Re: [PATCH RFC] [X86] performance improvement for memcpy_64.S by fast string.

H. Peter Anvin
2009-11-06T17:08:00Z

On 11/06/2009 01:41 AM, ling.ma@intel.com wrote:
>
> Performance counter stats for './static_orig' (10 runs):
>
> 2835.650105 task-clock-msecs # 0.999 CPUs ( +- 0.051% )
> 3 context-switches # 0.000 M/sec ( +- 6.503% )
> 0 CPU-migrations # 0.000 M/sec ( +- nan% )
> 4429 page-faults # 0.002 M/sec ( +- 0.003% )
> 7941098692 cycles # 2800.451 M/sec ( +- 0.051% )
> 10848100323 instructions # 1.366 IPC ( +- 0.000% )
> 322808 cache-references # 0.114 M/sec ( +- 1.467% )
> 280716 cache-misses # 0.099 M/sec ( +- 0.618% )
>
> 2.838006377 seconds time elapsed ( +- 0.051% )
>
> 'perf stat --repeat 10 ./static_new' command get data after patch:
>
> Performance counter stats for './static_new' (10 runs):
>
> 7401.423466 task-clock-msecs # 0.999 CPUs ( +- 0.108% )
> 10 context-switches # 0.000 M/sec ( +- 2.797% )
> 0 CPU-migrations # 0.000 M/sec ( +- nan% )
> 4428 page-faults # 0.001 M/sec ( +- 0.003% )
> 20727280183 cycles # 2800.445 M/sec ( +- 0.107% )
> 1472673654 instructions # 0.071 IPC ( +- 0.013% )
> 1092221 cache-references # 0.148 M/sec ( +- 12.414% )
> 290550 cache-misses # 0.039 M/sec ( +- 1.577% )
>
> 7.407006046 seconds time elapsed ( +- 0.108% )
>

I assume these are backwards? If so, it's a dramatic performance
improvement.

Where did the 1024 byte threshold come from? It seems a bit high to me,
and is at the very best a CPU-specific tuning factor.

Andi is of course correct that older CPUs might suffer (sadly enough),
which is why we'd at the very least need some idea of what the
performance impact on those older CPUs would look like -- at that point
we can make a decision to just unconditionally do the rep movs or
consider some system where we point at different implementations for
different processors -- memcpy is probably one of the very few
operations for which something like that would make sense.

-hpa

--
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel. I don't speak on their behalf.