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

[Richard Henderson <richard.henderson@linaro.org>]

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


r~