[PATCH v3] Reversing calculation of __x86_shared_non_temporal_threshold
H.J. Lu
hjl.tools@gmail.com
Thu Apr 20 16:17:41 GMT 2023
On Wed, Apr 19, 2023 at 5:27 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
>
> On Wed, Apr 19, 2023 at 7:12 PM H.J. Lu <hjl.tools@gmail.com> wrote:
> >
> > On Wed, Apr 19, 2023 at 4:24 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
> > >
> > > On Wed, Apr 19, 2023 at 5:43 PM H.J. Lu <hjl.tools@gmail.com> wrote:
> > > >
> > > > 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
> > >
> > > That's the same as a normal memcpy benchmark no? Its just calling
> > > something like `tuned_STREAM_Copy()` (memcpy) in a loop maybe
> > > scattered with some other reads. Similar to what I was running to get:
> > > https://github.com/goldsteinn/memcpy-nt-benchmarks/blob/master/results-skx-pdf/skx-memcpy-4--read.pdf
> >
> > tuned_STREAM_Copy doesn't use OPENMP pragma:
> >
> > #ifdef TUNED
> > tuned_STREAM_Copy();
> > #else
> > #pragma omp parallel for
> > for (j=0; j<STREAM_ARRAY_SIZE; j++)
> > c[j] = a[j];
> > #endif
> >
> > It is single-threaded.
> >
> ```
> #define do_copy //do_copy_erms / do_copy_nt
> void tuned_STREAM_Copy()
> {
> ssize_t j;
> #pragma omp parallel for
> for (j=0; j<NTHREADS; j++)
> do_copy(c + j * THREAD_CHUNK_SIZE, a + j *
> THREAD_CHUNK_SIZE, THREAD_CHUNK_SIZE);
> }
> ```
>
> > > Either way on my ICL using the benchmark:
> > >
> > > ```
> > > ERMS (L3)
> > > Function Best Rate MB/s Avg time Min time Max time
> > > Copy: 323410.5 0.001262 0.001245 0.001285
> > > Scale: 26367.3 0.017114 0.015271 0.029576
> > > Add: 29635.9 0.022948 0.020380 0.032384
> > > Triad: 29401.0 0.021522 0.020543 0.024977
> > >
> > > NT (L3)
> > > Function Best Rate MB/s Avg time Min time Max time
> > > Copy: 285375.1 0.001421 0.001411 0.001443
> > > Scale: 26457.3 0.015358 0.015219 0.015730
> > > Add: 29753.9 0.020656 0.020299 0.022881
> > > Triad: 29594.0 0.020732 0.020409 0.022240
> > >
> > >
> > > ERMS (L3 / 2)
> > > Function Best Rate MB/s Avg time Min time Max time
> > > Copy: 431049.0 0.000620 0.000467 0.001749
> > > Scale: 27071.0 0.007996 0.007437 0.010018
> > > Add: 31005.5 0.009864 0.009740 0.010432
> > > Triad: 30359.7 0.010061 0.009947 0.010434
> > >
> > > NT (L3 / 2)
> > > Function Best Rate MB/s Avg time Min time Max time
> > > Copy: 277315.2 0.000746 0.000726 0.000803
> > > Scale: 27511.1 0.007540 0.007318 0.008739
> > > Add: 30423.9 0.010116 0.009926 0.011031
> > > Triad: 30430.5 0.009980 0.009924 0.010097
> > > ```
> > > Seems to suggest ERMS is favorable.
> > >
If we don't have a workload to support the current threshold, should we
restore the old threshold for processors with ERMS?
--
H.J.
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