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

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Date:   Thu, 2 Dec 2021 07:01:13 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     Noah Goldstein <goldstein.w.n@...il.com>,
        "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>,
        netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH v1] x86/lib: Optimize 8x loop and memory clobbers in csum_partial.c

On Thu, Dec 2, 2021 at 6:24 AM David Laight <David.Laight@...lab.com> wrote:
>
> I've dug out my test program and measured the performance of
> various copied of the inner loop - usually 64 bytes/iteration.
> Code is below.
>
> It uses the hardware performance counter to get the number of
> clocks the inner loop takes.
> This is reasonable stable once the branch predictor has settled down.
> So the different in clocks between a 64 byte buffer and a 128 byte
> buffer is the number of clocks for 64 bytes.
> (Unlike the TSC the pmc count doesn't depend on the cpu frequency.)
>
> What is interesting is that even some of the trivial loops appear
> to be doing 16 bytes per clock for short buffers - which is impossible.
> Checksum 1k bytes and you get an entirely different answer.
> The only loop that really exceeds 8 bytes/clock for long buffers
> is the adxc/adoc one.
>
> What is almost certainly happening is that all the memory reads and
> the dependant add/adc instructions are all queued up in the 'out of
> order' execution unit.
> Since 'rdpmc' isn't a serialising instruction they can still be
> outstanding when the function returns.
> Uncomment the 'rdtsc' and you get much slower values for short buffers.
>
> When testing the full checksum function the queued up memory
> reads and adc are probably running in parallel with the logic
> that is handling lengths that aren't multiples of 64.
>
> I also found nothing consistently different for misaligned reads.
>
> These were all tested on my i7-7700 cpu.
>

I usually do not bother timing each call.
I instead time a loop of 1,000,000,000 calls.
Yes, this includes loop cost, but this is the same cost for all variants.
   for (i = 0; i < 100*1000*1000; i++) {
        res += csum_partial((void *)frame + 14 + 64*0, 40, 0);
        res += csum_partial((void *)frame + 14 + 64*1, 40, 0);
        res += csum_partial((void *)frame + 14 + 64*2, 40, 0);
        res += csum_partial((void *)frame + 14 + 64*3, 40, 0);
        res += csum_partial((void *)frame + 14 + 64*4, 40, 0);
        res += csum_partial((void *)frame + 14 + 64*5, 40, 0);
        res += csum_partial((void *)frame + 14 + 64*6, 40, 0);
        res += csum_partial((void *)frame + 14 + 64*7, 40, 0);
        res += csum_partial((void *)frame + 14 + 64*8, 40, 0);
        res += csum_partial((void *)frame + 14 + 64*9, 40, 0);
    }

Then use " perf stat ./bench"   or similar.