public inbox for glibc-cvs@sourceware.org help / color / mirror / Atom feed
From: Noah Goldstein <nwg@sourceware.org> To: glibc-cvs@sourceware.org Subject: [glibc/release/2.29/master] x86: Use `3/4*sizeof(per-thread-L3)` as low bound for NT threshold. Date: Tue, 12 Sep 2023 03:48:53 +0000 (GMT) [thread overview] Message-ID: <20230912034853.C1C0F3858C2D@sourceware.org> (raw) https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=83ce856e77fc50e213195a3f59aefb1e7dde6d71 commit 83ce856e77fc50e213195a3f59aefb1e7dde6d71 Author: Noah Goldstein <goldstein.w.n@gmail.com> Date: Fri Aug 11 18:47:17 2023 -0500 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) Diff: --- sysdeps/x86/cacheinfo.c | 11 ++++++++++- 1 file changed, 10 insertions(+), 1 deletion(-) diff --git a/sysdeps/x86/cacheinfo.c b/sysdeps/x86/cacheinfo.c index 62986a7327..0080e8dad5 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
reply other threads:[~2023-09-12 3:48 UTC|newest] Thread overview: [no followups] expand[flat|nested] mbox.gz Atom feed
Reply instructions: You may reply publicly to this message via plain-text email using any one of the following methods: * Save the following mbox file, import it into your mail client, and reply-to-all from there: mbox Avoid top-posting and favor interleaved quoting: https://en.wikipedia.org/wiki/Posting_style#Interleaved_style * Reply using the --to, --cc, and --in-reply-to switches of git-send-email(1): git send-email \ --in-reply-to=20230912034853.C1C0F3858C2D@sourceware.org \ --to=nwg@sourceware.org \ --cc=glibc-cvs@sourceware.org \ /path/to/YOUR_REPLY https://kernel.org/pub/software/scm/git/docs/git-send-email.html * If your mail client supports setting the In-Reply-To header via mailto: links, try the mailto: linkBe sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions for how to clone and mirror all data and code used for this inbox; as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).