On Mon, Jun 23, 2014 at 11:17 AM, Andi Kleen <a...@firstfloor.org> wrote: > Marc Glisse <marc.gli...@inria.fr> writes: > >> Hello, >> >> this is a stage 1 patch, and I'll ping it then, but if you have >> comments now... > > FWIW i believe the transformation will break a large variety of micro > benchmarks. > > calloc internally knows that memory fresh from the OS is zeroed. > But the memory may not be faulted in yet. > > memset always faults in the memory. > > So if you have some test like > > buf = malloc(...) > memset(buf, ...) > start = get_time(); > ... do something with buf > end = get_time() > > Now the times will be completely off because the measured times includes > the page faults.
Easy way for these benchmarks to get around this. volatile char *vbuf = (char*)buf; for(i=0;i<bufsize;i++) *vbuf = 0; before get_time (); Now there is no way for the compiler to optimize away the inlined memset and will always be 100% correct in the future. Also micro-benchmarking is going to have issues like this too with future optimizations. Thanks, Andrew > > -Andi > > -- > a...@linux.intel.com -- Speaking for myself only
