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

[Noah Goldstein <goldstein.w.n@gmail.com>]

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?
>
> --
> H.J.