Re: [PATCH v2] Reversing calculation of __x86_shared_non_temporal_threshold

["H.J. Lu" <hjl.tools@gmail.com>]

On Wed, Sep 23, 2020 at 1:10 PM 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 | 12 +++++++-----
>  2 files changed, 12 insertions(+), 6 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..c6767d9 100644
> --- a/sysdeps/x86/cacheinfo.c
> +++ b/sysdeps/x86/cacheinfo.c
> @@ -778,14 +778,16 @@ intel_bug_no_cache_info:
>        __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. While higher
> +     single thread performance may be observed with a higher
> +     threshold, having a single thread use more than it's share
> +     of the cache will negatively impact 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);
>  }
>

Can we tune it with the number of threads and/or total cache
size?

-- 
H.J.