On Thu, Feb 1, 2024 at 7:34 AM Florian Weimer <fweimer@xxxxxxxxxx> wrote:
>
> * Yang Shi:
>
> > On Tue, Jan 30, 2024 at 11:53 PM Florian Weimer <fweimer@xxxxxxxxxx> wrote:
> >>
> >> * Yang Shi:
> >>
> >> > From: Yang Shi <yang@xxxxxxxxxxxxxxxxxxxxxx>
> >> >
> >> > The commit efa7df3e3bb5 ("mm: align larger anonymous mappings on THP
> >> > boundaries") incured regression for stress-ng pthread benchmark [1].
> >> > It is because THP get allocated to pthread's stack area much more possible
> >> > than before. Pthread's stack area is allocated by mmap without VM_GROWSDOWN
> >> > or VM_GROWSUP flag, so kernel can't tell whether it is a stack area or not.
> >> >
> >> > The MAP_STACK flag is used to mark the stack area, but it is a no-op on
> >> > Linux. Mapping MAP_STACK to VM_NOHUGEPAGE to prevent from allocating
> >> > THP for such stack area.
> >>
> >> Doesn't this introduce a regression in the other direction, where
> >> workloads expect to use a hugepage TLB entry for the stack?
> >
> > Maybe, it is theoretically possible. But AFAICT, the real life
> > workloads performance usually gets hurt if THP is used for stack.
> > Willy has an example:
> >
> > https://lore.kernel.org/linux-mm/ZYPDwCcAjX+r+g6s@xxxxxxxxxxxxxxxxxxxx/#t
> >
> > And avoiding THP on stack is not new, VM_GROWSDOWN | VM_GROWSUP areas
> > have been applied before, this patch just extends this to MAP_STACK.
>
> If it's *always* beneficial then we should help it along in glibc as
> well. We've started to offer a tunable in response to this observation
> (also paper over in OpenJDK):
>
> Make thread stacks not use huge pages
> <https://bugs.openjdk.org/browse/JDK-8303215>
>
> But this is specifically about RSS usage, and not directly about
> reducing TLB misses etc.
Thanks for the data point. Out of curiosity, what mmap flags are used
by JVM to indicate a stack? MAP_STACK? If so it should get
VM_NOHUGEPAGE due to this patch (of course, on older kernel
MADV_NOHUGEPAGE must be called by JVM).
Letting others, for example, glibc, call MADV_NOHUGEPAGE explicitly on
stack area is fine too, but it may take some time to get there...
>
> Thanks,
> Florian
>
