RFR: 8257145: Performance regression with -XX:-ResizePLAB after JDK-8079555

[Thomas Schatzl]

On Fri, 27 Nov 2020 11:14:29 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

>> Just one note, if we run with `UseDynamicNumberOfGCThreads` the PLAB size might still be "resized" between GCs if they use different number of threads. This is still an improvement since the old value is wrong and will give a way to small PLAB size. But we might want to look at a solution that will always return a fixed PLAB size with `-ResizePLAB`.
>
> I agree and noted that in the CR already. As you mention, it is still an improvement, and with `-XX:+ResizePLAB` G1 will automatically resize the PLABs back to more reasonable if needed (as G1 previously used a way too small PLAB initially).
> 
> Giving that typical number or threads correlate somewhat with heap size, and the policy for `-XX:+UseDynamicNumberOfThreads` is fairly conservative anyway, this change will "always" return the (correct) fixed PLAB size with `-XX:-ResizePLAB` now.

I.e. if you want (and think it is a good idea), we can do both changes, first the one suggested originally, and this one so that we do not need to review + test twice.

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PR: https://git.openjdk.java.net/jdk/pull/1474