bool, ShenandoahTerminationTrace, false, \
+ "Tracing task termination timings") \
+ \
+ develop(bool, ShenandoahVerifyObjectEquals, false, \
+ "Verify that == and != are not used on oops. Only in fastdebug") \
+ \
+ diagnostic(bool, ShenandoahAlwaysPreTouch, false, \
+ "Pre-touch heap memory, overrides global AlwaysPreTouch") \
+ \
+ experimental(intx, ShenandoahMarkScanPrefetch, 32, \
+ "How many objects to prefetch ahead when traversing mark bitmaps."\
+ "Set to 0 to disable prefetching.") \
+ range(0, 256) \
+ \
+ experimental(uintx, ShenandoahMarkLoopStride, 1000, \
+ "How many items are processed during one marking step") \
+ \
+ experimental(uintx, ShenandoahParallelRegionStride, 1024, \
+ "How many regions are processed in one stride during parallel " \
+ "iteration.") \
+ \
+ experimental(size_t, ShenandoahSATBBufferSize, 1 * K, \
+ "Number of entries in an SATB log buffer.") \
+ range(1, max_uintx) \
+ \
+ experimental(uintx, ShenandoahSATBBufferFlushInterval, 100, \
+ "Forcefully flush non-empty SATB buffers at this interval. " \
+ "Time is in milliseconds.") \
+ \
+ experimental(uint, ShenandoahParallelSafepointThreads, 4, \
+ "Number of parallel threads used for safepoint prolog/epilog") \
+ \
+ experimental(bool, ShenandoahPreclean, true, \
+ "Do concurrent preclean phase before final mark: process " \
+ "definitely alive references to avoid dealing with them during " \
+ "pause.") \
+ \
+ experimental(bool, ShenandoahSuspendibleWorkers, false, \
+ "Suspend concurrent GC worker threads at safepoints") \
+ \
+ diagnostic(bool, ShenandoahSATBBarrier, true, \
+ "Turn on/off SATB barriers in Shenandoah") \
+ \
+ diagnostic(bool, ShenandoahKeepAliveBarrier, true, \
+ "Turn on/off keep alive barriers in Shenandoah") \
+ \
+ diagnostic(bool, ShenandoahWriteBarrier, true, \
+ "Turn on/off write barriers in Shenandoah") \
+ \
+ diagnostic(bool, ShenandoahReadBarrier, true, \
+ "Turn on/off read barriers in Shenandoah") \
+ \
+ diagnostic(bool, ShenandoahStoreValEnqueueBarrier, false, \
+ "Turn on/off enqueuing of oops for storeval barriers") \
+ \
+ diagnostic(bool, ShenandoahStoreValReadBarrier, true, \
+ "Turn on/off store val read barriers in Shenandoah") \
+ \
+ diagnostic(bool, ShenandoahCASBarrier, true, \
+ "Turn on/off CAS barriers in Shenandoah") \
+ \
+ diagnostic(bool, ShenandoahAcmpBarrier, true, \
+ "Turn on/off acmp barriers in Shenandoah") \
+ \
+ diagnostic(bool, ShenandoahCloneBarrier, true, \
+ "Turn on/off clone barriers in Shenandoah") \
+ \
+ diagnostic(bool, ShenandoahStoreCheck, false, \
+ "Emit additional code that checks objects are written to only" \
+ " in to-space") \
+ \
+ experimental(bool, ShenandoahConcurrentScanCodeRoots, true, \
+ "Scan code roots concurrently, instead of during a pause") \
+ \
+ experimental(uintx, ShenandoahCodeRootsStyle, 2, \
+ "Use this style to scan code cache:" \
+ " 0 - sequential iterator;" \
+ " 1 - parallel iterator;" \
+ " 2 - parallel iterator with cset filters;") \
+ \
+ experimental(bool, ShenandoahOptimizeStaticFinals, true, \
+ "Optimize barriers on static final fields. " \
+ "Turn it off for maximum compatibility with reflection or JNI " \
+ "code that manipulates final fields.") \
+ \
+ experimental(bool, ShenandoahOptimizeInstanceFinals, false, \
+ "Optimize barriers on final instance fields." \
+ "Turn it off for maximum compatibility with reflection or JNI " \
+ "code that manipulates final fields.") \
+ \
+ experimental(bool, ShenandoahOptimizeStableFinals, false, \
+ "Optimize barriers on stable fields." \
+ "Turn it off for maximum compatibility with reflection or JNI " \
+ "code that manipulates final fields.") \
+ \
+ diagnostic(bool, ShenandoahDecreaseRegisterPressure, false, \
+ "Try to reuse after-barrier values to reduce register pressure") \
+ \
+ experimental(bool, ShenandoahCommonGCStateLoads, false, \
+ "Enable commonming for GC state loads in generated code.") \
+ \
+ develop(bool, ShenandoahVerifyOptoBarriers, false, \
+ "Verify no missing barriers in C2") \
+ \
+ experimental(bool, ShenandoahDontIncreaseWBFreq, true, \
+ "Common 2 WriteBarriers or WriteBarrier and a ReadBarrier only " \
+ "if the resulting WriteBarrier isn't executed more frequently") \
+ \
+ experimental(bool, ShenandoahLoopOptsAfterExpansion, true, \
+ "Attempt more loop opts after write barrier expansion") \
+
See also proposed webrev for review:
http://cr.openjdk.java.net/~rkennke/shenandoah-upstream/00/shenandoah-complete/