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

Back from the holidays and actually looking at our use of large pages from a other perspecive as well. I think the approach here has been simplified a lot from the first suggestion and I like it. Just a few small additional comments.

src/hotspot/os/linux/os_linux.cpp line 3750:

> 3748:     }
> 3749:   }
> 3750:   closedir(dir);

It would be nice to add some logging here using the `pagesize` tag. The new PageSizes class has a `print_on()` that we could use. I'm thinking something like:
  LogTarget(Info, pagesize) lt;
  if (lt.is_enabled()) {
    LogStream ls(lt);
    ls.print("Available page sizes: ");
    _page_sizes.print_on(&ls);
  }

src/hotspot/os/linux/os_linux.cpp line 4013:

> 4011:   assert(UseLargePages && UseHugeTLBFS, "only for Huge TLBFS large pages");
> 4012:   assert(is_aligned(bytes, large_page_size), "Unaligned size");
> 4013:   assert(is_aligned(req_addr, large_page_size), "Unaligned address");

Adding an assert here that `large_page_size` is larger than os::vm_page_size (small page size) to ensure we actually get a large page size from `page_size_for_region_aligned()`. Otherwise the passed in a size wasn't correctly aligned.

src/hotspot/os/linux/os_linux.cpp line 4047:

> 4045:   // that is smaller than size_t bytes
> 4046:   size_t large_page_size = os::page_size_for_region_aligned(bytes, 1);
> 4047: 

Adding the same assert as suggested above here.

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

Changes requested by sjohanss (Reviewer).

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