RFR: 8254699: Suboptimal PreTouchParallelChunkSize defaults and limits [v2]

[Amit Pawar]

On Wed, 2 Dec 2020 10:04:32 GMT, Stefan Johansson <sjohanss at openjdk.org> wrote:

>> Thanks Thomas and Stefan for reviewing and approving the changes. Will wait until csr approval.
>
> I did some performance runs and found that on Windows this change will not speed up pre-touching. I see some quite big regressions in some cases. So I don't think we can do this change for all platforms without doing more benchmarking. 
> 
> But since it looks good on Linux, one solution would be to make `PreTouchParallelChunkSize` a platform-dependent flag and set it to 4M for Linux and keep it at 1G for the others until we can do more investigations. 
> 
> For guidance on how to make it a platform-dependent flag you can look at how `UseLargePages` is handled.

I was doubtful about the improvement regarding other platforms and thanks for testing and verifying. I will make it platform-specific as per your suggestion. 

On other platform, this improvement is not seen for smaller or lesser memory range also right ? similar too SPECJbb_Summary sheet in Excel file.

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PR: https://git.openjdk.java.net/jdk/pull/1503