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| author | Noah Goldstein <goldstein.w.n@gmail.com> | 2023-07-18 10:27:59 -0500 |
|---|---|---|
| committer | Noah Goldstein <goldstein.w.n@gmail.com> | 2023-09-05 17:17:52 -0500 |
| commit | a07ab67a8815fd4b1b6029850b880f7880ec845f (patch) | |
| tree | 000ef6c9210982b90e1aac9b5720800c2c44fc0f | |
| parent | 521afc963799681dd2ff8309d7cd0cf6dc4bc0c3 (diff) | |
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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)
| -rw-r--r-- | sysdeps/x86/dl-cacheinfo.h | 11 |
1 files changed, 10 insertions, 1 deletions
diff --git a/sysdeps/x86/dl-cacheinfo.h b/sysdeps/x86/dl-cacheinfo.h index 3aad90b923..61791362dc 100644 --- a/sysdeps/x86/dl-cacheinfo.h +++ b/sysdeps/x86/dl-cacheinfo.h @@ -872,12 +872,21 @@ dl_init_cacheinfo (struct cpu_features *cpu_features) 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_FEATURE_USABLE_P (cpu_features, ERMS)) - non_temporal_threshold = shared_per_thread * 3 / 4; + non_temporal_threshold = non_temporal_threshold_lowbound; /* SIZE_MAX >> 4 because memmove-vec-unaligned-erms right-shifts the value of 'x86_non_temporal_threshold' by `LOG_4X_MEMCPY_THRESH` (4) and it is best if that operation cannot overflow. Minimum of 0x4040 (16448) because the |