Re: [PATCH v5 10/10] tests/bench: Add bufferiszero-bench

[Daniel P . Berrangé]

On Sat, Feb 17, 2024 at 09:21:50AM -1000, Richard Henderson wrote:
> On 2/16/24 23:49, Alexander Monakov wrote:
> > 
> > On Fri, 16 Feb 2024, Richard Henderson wrote:
> > 
> > > Benchmark each acceleration function vs an aligned buffer of zeros.
> > > 
> > > Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> > > ---
> > > +
> > > +static void test(const void *opaque)
> > > +{
> > > +    size_t len = 64 * KiB;
> > 
> > This exceeds L1 cache capacity, so the performance ceiling of L2 cache
> > throughput is easier to hit with a suboptimal implementation. It also
> > seems to vastly exceed typical buffer sizes in Qemu.
> > 
> > When preparing the patch we mostly tested at 8 KiB. The size decides
> > whether the branch exiting the loop becomes perfectly predictable in
> > the microbenchmark, e.g. at 128 bytes per iteration it exits on the
> > 63'rd iteration, which Intel predictors cannot track, so we get
> > one mispredict per call.
> > 
> > (so perhaps smaller sizes like 2 or 4 KiB are better)
> 
> Fair.  I've adjusted to loop over 1, 4, 16, 64 KiB.
> 
> # Start of bufferiszero tests
> # buffer_is_zero #0: 1KB 49227.29 MB/sec
> # buffer_is_zero #0: 4KB 137461.28 MB/sec
> # buffer_is_zero #0: 16KB 224220.41 MB/sec
> # buffer_is_zero #0: 64KB 142461.00 MB/sec
> # buffer_is_zero #1: 1KB 45423.59 MB/sec
> # buffer_is_zero #1: 4KB 91409.69 MB/sec
> # buffer_is_zero #1: 16KB 123819.94 MB/sec
> # buffer_is_zero #1: 64KB 71173.75 MB/sec
> # buffer_is_zero #2: 1KB 35465.03 MB/sec
> # buffer_is_zero #2: 4KB 56110.46 MB/sec
> # buffer_is_zero #2: 16KB 68852.28 MB/sec
> # buffer_is_zero #2: 64KB 39043.80 MB/sec

Totally nit-picking, but it would be easier to read with a little
alignment and blanks lines:

 # buffer_is_zero #0:  1KB  49227.29 MB/sec
 # buffer_is_zero #0:  4KB 137461.28 MB/sec
 # buffer_is_zero #0: 16KB 224220.41 MB/sec
 # buffer_is_zero #0: 64KB 142461.00 MB/sec
 
 # buffer_is_zero #1:  1KB  45423.59 MB/sec
 # buffer_is_zero #1:  4KB  91409.69 MB/sec
 # buffer_is_zero #1: 16KB 123819.94 MB/sec
 # buffer_is_zero #1: 64KB  71173.75 MB/sec
 
 # buffer_is_zero #2:  1KB  35465.03 MB/sec
 # buffer_is_zero #2:  4KB  56110.46 MB/sec
 # buffer_is_zero #2: 16KB  68852.28 MB/sec
 # buffer_is_zero #2: 64KB  39043.80 MB/sec

With regards,
Daniel
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