Re: [PATCH v1] x86: Use 64MB as nt-store threshold if no cacheinfo [BZ #30429]

[Noah Goldstein <goldstein.w.n@gmail.com>]

On Tue, May 9, 2023 at 3:56 AM Florian Weimer <fweimer@redhat.com> wrote:
>
> * Noah Goldstein via Libc-alpha:
>
> > 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.
>
> I think that's quite surprising for GLIBC_TUNABLES.  Maybe that logic
> should only activate if the default was set from cache sizes?
>
I don't quite understand what you mean by "only active if the default
was set from cache sizes"?

This logic only triggers if total_cachesize / 8 < ~16kb. I think this should
only ever really happen if we failed to read cache info. What is the
surprise?

> Thanks,
> Florian
>