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author | Noah Goldstein <goldstein.w.n@gmail.com> | 2023-08-11 18:47:17 -0500 |
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committer | Noah Goldstein <goldstein.w.n@gmail.com> | 2023-09-11 22:48:44 -0500 |
commit | 68a2030415c9c6211d7fea1f48f4922dab132335 (patch) | |
tree | 3789d1c379085059f51e2362efc59805ccba2d05 /sysdeps | |
parent | 402324a71073ac36d334845a71a5f97350aebbc3 (diff) | |
download | glibc-68a2030415c9c6211d7fea1f48f4922dab132335.tar.gz glibc-68a2030415c9c6211d7fea1f48f4922dab132335.tar.xz glibc-68a2030415c9c6211d7fea1f48f4922dab132335.zip |
x86: Use `3/4*sizeof(per-thread-L3)` as low bound for NT threshold.
On some machines we end up with incomplete cache information. This can make the new calculation of `sizeof(total-L3)/custom-divisor` end up lower than intended (and lower than the prior value). So reintroduce the old bound as a lower bound to avoid potentially regressing code where we don't have complete information to make the decision. Reviewed-by: DJ Delorie <dj@redhat.com> (cherry picked from commit 8b9a0af8ca012217bf90d1dc0694f85b49ae09da)
Diffstat (limited to 'sysdeps')
-rw-r--r-- | sysdeps/x86/cacheinfo.c | 11 |
1 files changed, 10 insertions, 1 deletions
diff --git a/sysdeps/x86/cacheinfo.c b/sysdeps/x86/cacheinfo.c index 6baec4d7ed..f5d36573b3 100644 --- a/sysdeps/x86/cacheinfo.c +++ b/sysdeps/x86/cacheinfo.c @@ -797,12 +797,21 @@ init_cacheinfo (void) modern HW detects streaming patterns and provides proper LRU hints so that the maximum thrashing capped at 1/associativity. */ unsigned long int non_temporal_threshold = shared / 4; + + /* If the computed non_temporal_threshold <= 3/4 * per-thread L3, we most + likely have incorrect/incomplete cache info in which case, default to + 3/4 * per-thread L3 to avoid regressions. */ + unsigned long int non_temporal_threshold_lowbound + = shared_per_thread * 3 / 4; + if (non_temporal_threshold < non_temporal_threshold_lowbound) + non_temporal_threshold = non_temporal_threshold_lowbound; + /* If no ERMS, we use the per-thread L3 chunking. Normal cacheable stores run a higher risk of actually thrashing the cache as they don't have a HW LRU hint. As well, their performance in highly parallel situations is noticeably worse. */ if (!CPU_FEATURES_CPU_P (cpu_features, ERMS)) - non_temporal_threshold = shared_per_thread * 3 / 4; + non_temporal_threshold = non_temporal_threshold_lowbound; __x86_shared_non_temporal_threshold = (cpu_features->non_temporal_threshold != 0 |