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

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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.
Reviewed-by: Florian Weimer <fweimer@redhat.com>
This commit is contained in:
Noah Goldstein 2023-05-08 22:10:20 -05:00
parent 9ffdcf5b79
commit ed2f9dc942
1 changed files with 9 additions and 1 deletions
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10
sysdeps/x86/dl-cacheinfo.h
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@ -749,8 +749,16 @@ 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;