RFR: 8244752: Enable Linux support for multiple huge page sizes -XX:LargePageSizeInBytes

[Ivan Walulya]

Thanks Thomas

Please find new webrev:

https://cr.openjdk.java.net/~iwalulya/8244752/01/



> On 12 May 2020, at 10:50, Thomas Schatzl <thomas.schatzl at oracle.com> wrote:
> 
> Hi,
> 
> On 11.05.20 18:46, Ivan Walulya wrote:
>> Hi all,
>> Please review this enhancement to enable selecting a desired huge page size on Linux systems that support multiple huge page sizes.
>> JBS: https://bugs.openjdk.java.net/browse/JDK-8244752
>> Webrev: http://cr.openjdk.java.net/~iwalulya/8244752/00/
>> Testing: Tier 1 - 3
> 
> CI does not have large page enabled machines at the moment. What local testing did you perform?

Local tests with both 1Gb and 2Mb (Linux 2.6.32 and 4.15.0), and also run specjbb2015 with largepages. Note: all done on x86. 

> 
>> //Ivan
> 
> - in the comment with the MAP_HUGE* flags, please add the information that you got this from the mmap(2) man page, and that this is since Linux 3.8. (I understand it's "obvious", but just to make it clear)

Noted

> 
> - what happens when run on < Linux 3.8 and the flags are used? (not sure Hotspot minimal requirements _are_ Linux 3.8+, at least for hugetlbfs support).
Flags would have no effect and with the new modifications, they wouldn’t be set if not supported

> 
> - os_linux.cpp:3846: "if(entry->d_type == DT_DIR" missing space after the "if". At the end of the condition there is an unusual extra space.

Noted

> 
> - in os::Linux::reserve_memory_special_huge_tlbfs_only, are you sure that on non-x64 (e.g. arm64) the only supported page sizes for hugetlbfs are 2M and 1GB?
> From what I understand, on e.g. arm64 this is only the case with 4k "small" page size. I do not know if that is different with e.g. 64k pages (as in: if you have 16k/64k pages, then the hugetlbfs page sizes change too).
> 
> Looking at https://wiki.debian.org/Hugepages, in the arm64 "Multiple huge page size support" section, it mentions:
> 
> "If one elects to build their own Debian arm64 kernel with CONFIG_ARM64_64K_PAGES=y, then only 512MB HugeTLB (and THP) pages are available. These are available at run time. "
> 
> So I recommend calculating the flag value based on the LargePageSizeInBytes value, not strictly using the constants (which then can be removed). Still there is some concern that on Linux < 3.8 this won't work (or fail).

True, this has been changed. 

> 
> Thanks,
>  Thomas

//Ivan