[PATCH] Reversing calculation of __x86_shared_non_temporal_threshold
H.J. Lu
hjl.tools@gmail.com
Wed Sep 23 17:19:06 GMT 2020
On Wed, Sep 23, 2020 at 8:14 AM Patrick McGehearty via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
> 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
> ---
> manual/tunables.texi | 6 +++++-
> sysdeps/x86/cacheinfo.c | 2 +-
> 2 files changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/manual/tunables.texi b/manual/tunables.texi
> index b6bb54d..94d4fbd 100644
> --- a/manual/tunables.texi
> +++ b/manual/tunables.texi
> @@ -364,7 +364,11 @@ set shared cache size in bytes for use in memory and string routines.
>
> @deftp Tunable glibc.tune.x86_non_temporal_threshold
> The @code{glibc.tune.x86_non_temporal_threshold} tunable allows the user
> -to set threshold in bytes for non temporal store.
> +to set threshold in bytes for non temporal store. Non temporal stores
> +give a hint to the hardware to move data directly to memory without
> +displacing other data from the cache. This tunable is used by some
> +platforms to determine when to use non temporal stores in operations
> +like memmove and memcpy.
>
> This tunable is specific to i386 and x86-64.
> @end deftp
> diff --git a/sysdeps/x86/cacheinfo.c b/sysdeps/x86/cacheinfo.c
> index b9444dd..5c5192a 100644
> --- a/sysdeps/x86/cacheinfo.c
> +++ b/sysdeps/x86/cacheinfo.c
> @@ -785,7 +785,7 @@ intel_bug_no_cache_info:
> __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);
> }
>
You need to update the comment for __x86_shared_non_temporal_threshold.
--
H.J.
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