RFR: 8269870: PS: Membar in PSPromotionManager::copy_unmarked_to_survivor_space could be relaxed

[Kim Barrett]

On Mon, 12 Jul 2021 02:43:29 GMT, Hamlin Li <mli at openjdk.org> wrote:

> Membar in PSPromotionManager::copy_unmarked_to_survivor_space could be relaxed.
> ( Please also refer to the similar membar relaxed in G1ParScanThreadState::do_copy_to_survivor_space )
> 
> Per the specjbb2015 test results, it has about 8% improvement in critical.
> https://bugs.openjdk.java.net/secure/attachment/95445/jdk17_default_options.v2.png

A change of this kind requires careful analysis and supporting arguments that it is correct.  As mentioned in the bug comments, this same change was proposed and extensively discussed previously (JDK-8154736), and rejected.  What has changed that makes this even worth discussing again?  It is the responsibility of the author to make those arguments and provide the needed evidence.

Simply referring to a similar part of G1 is not sufficient evidence for the correctness of this change.  G1 and Parallel have significant differences; it cannot be assumed that something that works for one will work for the other.

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PR: https://git.openjdk.java.net/jdk/pull/4751