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

[Stefan Johansson]

On Wed, 11 Nov 2020 01:48:46 GMT, Marcus G K Williams <github.com+168222+mgkwill at openjdk.org> wrote:

> When using LargePageSizeInBytes=1G, os::Linux::reserve_memory_special_huge_tlbfs* cannot select large pages smaller than 1G. Code heap usually uses less than 1G, so currently the code precludes code heap from using
> Large pages in this circumstance and when os::Linux::reserve_memory_special_huge_tlbfs* is called page sizes fall back to Linux::page_size() (usually 4k).
> 
> This change allows the above use case by populating all large_page_sizes present in /sys/kernel/mm/hugepages in _page_sizes upon calling os::Linux::setup_large_page_size().
> 
> In os::Linux::reserve_memory_special_huge_tlbfs* we then select the largest large page size available in _page_sizes that is smaller than bytes being reserved.

@mgkwill, I've been doing some measurements trying to see what kind of improvements to expect from backing the code-cache with 2m pages and the heap with 1g pages. Can you share what benchmarks you've used when analyzing the performance of this change and also what kind of setups you've used (heap-size, code-cache size, etc).

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