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

[Hamlin-Li]

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

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Progress

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Issue

• JDK-8269870: PS: Membar in PSPromotionManager::copy_unmarked_to_survivor_space could be relaxed

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[mlbridge]

Webrevs

• 00: Full (744ffdcc)

[kimbarrett]

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.

[Hamlin-Li]

I thought it's because of some log message in PS which needs to refer to forwardee's content, and the log messages is now gone.
But let me do a more comprehensive investigation, will update later.

[kimbarrett]

I thought it's because of some log message in PS which needs to refer to forwardee's content, and the log messages is now gone.

Those were the immediately obvious reasons why it didn't work. But there was also discussion about the problems that could arise because the forwardee was being returned to various places that might also have problems. And even if they didn't at the time, there was concern over the fragility that would result, as those places using the "copied" object would need to always be careful about touching the contents of the object.

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