Huge difference in GC pause time and total application pause time.

[Naman Jain]

Hey Everyone,
G1 GC log:2015-10-05T14:54:35.518+0000: 549629.568: [GC pause (G1 Evacuation Pause) (young)Desired survivor size 322961408 bytes, new threshold 15 (max 15)- age   1:    5292232 bytes,    5292232 total- age   2:    1201264 bytes,    6493496 total- age   3:     377376 bytes,    6870872 total- age   4:     440752 bytes,    7311624 total- age   5:     201976 bytes,    7513600 total- age   6:      77616 bytes,    7591216 total- age   7:      28824 bytes,    7620040 total- age   8:      55312 bytes,    7675352 total- age   9:      55432 bytes,    7730784 total- age  10:      14272 bytes,    7745056 total- age  11:      16872 bytes,    7761928 total- age  12:      18208 bytes,    7780136 total- age  13:      11656 bytes,    7791792 total- age  14:      15800 bytes,    7807592 total- age  15:      33960 bytes,    7841552 total 549629.569: [G1Ergonomics (CSet Construction) start choosing CSet, _pending_cards: 14904, predicted base time: 20.98 ms, remaining time: 19.02 ms, target pause time: 40.00 ms] 549629.569: [G1Ergonomics (CSet Construction) add young regions to CSet, eden: 1224 regions, survivors: 4 regions, predicted young region time: 10.73 ms] 549629.569: [G1Ergonomics (CSet Construction) finish choosing CSet, eden: 1224 regions, survivors: 4 regions, old: 0 regions, predicted pause time: 31.70 ms, target pause time: 40.00 ms], 0.0355102 secs]   [Parallel Time: 21.7 ms, GC Workers: 18]      [GC Worker Start (ms): Min: 549629569.6, Avg: 549629569.8, Max: 549629569.9, Diff: 0.3]      [Ext Root Scanning (ms): Min: 3.6, Avg: 4.3, Max: 12.7, Diff: 9.0, Sum: 78.1]      [Update RS (ms): Min: 0.0, Avg: 4.1, Max: 4.7, Diff: 4.7, Sum: 74.6]         [Processed Buffers: Min: 0, Avg: 60.3, Max: 79, Diff: 79, Sum: 1086]      [Scan RS (ms): Min: 0.1, Avg: 2.2, Max: 2.7, Diff: 2.5, Sum: 40.4]      [Code Root Scanning (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.1]      [Object Copy (ms): Min: 8.0, Avg: 10.2, Max: 10.5, Diff: 2.5, Sum: 183.2]      [Termination (ms): Min: 0.0, Avg: 0.1, Max: 0.2, Diff: 0.2, Sum: 2.5]      [GC Worker Other (ms): Min: 0.0, Avg: 0.1, Max: 0.2, Diff: 0.2, Sum: 2.0]      [GC Worker Total (ms): Min: 20.9, Avg: 21.2, Max: 21.4, Diff: 0.5, Sum: 381.0]      [GC Worker End (ms): Min: 549629590.9, Avg: 549629590.9, Max: 549629591.1, Diff: 0.2]   [Code Root Fixup: 0.3 ms]   [Code Root Purge: 0.0 ms]   [Clear CT: 0.8 ms]   [Other: 12.6 ms]      [Choose CSet: 0.0 ms]      [Ref Proc: 6.2 ms]      [Ref Enq: 0.5 ms]      [Redirty Cards: 0.4 ms]      [Humongous Reclaim: 0.0 ms]      [Free CSet: 2.9 ms]   [Eden: 4896.0M(4896.0M)->0.0B(4892.0M) Survivors: 16.0M->20.0M Heap: 6505.9M(8192.0M)->1611.1M(8192.0M)] [Times: user=0.38 sys=0.02, real=0.75 secs] 2015-10-05T14:54:36.272+0000: 549630.322: Total time for which application threads were stopped: 0.7695049 seconds, Stopping threads took: 0.0004596 seconds
As you can see that young GC took around 35ms but the total time for which application threads were stopped is 769ms. Can anyone tell me what other pauses are included in this total time ?Also why pauses other then GC pauses were included in GC logs ?Earlier I was using CMS. But in CMS I never saw such differences in GC time and Application thread pause time. Can anyone tell me what is the reason behind such behaviour ?
Thanks
-Naman
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