75 lines
3.3 KiB
Diff
75 lines
3.3 KiB
Diff
From af0606f5d626b92d6e59da3a797548e9daab5580 Mon Sep 17 00:00:00 2001
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From: Qingqing Li <liqingqing3@huawei.com>
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Date: Sat, 25 Jun 2022 15:36:44 +0800
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Subject: [PATCH] x86: use total l3cache for non_temporal_threshold
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Below glibc upstream patch modified the default behavoir for large size of memcpy,
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such as 1M~10M. revert it and use GLIBC_TUNABLES="glibc.cpu.x86_non_temporal_threshold=xxx"
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to tune the application when needed.
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d3c57027470b78dba79c6d931e4e409b1fecfc80
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Author: Patrick McGehearty <patrick.mcgehearty@oracle.com>
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Date: Mon Sep 28 20:11:28 2020 +0000
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Reversing calculation of __x86_shared_non_temporal_threshold
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The __x86_shared_non_temporal_threshold determines when memcpy on x86
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uses non_temporal stores to avoid pushing other data out of the last
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level cache.
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uses non_temporal stores to avoid pushing other data out of the last
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level cache.
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This patch proposes to revert the calculation change made by H.J. Lu's
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patch of June 2, 2017.
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H.J. Lu's patch selected a threshold suitable for a single thread
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getting maximum performance. It was tuned using the single threaded
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large memcpy micro benchmark on an 8 core processor. The last change
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changes the threshold from using 3/4 of one thread's share of the
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cache to using 3/4 of the entire cache of a multi-threaded system
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before switching to non-temporal stores. Multi-threaded systems with
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more than a few threads are server-class and typically have many
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active threads. If one thread consumes 3/4 of the available cache for
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all threads, it will cause other active threads to have data removed
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from the cache. Two examples show the range of the effect. John
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McCalpin's widely parallel Stream benchmark, which runs in parallel
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and fetches data sequentially, saw a 20% slowdown with this patch on
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an internal system test of 128 threads. This regression was discovered
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when comparing OL8 performance to OL7. An example that compares
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normal stores to non-temporal stores may be found at
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https://vgatherps.github.io/2018-09-02-nontemporal/. A simple test
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shows performance loss of 400 to 500% due to a failure to use
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nontemporal stores. These performance losses are most likely to occur
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when the system load is heaviest and good performance is critical.
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The tunable x86_non_temporal_threshold can be used to override the
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default for the knowledgable user who really wants maximum cache
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allocation to a single thread in a multi-threaded system.
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The manual entry for the tunable has been expanded to provide
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more information about its purpose.
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modified: sysdeps/x86/cacheinfo.c
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modified: manual/tunables.texi
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---
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sysdeps/x86/dl-cacheinfo.h | 4 ++++++
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1 file changed, 4 insertions(+)
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diff --git a/sysdeps/x86/dl-cacheinfo.h b/sysdeps/x86/dl-cacheinfo.h
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index e6c94dfd..c5e8deb3 100644
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--- a/sysdeps/x86/dl-cacheinfo.h
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+++ b/sysdeps/x86/dl-cacheinfo.h
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@@ -926,6 +926,10 @@ dl_init_cacheinfo (struct cpu_features *cpu_features)
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if (tunable_size != 0)
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shared = tunable_size;
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+ /* keep x86 to use the same non_temporal_threshold like glibc2.28 */
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+ if (threads != 0)
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+ non_temporal_threshold *= threads;
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+
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tunable_size = TUNABLE_GET (x86_non_temporal_threshold, long int, NULL);
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/* NB: Ignore the default value 0. */
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if (tunable_size != 0)
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--
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2.30.0
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