[PATCH] Improve string benchtest timing

[Wilco Dijkstra]

Hi Siddhesh,
  
>On 21/05/19 7:28 PM, Wilco Dijkstra wrote:
>> Well the test doesn't actually test misaligned copies - both source and
>> destination are always mutually aligned, so any memcpy implementation which
>> aligns either source or destination will only do aligned copies.
>
> They were not intended to be mutually misaligned, that was not the
> intent of the benchmark since the target application it modeled did not
> have such inputs.
>
>> In any case I'm not sure what the test is supposed to measure - the scores are
>> identical across all memcpy implementations. The time taken for double the
>> copy size is exactly twice as much.
>
> Right, you'll probably only see differences in case of mutually
> misaligned inputs.

Well if I force the copies to be mutually unaligned, there is only about 1% difference
for a few of the memcpy implementations compared to them being always aligned
The others show identical performance whether aligned or not. This is not too
surprising since the test is basically waiting for DRAM most of the time.

So if we wanted to measure something useful we'd need to do it differently. Maybe
the goal was to measure DRAM bandwidth? If so we could modify it to compare
copy bandwidth for just a few different sizes (corresponding with typical L1/L2/L3 sizes).

Wilco