Re: [PATCH 2/2] Improve 64bit memcpy/memmove for Corei7 with avx2 instruction

[OndÅej BÃlka <neleai at seznam dot cz>]

On Sat, Jun 08, 2013 at 12:12:56AM +0800, Ling Ma wrote:
> > First it does not randomize size in any way. This will cause branches to
> > be predicted and as branch prediction can account to 20% of time results
> > you get will be 20% off.
> Ling: Because "A widely held rule of thumb is that a program spends
> 90% of its execution time in only 10% of the code",  so hardware
> implemented  branch prediction mechanism, stable pattern history
> provide benchmark(SPEC 2000) with average 95% correct prediction,
> fully reandom code will make it useless.
>
And are you sure that it is relevant for memcpy? Compile and run simple
program below.

gcc -fPIC -shared memcpy.c -o memcpy.so
LD_PRELOAD=./memcpy.so bash 2> memcpy_input
 
It will record alignments and sizes of each memcpy call you do in that
shell. You can see how random they are.

> > Fox example as you ran
> > ./memcpy-test-avx2-bench
> > cpy frequency could be 800MHz
> > then in
> > ./memcpy-test-new-bench
> > a governor can decide to switch to 2.5GHz making results above three
> > times worse than they are.
> Ling:  I can confirm it is not issue in my compare.html, but like to
> send out double-check result.
> 
> Ondra, if we can test real benchmark, that will more approximate our
> real world usage. So some people know good memcpy benchmarks which
> represent the real world applications, and could you please tell us ?
> 
One I posted. 
> Thanks & Best Regards
> Ling



#include <stdio.h>
#undef memcpy
void *memcpy(void *_x,const void *_y,size_t n){
        char *x=_x,*y=_y;
        int i;

  for(i=0;i<n;i++){
    x[i]=y[i];
  }
        fprintf(stderr,"memcpy:%i dest %i src %i
size\n",((int)_x)%64,((int)_y)%64,n);
        return x;
}