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

[Thomas Stuefe]

On Fri, 15 Jan 2021 20:13:42 GMT, Stefan Johansson <sjohanss at openjdk.org> wrote:

>> Marcus G K Williams has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 23 commits:
>> 
>>  - Merge branch 'master' into update_hlp
>>  - Merge branch 'master' into update_hlp
>>  - Remove extraneous ' from warning
>>    
>>    Signed-off-by: Marcus G K Williams <marcus.williams at intel.com>
>>  - Merge branch 'master' into update_hlp
>>  - Merge branch 'master' into update_hlp
>>  - Merge branch 'master' into update_hlp
>>  - Fix os::large_page_size() in last update
>>    
>>    Signed-off-by: Marcus G K Williams <marcus.williams at intel.com>
>>  - Ivan W. Requested Changes
>>    
>>    Removed os::Linux::select_large_page_size and
>>    use os::page_size_for_region instead
>>    
>>    Removed Linux::find_large_page_size and use
>>    register_large_page_sizes. Streamlined
>>    Linux::setup_large_page_size
>>    
>>    Signed-off-by: Marcus G K Williams <marcus.williams at intel.com>
>>  - Fix space format, use Linux:: for local func.
>>    
>>    Signed-off-by: Marcus G K Williams <marcus.williams at intel.com>
>>  - Merge branch 'update_hlp' of github.com:mgkwill/jdk into update_hlp
>>  - ... and 13 more: https://git.openjdk.java.net/jdk/compare/da2415fe...d73e7a4c
>
> Did some more testing with the code. I'm using Parallel for testing becuase G1 does a better job aligning sizes and avoiding some problems.
> 
> I found that this change has a problem with mapping using both small and large pages (`reserve_memory_special_huge_tlbfs_mixed()`). I'm currently investigating if we can remove these type of mixed-mappings, and instead make sure we only use large pages when properly aligned, so in the future we might be able get rid of some code in this area. For know see my comments below.

Since we are not shipping this with JDK16, I'm more relaxed now. This will have time to cook before JDK17 is shipped, which takes care of my third point (doing more tests).

About the jtreg test. I originally wrote:

>> one jtreg test to test that the VM comes up with -XX:+UseLargePages -XX:LargePageSizeInBytes=1G and allocates small-large-pages as expected. This is not only needed as a function proof but to prevent regressions when we reform the code (which will happen)

Not sure if that was too vague. An easy way would be to add some tracing to the VM in the allocation path, eg with `log_info(os)(...)`, then in the test start a VM with `-XX:+UseLargePages -XX:LargePageSizeInBytes=1G -Xlog=os`, and scan its output. There are many tests which do this, for an easy example see e.g. runtime/os/TestUseCpuAllocPath.java.

I'll take a closer look next week but will wait until Stefan had his go.

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