Unexplained long stop the world pauses during concurrent marking step in G1 Collector

[Krishnamurthy, Kannan]

Greetings,

We are experiencing unexplained/unknown long pauses (8 seconds) during concurrent marking step of G1 collector.

    2014-08-07T13:42:30.552-0400: 92183.303: [GC concurrent-root-region-scan-start]
    2014-08-07T13:42:30.555-0400: 92183.305: [GC concurrent-root-region-scan-end, 0.0025230 secs]
    **2014-08-07T13:42:30.555-0400: 92183.305: [GC concurrent-mark-start]**
    **2014-08-07T13:42:39.598-0400: 92192.348: Application time: 9.0448670 seconds**
    2014-08-07T13:42:39.601-0400: 92192.351: Total time for which application threads were stopped: 0.0029740 seconds
    2014-08-07T13:42:39.603-0400: 92192.353: [GC pause (G1 Evacuation Pause) (young) 92192.354: [G1Ergonomics (CSet Construction) start choosing CSet, _pending_cards: 7980, predicted base time: 28.19 ms, remaining time: 71.81 ms, target pause time: 100.00 ms


`2014-08-07T13:42:30.555-0400: 92183.305` is when the concurrent mark starts, approximately after 2 seconds of this step the application starts to pause. However the GC logs claims the application was not paused during this window.
Linux "top" shows single CPU at 100% and rest of the CPUs at 0% during the pause.

Any help in understanding the root cause of this issue is appreciated.

Our target JVMS:

    java version "1.7.0_04"
    Java(TM) SE Runtime Environment (build 1.7.0_04-b20)
    Java HotSpot(TM) 64-Bit Server VM (build 23.0-b21, mixed mode)

    java version "1.8.0_11"
    Java(TM) SE Runtime Environment (build 1.8.0_11-b12)
    Java HotSpot(TM) 64-Bit Server VM (build 25.11-b03, mixed mode)


Our JVM options :

    -Xms20G -Xmx20G -Xss10M -XX:PermSize=128M -XX:MaxPermSize=128M -XX:MarkStackSize=16M
    -XX:+UnlockDiagnosticVMOptions -XX:+G1PrintRegionLivenessInfo -XX:+TraceGCTaskThread
    -XX:+G1SummarizeConcMark -XX:+G1SummarizeRSetStats -XX:+G1TraceConcRefinement
    -XX:+UseG1GC -XX:MaxGCPauseMillis=100 -XX:InitiatingHeapOccupancyPercent=65
    -XX:ParallelGCThreads=24 -verbose:gc -XX:+PrintGC -XX:+PrintGCDetails
    -XX:+PrintGCDateStamps -XX:+PrintAdaptiveSizePolicy -XX:+PrintTenuringDistribution
    -XX:+PrintGCApplicationStoppedTime -XX:+PrintGCApplicationConcurrentTime
    -Xloggc:/common/logs/ocean-partition-gc.log


Thanks and regards,
Kannan