linux-kernel - Re: [PATCH v2] x86/lib: Optimize 8x loop and memory clobbers in csum

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Date:   Sat, 27 Nov 2021 00:38:58 -0600
From:   Noah Goldstein <goldstein.w.n@...il.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     tglx@...utronix.de, mingo@...hat.com,
        Borislav Petkov <bp@...en8.de>, dave.hansen@...ux.intel.com,
        X86 ML <x86@...nel.org>, hpa@...or.com, peterz@...radead.org,
        alexanderduyck@...com, open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] x86/lib: Optimize 8x loop and memory clobbers in csum_partial.c

On Sat, Nov 27, 2021 at 12:03 AM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Fri, Nov 26, 2021 at 8:25 PM Noah Goldstein <goldstein.w.n@...il.com> wrote:
> >
> > Modify the 8x loop to that it uses two independent
> > accumulators. Despite adding more instructions the latency and
> > throughput of the loop is improved because the `adc` chains can now
> > take advantage of multiple execution units.
> >
> > Make the memory clobbers more precise. 'buff' is read only and we know
> > the exact usage range. There is no reason to write-clobber all memory.
> >
> > Relative performance changes on Tigerlake:
> >
> > Time Unit: Ref Cycles
> > Size Unit: Bytes
> >
> > size, lat old, lat new,    tput old,    tput new
> >    0,   4.961,   4.901,       4.887,       4.951
> >    8,   5.590,   5.620,       4.227,       4.252
> >   16,   6.182,   6.202,       4.233,       4.278
> >   24,   7.392,   7.380,       4.256,       4.279
> >   32,   7.371,   7.390,       4.550,       4.537
> >   40,   8.621,   8.601,       4.862,       4.836
> >   48,   9.406,   9.374,       5.206,       5.234
> >   56,  10.535,  10.522,       5.416,       5.447
> >   64,  10.000,   7.590,       6.946,       6.989
> >  100,  14.218,  12.476,       9.429,       9.441
> >  200,  22.115,  16.937,      13.088,      12.852
> >  300,  31.826,  24.640,      19.383,      18.230
> >  400,  39.016,  28.133,      23.223,      21.304
> >  500,  48.815,  36.186,      30.331,      27.104
> >  600,  56.732,  40.120,      35.899,      30.363
> >  700,  66.623,  48.178,      43.044,      36.400
> >  800,  73.259,  51.171,      48.564,      39.173
> >  900,  82.821,  56.635,      58.592,      45.162
> > 1000,  90.780,  63.703,      65.658,      48.718
> >
> > Signed-off-by: Noah Goldstein <goldstein.w.n@...il.com>
> >
> > tmp
>
> SGTM (not sure what this 'tmp' string means here :) )
>
> Reviewed-by: Eric Dumazet <edumazet@...gle.com>

Poor rebasing practices :/

Fixed in V3 (only change).