RFR: 8271195: Use largest available large page size smaller than LargePageSizeInBytes when available [v2]

[Albert Mingkun Yang]

On Fri, 4 Feb 2022 11:35:55 GMT, Swati Sharma <duke at openjdk.java.net> wrote:

>> Hi Team,
>> 
>> In this patch I have fixed two issues related to large pages, following is the summary of changes :-
>> 
>> 1. Patch fixes existing large page allocation functionality where if a commit over 1GB pages fails allocation should happen over next small page size i.e. 2M where as currently its happening over 4kb pages resulting into significant TLB miss penalty.
>> Patch includes new JTREG Test case covering various scenarios for checking the correct explicit page allocation according ​to the 1G, 2M, 4K priority.
>> 2. While attempting commit over larger pages we first try to reserve requested bytes over the virtual address space, in case commit to large page fails we should be un reserving entire reservation to avoid leaving any leaks in virtual address space.
>> 
>> ​
>> Please find below the performance data with and without patch for the JMH benchmark included with the patch.
>> 
>> ![image](https://user-images.githubusercontent.com/96874289/152189587-4822a4ca-f5e2-4621-b405-0da941485143.png)
>> 
>> 
>> Please review and provide your valuable comments.
>> 
>> 
>> 
>> Thanks,
>> Swati Sharma
>> Runtime Software Development Engineer 
>> Intel
>
> Swati Sharma has updated the pull request incrementally with one additional commit since the last revision:
> 
>   8271195: Review comments resolved.

Thank you for the revision; some further comments/suggestions.

src/hotspot/os/linux/os_linux.cpp line 3497:

> 3495: bool os::Linux::hugetlbfs_sanity_check(bool warn, size_t page_size) {
> 3496:    bool large_page_found = false;
> 3497:   // Include the page size flag to ensure we sanity check the correct page size.

This comment should be moved.

src/hotspot/os/linux/os_linux.cpp line 3501:

> 3499:     if (os::Linux::try_commit_using_large_page(local_page_size) && !large_page_found) {
> 3500:       _large_page_size = local_page_size;
> 3501:       large_page_found = true;

I think one can use early-return here, which removes `large_page_found` and the `if (warn)` below is skipped automatically.

-------------

Changes requested by ayang (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/7326