RFR: 8319969: os::large_page_init() turns off THPs for ZGC [v2]

[Stefan Karlsson]

On Sat, 2 Dec 2023 06:51:07 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

> Would it be not clearer to define when to warn, as we do in warn_no_large_pages?

I don't understand what you are suggesting with this question / request, so I'm not sure exactly what you are looking for. Instead, I made my own version of the pseudo code you posted.

This is the warnings I get with that change:

Without ZGC:

$ thp never never
always madvise [never]
always within_size advise [never] deny force

$ java -XX:+UseTransparentHugePages -version
[0.002s][warning][pagesize] Anonymous transparent huge pages are not enabled in the OS. Set /sys/kernel/mm/transparent_hugepage/enabled to 'madvise' to enable them.
[0.002s][warning][pagesize] UseTransparentHugePages disabled, transparent huge pages are not supported by the operating system.
...

$ thp never advise
always madvise [never]
always within_size [advise] never deny force

java -XX:+UseTransparentHugePages -version
[0.002s][warning][pagesize] Anonymous transparent huge pages are not enabled in the OS. Set /sys/kernel/mm/transparent_hugepage/enabled to 'madvise' to enable them.
[0.002s][warning][pagesize] UseTransparentHugePages disabled, transparent huge pages are not supported by the operating system.
...

$ thp madvise never
always [madvise] never
always within_size advise [never] deny force

$ java -XX:+UseTransparentHugePages -version
...

$ thp madvise advise
always [madvise] never
always within_size [advise] never deny force
$ java -XX:+UseTransparentHugePages -version
...

With ZGC:

$ thp never never
always madvise [never]
always within_size advise [never] deny force

$ java -XX:+UseTransparentHugePages -XX:+UseZGC -version
[0.002s][warning][pagesize] Shared memory transparent huge pages are not enabled in the OS. Set /sys/kernel/mm/transparent_hugepage/shmem_enabled to 'advise' to enable them.
[0.002s][warning][pagesize] Anonymous transparent huge pages are not enabled in the OS. Set /sys/kernel/mm/transparent_hugepage/enabled to 'madvise' to enable them.
[0.002s][warning][pagesize] UseTransparentHugePages disabled, transparent huge pages are not supported by the operating system.
...

$ thp never advise
always madvise [never]
always within_size [advise] never deny force

$ java -XX:+UseTransparentHugePages -XX:+UseZGC -version
[0.001s][warning][pagesize] Anonymous transparent huge pages are not enabled in the OS. Set /sys/kernel/mm/transparent_hugepage/enabled to 'madvise' to enable them.
[0.001s][warning][pagesize] UseTransparentHugePages disabled, transparent huge pages are not supported by the operating system.
...

$ thp madvise never
always [madvise] never
always within_size advise [never] deny force

$ java -XX:+UseTransparentHugePages -XX:+UseZGC -version
[0.002s][warning][pagesize] Shared memory transparent huge pages are not enabled in the OS. Set /sys/kernel/mm/transparent_hugepage/shmem_enabled to 'advise' to enable them.
...

$ thp madvise advise
always [madvise] never
always within_size [advise] never deny force

$ java -XX:+UseTransparentHugePages -XX:+UseZGC -version
...


Please take a look and see if this is an OK solution.

> src/hotspot/os/linux/os_linux.cpp line 3736:
> 
>> 3734:         ls.print_cr(". Default large page size: " EXACTFMT ".", EXACTFMTARGS(os::large_page_size()));
>> 3735:       } else {
>> 3736:         ls.print("Large page support %sdisabled.", uses_zgc_shmem_thp() ? "partially " : "");
> 
> I wonder whether we could make our life simpler by not supporting mixes: we could require that for ZGC, to use THP, both shmen and anon thps have to be active. Would that be acceptable or do you think there are too many misconfigured systems out there?

I would prefer to not force users to set both.

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

PR Review Comment: https://git.openjdk.org/jdk/pull/16690#discussion_r1413588574
PR Review Comment: https://git.openjdk.org/jdk/pull/16690#discussion_r1413591291