Question about memcpy

[bing zhu]

I agree !,just i think the problem is still there,memcpy is indeed faster
in kernel than in user,i've tried both ways .
schedule might be to blame.

2018-07-09 22:04 GMT+08:00 Himanshu Jha <himanshujha199640 at gmail.com>:

> Hi Bing,
>
> On Sun, Jul 08, 2018 at 10:03:48PM +0800, bing zhu wrote:
> > void *p = malloc(4096 * max);
> > start = usec();
> > for (i = 0; i < max; i++) {
> > memcpy(p + i * 4096, page, 4096);
> > }
> > end = usec();
> > printf("%s : %d time use %lu us \n", __func__, max,end - start）;
> >
> > static unsigned long usec(void)
> > {
> >         struct timeval tv;
> >         gettimeofday(&tv, 0);
> >         return (unsigned long)tv.tv_sec * 1000000 + tv.tv_usec;
> > }
>
> I think for these benchmarking stuff, to evaluate the cycles and time
> correctly you should use the __rdtscp(more info at "AMD64 Architecture
> Programmer’s Manual Volume 3: General-Purpose and System Instructions"
> Pg 401)
>
> Userspace:
> ----------------------------------------------------------------------
> #include <stdio.h>
> #include <time.h>
> #include <stdint.h>
> #include <x86intrin.h>
>
> volatile unsigned sink;
> unsigned int junk;
>
> int main (void)
> {
> clock_t start = clock();
> register uint64_t t=__rdtscp(&junk);
>
> for(size_t i=0; i<10000000; ++i)
>         sink++;
>
> t=__rdtscp(&junk)-t;
> clock_t end = clock();
> double cpu_time_used = ((double) (end - start)) / CLOCKS_PER_SEC;
>
> printf("for loop took %f seconds to execute %zu cylces\n", cpu_time_used,
> t);
> }
> ---------------------------------------------------------------------
>
> Kernelspace:
> If you want to dig more:
> https://www.intel.com/content/dam/www/public/us/en/
> documents/white-papers/ia-32-ia-64-benchmark-code-execution-paper.pdf
>
>
> Thanks
> --
> Himanshu Jha
> Undergraduate Student
> Department of Electronics & Communication
> Guru Tegh Bahadur Institute of Technology
>
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