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@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