RFR: JDK-8256155: os::Linux Populate all large_page_sizes, select smallest page size in reserve_memory_special_huge_tlbfs* [v15]

[Stefan Johansson]

On Tue, 19 Jan 2021 10:35:47 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>> Yes, we've done some experiments using `madvise`. Some results looked promising and others a bit surprising, but I didn't actually looked at how it would affect `HUGETLB` large pages. But yes, it might be a way to get better behavior for large pages.
>
> Maybe we also could take another look at the "never-remap-hugepages" rule added with JDK-8007074. I understand why stefank did that, but maybe if one added safety measures (eg before remapping making sure that we have enough huge pages in the pool with a large margin) and combined with a switch it would be safe enough.

Yes, and maybe take another look at if +UseLargePages could mean use THP, currently THP is only used if explicitly set and I'm not sure that is true with newer Linux kernels. We even have a comment about this: 
    // Don't try UseTransparentHugePages since there are known
    // performance issues with it turned on. This might change in the future.
    UseTransparentHugePages = false;

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PR: https://git.openjdk.java.net/jdk/pull/1153