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

[Stefan Johansson]

On Tue, 1 Dec 2020 16:51:20 GMT, Amit Pawar <github.com+71302734+amitdpawar at openjdk.org> wrote:

>> Lgtm.
>> 
>> We need to wait until the CSR has been approved. This typically happens on Thursdays.
>
> 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.

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