[PATCH v3 5/5] x86: Double size of ERMS rep_movsb_threshold in dl-cacheinfo.h

Noah Goldstein goldstein.w.n@gmail.com
Sat Nov 6 18:34:00 GMT 2021


On Sat, Nov 6, 2021 at 1:21 PM H.J. Lu <hjl.tools@gmail.com> wrote:
>
> On Sat, Nov 6, 2021 at 11:12 AM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
> >
> > On Sat, Nov 6, 2021 at 12:57 PM H.J. Lu <hjl.tools@gmail.com> wrote:
> > >
> > > On Sat, Nov 6, 2021 at 10:39 AM Noah Goldstein via Libc-alpha
> > > <libc-alpha@sourceware.org> wrote:
> > > >
> > > > No bug.
> > > >
> > > > This patch doubles the rep_movsb_threshold when using ERMS. Based on
> > > > benchmarks the vector copy loop, especially now that it handles 4k
> > > > aliasing, is better for these medium ranged.
> > > >
> > > > On Skylake with ERMS:
> > > >
> > > > Size,   Align1, Align2, dst>src,(rep movsb) / (vec copy)
> > > > 4096,   0,      0,      0,      0.975
> > > > 4096,   0,      0,      1,      0.953
> > > > 4096,   12,     0,      0,      0.969
> > > > 4096,   12,     0,      1,      0.872
> > > > 4096,   44,     0,      0,      0.979
> > > > 4096,   44,     0,      1,      0.83
> > > > 4096,   0,      12,     0,      1.006
> > > > 4096,   0,      12,     1,      0.989
> > > > 4096,   0,      44,     0,      0.739
> > > > 4096,   0,      44,     1,      0.942
> > > > 4096,   12,     12,     0,      1.009
> > > > 4096,   12,     12,     1,      0.973
> > > > 4096,   44,     44,     0,      0.791
> > > > 4096,   44,     44,     1,      0.961
> > > > 4096,   2048,   0,      0,      0.978
> > > > 4096,   2048,   0,      1,      0.951
> > > > 4096,   2060,   0,      0,      0.986
> > > > 4096,   2060,   0,      1,      0.963
> > > > 4096,   2048,   12,     0,      0.971
> > > > 4096,   2048,   12,     1,      0.941
> > > > 4096,   2060,   12,     0,      0.977
> > > > 4096,   2060,   12,     1,      0.949
> > > > 8192,   0,      0,      0,      0.85
> > > > 8192,   0,      0,      1,      0.845
> > > > 8192,   13,     0,      0,      0.937
> > > > 8192,   13,     0,      1,      0.939
> > > > 8192,   45,     0,      0,      0.932
> > > > 8192,   45,     0,      1,      0.927
> > > > 8192,   0,      13,     0,      0.621
> > > > 8192,   0,      13,     1,      0.62
> > > > 8192,   0,      45,     0,      0.53
> > > > 8192,   0,      45,     1,      0.516
> > > > 8192,   13,     13,     0,      0.664
> > > > 8192,   13,     13,     1,      0.659
> > > > 8192,   45,     45,     0,      0.593
> > > > 8192,   45,     45,     1,      0.575
> > > > 8192,   2048,   0,      0,      0.854
> > > > 8192,   2048,   0,      1,      0.834
> > > > 8192,   2061,   0,      0,      0.863
> > > > 8192,   2061,   0,      1,      0.857
> > > > 8192,   2048,   13,     0,      0.63
> > > > 8192,   2048,   13,     1,      0.629
> > > > 8192,   2061,   13,     0,      0.627
> > > > 8192,   2061,   13,     1,      0.62
> > > > ---
> > > >  sysdeps/x86/dl-cacheinfo.h | 8 ++++----
> > > >  1 file changed, 4 insertions(+), 4 deletions(-)
> > > >
> > > > diff --git a/sysdeps/x86/dl-cacheinfo.h b/sysdeps/x86/dl-cacheinfo.h
> > > > index e6c94dfd02..ceb3b53828 100644
> > > > --- a/sysdeps/x86/dl-cacheinfo.h
> > > > +++ b/sysdeps/x86/dl-cacheinfo.h
> > > > @@ -866,12 +866,12 @@ dl_init_cacheinfo (struct cpu_features *cpu_features)
> > > >    /* NB: The REP MOVSB threshold must be greater than VEC_SIZE * 8.  */
> > > >    unsigned int minimum_rep_movsb_threshold;
> > > >  #endif
> > > > -  /* NB: The default REP MOVSB threshold is 2048 * (VEC_SIZE / 16).  */
> > > > +  /* NB: The default REP MOVSB threshold is 4096 * (VEC_SIZE / 16).  */
> > > >    unsigned int rep_movsb_threshold;
> > > >    if (CPU_FEATURE_USABLE_P (cpu_features, AVX512F)
> > > >        && !CPU_FEATURE_PREFERRED_P (cpu_features, Prefer_No_AVX512))
> > > >      {
> > > > -      rep_movsb_threshold = 2048 * (64 / 16);
> > > > +      rep_movsb_threshold = 4096 * (64 / 16);
> > > >  #if HAVE_TUNABLES
> > > >        minimum_rep_movsb_threshold = 64 * 8;
> > > >  #endif
> > > > @@ -879,14 +879,14 @@ dl_init_cacheinfo (struct cpu_features *cpu_features)
> > > >    else if (CPU_FEATURE_PREFERRED_P (cpu_features,
> > > >                                     AVX_Fast_Unaligned_Load))
> > > >      {
> > > > -      rep_movsb_threshold = 2048 * (32 / 16);
> > > > +      rep_movsb_threshold = 4096 * (32 / 16);
> > > >  #if HAVE_TUNABLES
> > > >        minimum_rep_movsb_threshold = 32 * 8;
> > > >  #endif
> > > >      }
> > > >    else
> > > >      {
> > > > -      rep_movsb_threshold = 2048 * (16 / 16);
> > > > +      rep_movsb_threshold = 4096 * (16 / 16);
> > > >  #if HAVE_TUNABLES
> > > >        minimum_rep_movsb_threshold = 16 * 8;
> > > >  #endif
> > > > --
> > > > 2.25.1
> > > >
> > >
> > > You need to update comments for x86_rep_movsb_threshold
> > > in sysdeps/x86/dl-tunables.list
> >
> > Can do.
> >
> > Noticing that the original values were based on comparisons with SSE2 likely on
> > SnB or IVB. I don't have any indication that the 2048 value is not
> > optimal for those
> > processors. Should we keep 2048 / (VEC_SIZE / 16) for SSE2?
>
> Good idea.   So change the threshold to 2048 * (VEC_SIZE / 16) *
> (VEC_SIZE / 16)?

Done and updated the comments explaining the thresholds.

>
> --
> H.J.


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