RFR: 8253971: ZGC: Flush mark stacks after processing concurrent roots

[Erik Österlund]

On Sat, 3 Oct 2020 13:02:39 GMT, Per Liden <pliden at openjdk.org> wrote:

> I looked into why almost all mark cycles see non-zero "mark completions". In other words, we almost always have some
> amount of mark work left to handle in the mark end pause. It turns out that worker threads don't flush their mark
> stacks in ZMarkConcurrentRootsTask::work(), which means they can hide work (in their thread local mark stacks) until
> those stacks are finally flushed out in ZMark::try_end(). The reason work can be hidden is that the set of worker
> threads executing ZMarkConcurrentRootsTask is not necessarily the same set of worker threads executing ZMarkTask. As a
> result, the mark end pause often becomes longer than it otherwise would have.  After fixing this, I did some tests with
> Dacapo, which shows the following improvement:  Before: Mark End Pause (avg/max): 0.391 / 1.142 ms After: Mark End
> Pause (avg/max): 0.130 / 0.294 ms

Looks good. We might also want to poke out mark stacks on the VM thread when we handshake threads, using a concurrent
VM operation. But maybe that is a problem for another day.

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Marked as reviewed by eosterlund (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/495