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, 15 Dec 2020 18:48:05 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.
>
> 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

src/hotspot/os/linux/os_linux.cpp line 3746:

> 3744:       if (page_size * K > (size_t)Linux::page_size()) {
> 3745:           // Add each found Large Page Size to _page_sizes
> 3746:           _page_sizes.add(page_size * K);

Just realized one more thing, with this code we will enable all page sizes configured even if there are no pages "allocated" for the given size. 

Is that what we want or should we read the file nr_hugepages in the given director and only add it if the size is > 0?

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

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