Re: [PATCH] x86: Don't set Prefer_No_AVX512 for processors with AVX512 and AVX-VNNI

[Sunil Pandey <skpgkp2@gmail.com>]

On Tue, Dec 7, 2021 at 11:33 AM Noah Goldstein via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
> On Tue, Dec 7, 2021 at 9:53 AM H.J. Lu via Libc-alpha
> <libc-alpha@sourceware.org> wrote:
> >
> > On Tue, Dec 7, 2021 at 7:48 AM Florian Weimer <fweimer@redhat.com> wrote:
> > >
> > > * H. J. Lu via Libc-alpha:
> > >
> > > > On Tue, Dec 7, 2021 at 6:05 AM Florian Weimer <fweimer@redhat.com> wrote:
> > > >>
> > > >> * H. J. Lu via Libc-alpha:
> > > >>
> > > >> > Hongtao, Hongyu,  can you find a Rocket Lake to test?
> > > >>
> > > >> I've found a lab machine with an i7-11700 CPU.  Is there something I
> > > >> could test for you?
> > > >
> > > > You can enable AVX512 in glibc with:
> > > >
> > > > $ export GLIBC_TUNABLES=glibc.cpu.hwcaps=-Prefer_No_AVX512
> > > >
> > > > While bootstrapping GCC with -j8, track CPU frequency with turbostat.  If
> > > > there is no CPU frequency drop and build time is less comparing against
> > > > without GLIBC_TUNABLES, we can enable AVX512.
> > > >
> > > >> (This could be non-production silicon, though.)
> > > >>
> > > >
> > > > The frequency behavior of non-production silicon can be different.
> > >
> > > With that caveat, it seems that frequencies drop further with
> > > GLIBC_TUNABLES set as above, and the build is also a little bit slower
> > > (5m31s vs 5m23s, the AVX-512 build was run first, and the systems was a
> > > little bit warmer for the second run).
> > >
> > > Would it make sense to run more extensive tests, or should we wait for
> > > someone with production silicon to show up?
> >
> > GCC is a heavy user of memcpy/memset, which is a good proxy of
> > ZMM load/store impact on CPU frequency.   We need to run the same
> > test on a production Rocket Lake.
>
> I would think a microbenchmark would be better for determining if
> rocketlake actually has throttling.
>
> Testing the full j8 GCC build will add a bunch of frequency "noise"
> due to thermal throttling.
>
> >
> > --
> > H.J.

I would like to backport this patch to release branches.
Any comments or objections?

--Sunil