RFR: 8261029: Code heap page sizes not traced correctly using os::trace_page_sizes

[Stefan Johansson]

On Wed, 10 Feb 2021 06:45:51 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

> Are you currently working on this? If yes, what are your plans? I have a half finished prototype where I changed os::reserve_xxx() to return meta information about the reservation alongside the pointer, one of them the reserved page size.
> 

Yes, I also have a prototype that I've been playing around with. It currently uses an out-parameter to return the page size from `os::reserve_memory_special*` calls. The page sizes is then saved in the ReservedSpace for later use. 

> Have you guys decided whether its okay to remove the "multiple page sizes per reservation" feature?
> 

I've done some investigations but nothing have been decided. For now my prototype will return the smallest page size used by a mapping. Going forward I would like to do this, but it feels more urgent to get the other things in place first. 

> There is also another possible simplification I was thinking about, which is to remove the "UseSHM" feature from Linux. I honestly do not know why we still need it. That would simplify rework of large page handling on Linux a lot. I did ask around in December: https://mail.openjdk.java.net/pipermail/hotspot-dev/2020-December/046885.html , but did not get many answers.
> 

I know and I've been digging around in this area and agree even more now that it would be great to get rid of `UseSHM`. Not sure how if everybody agrees though and I'm currently working on small fix for `UseSHM` so that at least we don't leave it enabled everytime someone sets `+UseLargePages` without having any explicit large pages enabled ([PR#2488](https://github.com/openjdk/jdk/pull/2488)).

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PR: https://git.openjdk.java.net/jdk/pull/2481