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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
uses non_temporal stores to avoid pushing other data out of the last
level cache.

This patch proposes to revert the calculation change made by H.J. Lu's
patch of June 2, 2017.

H.J. Lu's patch selected a threshold suitable for a single thread
getting maximum performance. It was tuned using the single threaded
large memcpy micro benchmark on an 8 core processor. The last change
changes the threshold from using 3/4 of one thread's share of the
cache to using 3/4 of the entire cache of a multi-threaded system
before switching to non-temporal stores. Multi-threaded systems with
more than a few threads are server-class and typically have many
active threads. If one thread consumes 3/4 of the available cache for
all threads, it will cause other active threads to have data removed
from the cache. Two examples show the range of the effect. John
McCalpin's widely parallel Stream benchmark, which runs in parallel
and fetches data sequentially, saw a 20% slowdown with this patch on
an internal system test of 128 threads. This regression was discovered
when comparing OL8 performance to OL7.  An example that compares
normal stores to non-temporal stores may be found at
https://vgatherps.github.io/2018-09-02-nontemporal/.  A simple test
shows performance loss of 400 to 500% due to a failure to use
nontemporal stores. These performance losses are most likely to occur
when the system load is heaviest and good performance is critical.

The tunable x86_non_temporal_threshold can be used to override the
default for the knowledgable user who really wants maximum cache
allocation to a single thread in a multi-threaded system.
The manual entry for the tunable has been expanded to provide
more information about its purpose.

	modified: sysdeps/x86/cacheinfo.c
	modified: manual/tunables.texi
This commit is contained in:
Patrick McGehearty
2020-09-28 20:11:28 +00:00
parent b16f282cb0
commit d3c5702747
2 changed files with 16 additions and 6 deletions
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16
sysdeps/x86/cacheinfo.c
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@ -854,14 +854,20 @@ init_cacheinfo (void)
__x86_shared_cache_size = shared;
}
/* The large memcpy micro benchmark in glibc shows that 6 times of
shared cache size is the approximate value above which non-temporal
store becomes faster on a 8-core processor. This is the 3/4 of the
total shared cache size. */
/* The default setting for the non_temporal threshold is 3/4 of one
thread's share of the chip's cache. For most Intel and AMD processors
with an initial release date between 2017 and 2020, a thread's typical
share of the cache is from 500 KBytes to 2 MBytes. Using the 3/4
threshold leaves 125 KBytes to 500 KBytes of the thread's data
in cache after a maximum temporal copy, which will maintain
in cache a reasonable portion of the thread's stack and other
active data. If the threshold is set higher than one thread's
share of the cache, it has a substantial risk of negatively
impacting the performance of other threads running on the chip. */
__x86_shared_non_temporal_threshold
= (cpu_features->non_temporal_threshold != 0
? cpu_features->non_temporal_threshold
: __x86_shared_cache_size * threads * 3 / 4);
: __x86_shared_cache_size * 3 / 4);
/* NB: The REP MOVSB threshold must be greater than VEC_SIZE * 8. */
unsigned int minimum_rep_movsb_threshold;