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  1. JDK
  2. JDK-8244278

Excessive code cache flushes and sweeps

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    • Subcomponent:
    • Introduced In Version:
      9
    • Resolved In Build:
      b26

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        Description

        We have noticed increased deoptimizations and recompilations due to more frequent code cache flushing, during migration from JDK8 to JDK11. As a result, applications experienced latency and CPU usage regressions.

        The aggressive flushing behavior is likely introduced in JDK-8046809. The new option -XX:StartAggressiveSweepingAt is not fully respected, because NMethodSweeper::notify() should use MAX2(start_threshold, 1.1) instead of MIN2.

        This behavior can be reproduced with DaCapo's tradesoap benchmark, which could use ~20MB of code cache:
        $ java -Xms2g -Xmx2g -XX:ReservedCodeCacheSize=40m -XX:StartAggressiveSweepingAt=<N> -Xlog:codecache+sweep*=debug -XX:+UnlockDiagnosticVMOptions -XX:+PrintMethodFlushingStatistics -jar dacapo-9.12-MR1-bach.jar tradesoap -n 5 -t 16

        The log shows that code cache is flushed when the usage is far below (100 - StartAggressiveSweepingAt)%, which is 90% by default.

        Counting lines with "CodeCache flushing" is the number of completed flushes (NMethodSweeper::sweep_code_cache()).
        "Total number of full sweeps" at the end of the output is the number of attempted sweeps (NMethodSweeper::possibly_sweep()), i.e. number of times the sweeper thread is woken up.

        Without the fix:
        StartAggressiveSweepingAt | completed_flushes | attempted_sweeps
        2 | 418 | 12768
        10 (default) | 405 | 12907
        50 | 419 | 11933

        With the fix:
        StartAggressiveSweepingAt | completed_flushes | attempted_sweeps
        2 | 3 | 3
        10 (default) | 6 | 6
        50 | 315 | 315

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                • Assignee:
                  manc Man Cao
                  Reporter:
                  manc Man Cao
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