[jdk17] RFR: 8268265: MutableSpaceUsedHelper::take_sample() hits assert(left >= right) failed: avoid overflow

Thomas Schatzl tschatzl at openjdk.java.net
Wed Jun 16 09:33:34 UTC 2021

On Tue, 15 Jun 2021 21:37:06 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

> Please review this change to PerfData-based heap usage reporting by SerialGC
> and ParallelGC.
> For both of these collectors, the periodic heap usage sampling and reporting
> by the Watcher thread involves sampling the difference between each space's
> current allocation pointer and its base pointer.  But the Watcher thread
> runs during GC safepoints, which means that sampling may occasionally occur
> while the GC is in the process of updating the space being sampled.  This
> can result in a sample with a bogus value or even an assertion failure after
> JDK-8260046.
> Other collectors seem to avoid this by sampling and recording the heap usage
> value at specific synchronous times rather than asynchronously by the
> Watcher thread.
> There isn't a particularly good place for synchronous updates for ParallelGC
> and SerialGC, so this change doesn't do so.  Instead, it makes the existing
> asynchronous updates safe from interference by an in-progress GC.  This is
> done by having the update done under the Heap_lock.  Introducing a more
> fine-grained lock has the same problem as doing synchronous updates.  Rather
> than possibly blocking the sampling Watcher thread for the duration of a GC,
> it tries to lock the Heap_lock and skips sampling if it's not available.
> Testing:
> mach5 tier1-3, 5, 7 (3,5,7 are where focused Serial and ParallelGC testing is done).



Marked as reviewed by tschatzl (Reviewer).

PR: https://git.openjdk.java.net/jdk17/pull/65

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