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From:   Noah Goldstein <goldstein.w.n@gmail.com>
Date:   Thu, 2 Dec 2021 14:19:19 -0600
Message-ID: <CAFUsyfJticWKb3fv12r5L5QZ0AVxytWqtPVkYKeFYLW3K1SMNw@mail.gmail.com>
Subject: Re: [PATCH v1] x86/lib: Optimize 8x loop and memory clobbers in csum_partial.c
To:     Eric Dumazet <edumazet@google.com>
Cc:     David Laight <David.Laight@aculab.com>,
        "tglx@linutronix.de" <tglx@linutronix.de>,
        "mingo@redhat.com" <mingo@redhat.com>,
        Borislav Petkov <bp@alien8.de>,
        "dave.hansen@linux.intel.com" <dave.hansen@linux.intel.com>,
        X86 ML <x86@kernel.org>, "hpa@zytor.com" <hpa@zytor.com>,
        "peterz@infradead.org" <peterz@infradead.org>,
        "alexanderduyck@fb.com" <alexanderduyck@fb.com>,
        open list <linux-kernel@vger.kernel.org>,
        netdev <netdev@vger.kernel.org>
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On Thu, Dec 2, 2021 at 9:01 AM Eric Dumazet <edumazet@google.com> wrote:
>
> On Thu, Dec 2, 2021 at 6:24 AM David Laight <David.Laight@aculab.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.

Intuitively 10 passes is a bit low. Also you might consider aligning
the `csum64` function and possibly the loops.

There a reason you put ` jrcxz` at the beginning of the loops instead
of the end?

> > (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.

Maybe add an `lfence` before / after `csum64`
> >
> > 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);
>     }

+ 1. You can also feed `res` from previous iteration to the next
iteration to measure latency cheaply if that is better
predictor of performance.

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