From: "H.J. Lu" <hjl.tools@gmail.com>
To: Noah Goldstein <goldstein.w.n@gmail.com>
Cc: libc-alpha@sourceware.org, carlos@systemhalted.org
Subject: Re: [PATCH v1] x86: Use 64MB as nt-store threshold if no cacheinfo [BZ #30429]
Date: Mon, 5 Jun 2023 10:45:46 -0700 [thread overview]
Message-ID: <CAMe9rOprKrNHxZzFiSQbAmTBZm=tFb53XJZAhj5w4Pz5ms2cpg@mail.gmail.com> (raw)
In-Reply-To: <CAFUsyf+8T=7qa_Tte1YKznKRfCL5bb8XmB2+F+U3gUf-b3JcRw@mail.gmail.com>
On Mon, Jun 5, 2023 at 10:15 AM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
>
> On Mon, May 8, 2023 at 10:10 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
> >
> > If `non_temporal_threshold` is below `minimum_non_temporal_threshold`,
> > it almost certainly means we failed to read the systems cache info.
> >
> > In this case, rather than defaulting the minimum correct value, we
> > should default to a value that gets at least reasonable
> > performance. 64MB is chosen conservatively to be at the very high
> > end. This should never cause non-temporal stores when, if we had read
> > cache info, we wouldn't have otherwise.
> > ---
> > sysdeps/x86/dl-cacheinfo.h | 10 +++++++++-
> > 1 file changed, 9 insertions(+), 1 deletion(-)
> >
> > diff --git a/sysdeps/x86/dl-cacheinfo.h b/sysdeps/x86/dl-cacheinfo.h
> > index 864b00a521..6225c852f6 100644
> > --- a/sysdeps/x86/dl-cacheinfo.h
> > +++ b/sysdeps/x86/dl-cacheinfo.h
> > @@ -771,8 +771,16 @@ dl_init_cacheinfo (struct cpu_features *cpu_features)
> > reflected in the manual. */
> > unsigned long int maximum_non_temporal_threshold = SIZE_MAX >> 4;
> > unsigned long int minimum_non_temporal_threshold = 0x4040;
> > +
> > + /* If `non_temporal_threshold` less than `minimum_non_temporal_threshold`
> > + it most likely means we failed to detect the cache info. We don't want
> > + to default to `minimum_non_temporal_threshold` as such a small value,
> > + while correct, has bad performance. We default to 64MB as reasonable
> > + default bound. 64MB is likely conservative in that most/all systems would
> > + choose a lower value so it should never forcing non-temporal stores when
> > + they otherwise wouldn't be used. */
> > if (non_temporal_threshold < minimum_non_temporal_threshold)
> > - non_temporal_threshold = minimum_non_temporal_threshold;
> > + non_temporal_threshold = 64 * 1024 * 1024;
> > else if (non_temporal_threshold > maximum_non_temporal_threshold)
> > non_temporal_threshold = maximum_non_temporal_threshold;
> >
> > --
> > 2.34.1
> >
>
> I want to backport down to 2.28.
> Thoughts?
Who will use such backport?
--
H.J.
next prev parent reply other threads:[~2023-06-05 17:46 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-05-09 3:10 Noah Goldstein
2023-05-09 8:56 ` Florian Weimer
2023-05-09 16:01 ` Noah Goldstein
2023-05-11 18:15 ` Noah Goldstein
2023-05-25 8:11 ` Florian Weimer
2023-05-25 18:36 ` H.J. Lu
2023-06-05 17:14 ` Noah Goldstein
2023-06-05 17:45 ` H.J. Lu [this message]
2023-06-05 20:28 ` Noah Goldstein
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