linux-kernel - RE: [PATCH v1] x86/lib: Optimize 8x loop and memory clobbers in csum

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Date:   Sun, 28 Nov 2021 19:47:49 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Noah Goldstein' <goldstein.w.n@...il.com>,
        Eric Dumazet <edumazet@...gle.com>
CC:     "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        X86 ML <x86@...nel.org>, "hpa@...or.com" <hpa@...or.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "alexanderduyck@...com" <alexanderduyck@...com>,
        "open list" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v1] x86/lib: Optimize 8x loop and memory clobbers in
 csum_partial.c

...
> Regarding the 32 byte case, adding two accumulators helps with the latency
> numbers but causes a regression in throughput for the 40/48 byte cases. Which
> is the more important metric for the usage of csum_partial()?
> 
> Here are the numbers for the smaller sizes:
> 
> size, lat old,    lat ver2,    lat ver1,    tput old,   tput ver2,   tput ver1
>    0,   4.961,       4.503,       4.901,       4.887,       4.399,       4.951
>    8,   5.590,       5.594,       5.620,       4.227,       4.110,       4.252
>   16,   6.182,       6.398,       6.202,       4.233,       4.062,       4.278
>   24,   7.392,       7.591,       7.380,       4.256,       4.246,       4.279
>   32,   7.371,       6.366,       7.390,       4.550,       4.900,       4.537
>   40,   8.621,       7.496,       8.601,       4.862,       5.162,       4.836
>   48,   9.406,       8.128,       9.374,       5.206,       5.736,       5.234
>   56,  10.535,       9.189,      10.522,       5.416,       5.772,       5.447
>   64,  10.000,       7.487,       7.590,       6.946,       6.975,       6.989
>   72,  11.192,       8.639,       8.763,       7.210,       7.311,       7.277
>   80,  11.734,       9.179,       9.409,       7.605,       7.620,       7.548
>   88,  12.933,      10.545,      10.584,       7.878,       7.902,       7.858
>   96,  12.952,       9.331,      10.625,       8.168,       8.470,       8.206
>  104,  14.206,      10.424,      11.839,       8.491,       8.785,       8.502
>  112,  14.763,      11.403,      12.416,       8.798,       9.134,       8.771
>  120,  15.955,      12.635,      13.651,       9.175,       9.494,       9.130
>  128,  15.271,      10.599,      10.724,       9.726,       9.672,       9.655
> 
> 'ver2' uses two accumulators for 32 byte case and has better latency numbers
> but regressions in tput compared to 'old' and 'ver1'. 'ver1' is the
> implementation
> posted which has essentially the same numbers for tput/lat as 'old'
> for sizes [0, 63].

Which cpu are you testing on - it will make a big difference ?
And what are you measing throughput in?
And are you testing aligned or mis-aligned 64bit reads?

I think one of the performance counters will give 'cpu clocks'.

I did some tests early last year and got 8 bytes/clock on broadwell/haswell
with code that 'loop carried' the carry flag (a single adc chain).
On the older Intel cpu (Ivy bridge onwards) 'adc' has a latency of 2
for the result, but the carry flag is available earlier.
So alternating the target register in the 'adc' chain will give (nearly)
8 bytes/clock - I think I got to 7.5.

That can all be done with only 4 reads per interaction.
IIRC broadwell/haswell only need 2 reads/iteration.

It is actually likely (certainly worth checking) that haswell/broadwell
can do two misaligned memory reads every clock.
So it may not be worth aligning the reads (which the old code did).
In any case aren't tx packets likely to be aligned, and rx ones
misaligned to some known 4n+2 boundary?

Using adxc/adxo together is a right PITA.
I did get (about) 12 bytes/clock fo long buffers while loop carrying
both the overflow and carry flags.

Also is there a copy of the patched code anywhere?
I think I've missed some of the patches and they are difficult to follow.

	David

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