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

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Message-ID: <CANn89iKaTyrNZHCg7i46-db_zudXs0V-BvmwM-5fYLZT2yQr4Q@mail.gmail.com>
Date:   Fri, 26 Nov 2021 22:03:17 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     Noah Goldstein <goldstein.w.n@...il.com>
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
        peterz@...radead.org, alexanderduyck@...com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] x86/lib: Optimize 8x loop and memory clobbers in csum_partial.c

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>