Re: [PATCH v3] Reversing calculation of __x86_shared_non_temporal_threshold

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

On Wed, Apr 19, 2023 at 3:30 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
>
> On Wed, Apr 19, 2023 at 5:26 PM H.J. Lu <hjl.tools@gmail.com> wrote:
> >
> > ---------- Forwarded message ---------
> > From: Patrick McGehearty via Libc-alpha <libc-alpha@sourceware.org>
> > Date: Fri, Sep 25, 2020 at 3:21 PM
> > Subject: [PATCH v3] Reversing calculation of __x86_shared_non_temporal_threshold
> > To: <libc-alpha@sourceware.org>
> >
> >
> > 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 | 16 +++++++++++-----
> >  2 files changed, 16 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..42b468d 100644
> > --- a/sysdeps/x86/cacheinfo.c
> > +++ b/sysdeps/x86/cacheinfo.c
> > @@ -778,14 +778,20 @@ 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. 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);
> >  }
> >
> >  #endif
> > --
> > 1.8.3.1
> >
> >
> >
> > --
> > H.J.
>
>
> I am looking into re-tuning the NT store threshold which appears to be
> too low in many cases.
>
> I've played around with some micro-benchmarks:
> https://github.com/goldsteinn/memcpy-nt-benchmarks
>
> I am finding that for the most part, ERMS stays competitive with
> NT-Stores even as core count increases with heavy read workloads going
> on on other threads.
> See: https://github.com/goldsteinn/memcpy-nt-benchmarks/blob/master/results-skx-pdf/skx-memcpy-4--read.pdf
>
> I saw: https://vgatherps.github.io/2018-09-02-nontemporal/ although
> it's not clear how to reproduce the results in the blog. I also see it
> was only comparing vs standard temporal stores, not ERMS.
>
> Does anyone know of benchmarks or an application that can highlight
> the L3 clobbering issues brought up in this patch?

You can try this:

https://github.com/jeffhammond/STREAM

-- 
H.J.