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  1. JDK
  2. JDK-8229406

ZGC: Fix incorrect statistics

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    • Subcomponent:
      gc
    • Resolved In Build:
      b10

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        Description

        From Hao Tang <albert.th@alibaba-inc.com>:

        Hi, I found that column "total" of ZGC log's statistics always reports obviously incorrect average values after running program with ZGC for over 10 hours (36000s). See the GC log below.

        [36000.080s][info][gc,stats ] === Garbage Collection Statistics =======================================================================================================================
        [36000.080s][info][gc,stats ] Last 10s Last 10m Last 10h Total
        [36000.080s][info][gc,stats ] Avg / Max Avg / Max Avg / Max Avg / Max
        [36000.080s][info][gc,stats ] Collector: Garbage Collection Cycle 33.269 / 34.686 32.699 / 36.862 32.938 / 60.703 0.000 / 60.703 ms
        [36000.080s][info][gc,stats ] Contention: Mark Segment Reset Contention 0 / 0 0 / 0 0 / 0 1 / 0 ops/s
        [36000.080s][info][gc,stats ] Contention: Mark SeqNum Reset Contention 0 / 0 0 / 0 0 / 0 1 / 0 ops/s
        [36000.080s][info][gc,stats ] Contention: Relocation Contention 0 / 0 0 / 0 0 / 2277 1 / 2277 ops/s
        [36000.080s][info][gc,stats ] Critical: Allocation Stall 0.000 / 0.000 0.000 / 0.000 0.000 / 0.000 0.000 / 0.000 ms
        [36000.080s][info][gc,stats ] Critical: Allocation Stall 0 / 0 0 / 0 0 / 0 1 / 0 ops/s
        [36000.080s][info][gc,stats ] Critical: GC Locker Stall 0.000 / 0.000 0.000 / 0.000 0.000 / 0.000 0.000 / 0.000 ms
        [36000.080s][info][gc,stats ] Critical: GC Locker Stall 0 / 0 0 / 0 0 / 0 1 / 0 ops/s
        [36000.080s][info][gc,stats ] Memory: Allocation Rate 45 / 48 45 / 48 45 / 90 0 / 90 MB/s
        [36000.080s][info][gc,stats ] Memory: Heap Used After Mark 126 / 130 125 / 140 125 / 164 0 / 164 MB
        [36000.080s][info][gc,stats ] Memory: Heap Used After Relocation 48 / 52 47 / 52 47 / 54 0 / 54 MB
        [36000.080s][info][gc,stats ] Memory: Heap Used Before Mark 124 / 130 123 / 138 124 / 158 0 / 158 MB
        [36000.080s][info][gc,stats ] Memory: Heap Used Before Relocation 54 / 58 53 / 60 53 / 60 0 / 60 MB

        The bug is caused by mistakenly adding "nsamples" to ZStatSamplerData::_sum. I am applying the patch below to fix the bug. Please give advice on this fix.
        diff -r 8f067351c370 src/hotspot/share/gc/z/zStat.cpp
        --- a/src/hotspot/share/gc/z/zStat.cpp Mon Aug 05 16:27:30 2019 -0700
        +++ b/src/hotspot/share/gc/z/zStat.cpp Fri Aug 09 11:22:58 2019 +0800
        @@ -63,7 +63,7 @@
          void add(const ZStatSamplerData& new_sample) {
            _nsamples += new_sample._nsamples;
        - _sum += new_sample._nsamples;
        + _sum += new_sample._sum;
            _max = MAX2(_max, new_sample._max);
          }
        };
        This patch can enable ZGC to print correct statistics after 10 hours.

        [36000.080s][info][gc,stats ] === Garbage Collection Statistics =======================================================================================================================
        [36000.080s][info][gc,stats ] Last 10s Last 10m Last 10h Total
        [36000.080s][info][gc,stats ] Avg / Max Avg / Max Avg / Max Avg / Max
        [36000.080s][info][gc,stats ] Collector: Garbage Collection Cycle 32.794 / 32.985 32.865 / 38.454 33.212 / 80.477 33.212 / 80.477 ms
        [36000.080s][info][gc,stats ] Contention: Mark Segment Reset Contention 0 / 0 0 / 0 0 / 0 0 / 0 ops/s
        [36000.080s][info][gc,stats ] Contention: Mark SeqNum Reset Contention 0 / 0 0 / 0 0 / 0 0 / 0 ops/s
        [36000.080s][info][gc,stats ] Contention: Relocation Contention 0 / 0 0 / 0 0 / 5978 0 / 5978 ops/s
        [36000.080s][info][gc,stats ] Critical: Allocation Stall 0.000 / 0.000 0.000 / 0.000 0.000 / 0.000 0.000 / 0.000 ms
        [36000.080s][info][gc,stats ] Critical: Allocation Stall 0 / 0 0 / 0 0 / 0 0 / 0 ops/s
        [36000.080s][info][gc,stats ] Critical: GC Locker Stall 0.000 / 0.000 0.000 / 0.000 0.000 / 0.000 0.000 / 0.000 ms
        [36000.080s][info][gc,stats ] Critical: GC Locker Stall 0 / 0 0 / 0 0 / 0 0 / 0 ops/s
        [36000.080s][info][gc,stats ] Memory: Allocation Rate 45 / 46 45 / 48 45 / 92 45 / 92 MB/s
        [36000.080s][info][gc,stats ] Memory: Heap Used After Mark 124 / 126 125 / 148 126 / 164 126 / 164 MB
        [36000.080s][info][gc,stats ] Memory: Heap Used After Relocation 48 / 52 48 / 52 47 / 54 47 / 54 MB
        [36000.080s][info][gc,stats ] Memory: Heap Used Before Mark 122 / 124 123 / 148 124 / 158 124 / 158 MB
        [36000.080s][info][gc,stats ] Memory: Heap Used Before Relocation 54 / 58 54 / 60 53 / 60 53 / 60 MB

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                • Assignee:
                  pliden Per Liden
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                  pliden Per Liden
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