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

[Florian Weimer <fweimer@redhat.com>]

* Noah Goldstein:

> 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?

Right, my mistake.  The patch should work as-is.

Reviewed-by: Florian Weimer <fweimer@redhat.com>

Thanks,
Florian