RFR: 8271579: G1: Move copy before CAS in do_copy_to_survivor_space [v3]

[Thomas Schatzl]

On Fri, 6 Aug 2021 02:15:55 GMT, Hamlin Li <mli at openjdk.org> wrote:

>> Propose to move copy before CAS in do_copy_to_survivor_space, as we found this will improve G1 performance. Specjbb shows 3.7% in critical on aarch64, no change in max.
>> 
>> After this change copy_to_survivor in G1 is also aligned with PS's copy_to_survivor.
>
> Hamlin Li has refreshed the contents of this pull request, and previous commits have been removed. The incremental views will show differences compared to the previous content of the PR.

Sorry for the wait, but actually my testing just finished today - I initially had a few issues (needing to redo a few times, and due to the results re-testing to get better results), but I could not see any statistically significant difference in either maxjops or criticaljops (or any other benchmark I have tested) with Oracle cloud instances with Ampere Altra (dual-socket) machines at this time. Also tried just running on a single socket.

There is no discernible difference on x64 either.

I will discuss this with others how to proceed today; personally it may be worth aligning the code with Parallel GC anyway.

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PR: https://git.openjdk.java.net/jdk/pull/4983