Odd G1GC behavior on 8u91

Wed Aug 24 22:53:15 UTC 2016

Hi Bernd,

No, haven't tried that yet -- for now, I'm trying to understand the current
G1 behavior from the logs, and then hopefully figure out what needs
tweaking once I've understood the reason(s).

Thanks

On Wed, Aug 24, 2016 at 6:26 PM, <ecki at zusammenkunft.net> wrote:

> Hello,
>
>
>
> Did you try to use G1 with no constraints. Just setting heapsize and
> not-to-aggressive pause target. That at least should avoid the allocation
> failures.
>
>
>
> At least it would help tuning to know what patologies this would trigger
> with yout app and 100G heap.
>
>
> Gruss
> Bernd
> --
> http://bernd.eckenfels.net
> From Win 10 Mobile
>
>
>
> *Von: *Vitaly Davidovich <vitalyd at gmail.com>
> *Gesendet: *Donnerstag, 25. August 2016 00:02
> *An: *Eric Caspole <eric.caspole at oracle.com>
> *Cc: *hotspot-gc-use <hotspot-gc-use at openjdk.java.net>
> *Betreff: *Re: Odd G1GC behavior on 8u91
>
>
>
> Hi Eric,
>
>
>
> Thanks for the reply.  Yeah, we're experimenting with reducing the
> initiating heap occupancy to 55%, hopefully letting the concurrent marking
> complete before the allocation and promotion rate results in a Full GC.  In
> particular, I'm not sure how to interpret the gc log snippets I posted
> showing a young evac with an empty Young gen, and then followed by a Full
> GC due to heap expansion failure for a 32 byte allocation.  I'm likely
> missing something in my interpretation.
>
>
>
> I think you're also on point about the max tenuring threshold being too
> low - need to see how that value was arrived at.
>
>
>
> Thanks
>
>
>
> On Wed, Aug 24, 2016 at 5:20 PM, Eric Caspole <eric.caspole at oracle.com>
> wrote:
>
> I have not used G1 in JDK 8 that much but the two trouble spots to me are:
>
> -XX:InitiatingHeapOccupancyPercent=75
> -XX:MaxTenuringThreshold=2
>
> So this will tenure very quickly, filling up old gen and start the marking
> relatively late at 75%. This looks like it is pretty likely to end up in a
> STW full GC.
> Since you do have a huge amount of garbage getting collected in the full
> gc maybe try letting more of it die off in young gen with higher tenuring
> threshold and also start marking earlier than 75%.
>
> good luck,
> Eric
>
>
>
>
>
> On 08/24/2016 02:43 PM, Vitaly Davidovich wrote:
>
> Hi guys,
>
> Hoping someone could shed some light on G1 behavior (as seen from the gc
> log) that I'm having a hard time understanding.  The root problem is G1
> enters a Full GC that takes many tens of seconds, and need some advice
> on what could be causing it.
>
> First, some basic info:
> Java HotSpot(TM) 64-Bit Server VM (25.91-b14) for linux-amd64 JRE
> (1.8.0_91-b14), built on Apr  1 2016 00:57:21 by "java_re" with gcc
> 4.3.0 20080428 (Red Hat 4.3.0-8)
> Memory: 4k page, physical 264115728k(108464820k free), swap 0k(0k free)
> CommandLine flags: -XX:G1HeapWastePercent=5 -XX:G1MixedGCCountTarget=4
> -XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=<some_path>
> -XX:InitialCodeCacheSize=104857600 -XX:InitialHeapSize=103079215104
> -XX:InitialTenuringThreshold=2 -XX:InitiatingHeapOccupancyPercent=75
> -XX:+ManagementServer -XX:MaxGCPauseMillis=300
> -XX:MaxHeapSize=103079215104 -XX:MaxNewSize=32212254720
> -XX:MaxTenuringThreshold=2 -XX:NewSize=32212254720
> -XX:+ParallelRefProcEnabled -XX:+PrintAdaptiveSizePolicy
> -XX:+PrintCommandLineFlags -XX:+PrintCompilation
> -XX:PrintFLSStatistics=1 -XX:+PrintGC -XX:+PrintGCApplicationStoppedTime
> -XX:+PrintGCDateStamps -XX:+PrintGCDetails -XX:+PrintGCTimeStamps
> -XX:+PrintPromotionFailure -XX:+PrintReferenceGC
> -XX:+PrintSafepointStatistics -XX:PrintSafepointStatisticsCount=1
> -XX:+PrintTenuringDistribution -XX:ReservedCodeCacheSize=104857600
> -XX:SurvivorRatio=9 -XX:-UseAdaptiveSizePolicy -XX:+UseG1GC
>
> Swap is disabled.  THP is disabled.
>
> First issue I have a question about:
>
> 2016-08-24T15:29:12.302+0000: 17776.029: [GC pause (G1 Evacuation Pause)
> (young)
> Desired survivor size 1795162112 bytes, new threshold 2 (max 2)
>  17776.029: [G1Ergonomics (CSet Construction) start choosing CSet,
> _pending_cards: 0, predicted base time: 14.07 ms, remaining time: 285.93
> ms, target pause time: 300.00 ms]
>  17776.029: [G1Ergonomics (CSet Construction) add young regions to CSet,
> eden: 0 regions, survivors: 0 regions, predicted young region time: 0.00
> ms]
>  17776.029: [G1Ergonomics (CSet Construction) finish choosing CSet,
> eden: 0 regions, survivors: 0 regions, old: 0 regions, predicted pause
> time: 14.07 ms, target pause time: 300.00 ms]
> 2016-08-24T15:29:12.305+0000: 17776.033: [SoftReference, 0 refs,
> 0.0012417 secs]2016-08-24T15:29:12.307+0000: 17776.034: [WeakReference,
> 0 refs, 0.0007101 secs]2016-08-24T15:29:12.307+0000: 17776.035:
> [FinalReference, 0 refs, 0.0007027 secs]2016-08-24T15:29:12.308+0000:
> 17776.035: [PhantomReference, 0 refs, 0 refs, 0.0013585
> secs]2016-08-24T15:29:12.309+0000: 17776.037: [JNI Weak Reference,
> 0.0000118 secs], 0.0089758 secs]
>    [Parallel Time: 3.1 ms, GC Workers: 23]
>       [GC Worker Start (ms): Min: 17776029.2, Avg: 17776029.3, Max:
> 17776029.4, Diff: 0.2]
>       [Ext Root Scanning (ms): Min: 0.8, Avg: 1.1, Max: 2.8, Diff: 1.9,
> Sum: 24.2]
>       [Update RS (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.1]
>          [Processed Buffers: Min: 0, Avg: 0.1, Max: 1, Diff: 1, Sum: 2]
>       [Scan RS (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
>       [Code Root Scanning (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0,
> Sum: 0.0]
>       [Object Copy (ms): Min: 0.0, Avg: 0.1, Max: 0.1, Diff: 0.1, Sum: 1.2]
>       [Termination (ms): Min: 0.0, Avg: 1.6, Max: 1.8, Diff: 1.8, Sum:
> 37.9]
>          [Termination Attempts: Min: 1, Avg: 1.0, Max: 1, Diff: 0, Sum: 23]
>       [GC Worker Other (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0,
> Sum: 0.2]
>       [GC Worker Total (ms): Min: 2.7, Avg: 2.8, Max: 2.9, Diff: 0.2,
> Sum: 63.8]
>       [GC Worker End (ms): Min: 17776032.0, Avg: 17776032.0, Max:
> 17776032.1, Diff: 0.0]
>    [Code Root Fixup: 0.2 ms]
>    [Code Root Purge: 0.0 ms]
>    [Clear CT: 0.4 ms]
>    [Other: 5.3 ms]
>       [Choose CSet: 0.0 ms]
>       [Ref Proc: 4.4 ms]
>       [Ref Enq: 0.3 ms]
>       [Redirty Cards: 0.3 ms]
>       [Humongous Register: 0.1 ms]
>       [Humongous Reclaim: 0.1 ms]
>       [Free CSet: 0.0 ms]
>
>    *[Eden: 0.0B(30.0G)->0.0B(30.0G) Survivors: 0.0B->0.0B Heap:
> 95.2G(96.0G)->95.2G(96.0G)]*
>  [Times: user=0.08 sys=0.00, real=0.01 secs]
> 2016-08-24T15:29:12.311+0000: 17776.038: Total time for which
> application threads were stopped: 0.0103002 seconds, Stopping threads
> took: 0.0000566 seconds
>  17776.039: [G1Ergonomics (Heap Sizing) attempt heap expansion, reason:
> allocation request failed, allocation request: 32 bytes]
>  17776.039: [G1Ergonomics (Heap Sizing) expand the heap, requested
> expansion amount: 33554432 bytes, attempted expansion amount: 33554432
> bytes]
>  17776.039: [G1Ergonomics (Heap Sizing) did not expand the heap, reason:
> heap already fully expanded]
> 2016-08-24T15:29:12.312+0000: 17776.039: [Full GC (Allocation Failure)
> 2016-08-24T15:29:40.727+0000: 17804.454: [SoftReference, 5504 refs,
> 0.0012432 secs]2016-08-24T15:29:40.728+0000: 17804.456: [WeakReference,
> 1964 refs, 0.0003012 secs]2016-08-24T15:29:40.728+0000: 17804.456:
> [FinalReference, 3270 refs, 0.0033290 secs]2016-08-24T15:29:40.732+0000:
> 17804.459: [PhantomReference, 0 refs, 75 refs, 0.0000257
> secs]2016-08-24T15:29:40.732+0000: 17804.459: [JNI Weak Reference,
> 0.0000172 secs] 95G->38G(96G), 95.5305034 secs]
>    [Eden: 0.0B(30.0G)->0.0B(30.0G) Survivors: 0.0B->0.0B Heap:
> 95.2G(96.0G)->38.9G(96.0G)], [Metaspace: 104180K->103365K(106496K)]
> * [Times: user=157.02 sys=0.28, real=95.54 secs] *
> *
> *
> So here we have a lengthy full GC pause that collects quite a bit of old
> gen (expected).  Right before this is a young evac pause.
>
> Why is the heap sizing (bolded) reported after the evac pause showing
> empty Eden+Survivor?
>
> Why is ergonomic info reporting 0 regions selected (i.e. what's
> evacuated then)?
>
> Right before the Full GC, ergonomics report a failure to expand the heap
> due to an allocation request of 32 bytes.  Is this implying that a
> mutator tried to allocate 32 bytes but couldn't? How do I reconcile that
> with Eden+Survivor occupancy reported right above that?
>
> Young gen is sized to 30GB, total heap is 96GB.  Allocation rate of the
> application is roughly 1GB/s.  Am I correct in assuming that allocation
> is outpacing concurrent marking, based on the above? What tunable(s)
> would you advise to tweak to get G1 to keep up with the allocation rate?
> I'm ok taking some throughput hit to mitigate 90s+ pauses.
>
> Let me know if any additional info is needed (I have the full GC log,
> and can attach that if desired).
>
> Thanks
>
>
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