[petsc-dev] First call to cudaMalloc or cudaFree is very slow on summit

Zhang, Hong hongzhang at anl.gov
Fri Feb 7 12:23:39 CST 2020

Hi all,

Previously I have noticed that the first call to a CUDA function such as cudaMalloc and cudaFree in PETSc takes a long time (7.5 seconds) on summit. Then I prepared a simple example as attached to help OCLF reproduce the problem. It turned out that the problem was  caused by PETSc. The 7.5-second overhead can be observed only when the PETSc lib is linked. If I do not link PETSc, it runs normally. Does anyone have any idea why this happens and how to fix it?

Hong (Mr.)

bash-4.2$ cat ex_simple.c
#include <time.h>
#include <cuda_runtime.h>
#include <stdio.h>

int main(int argc,char **args)
  clock_t start,s1,s2,s3;
  double  cputime;
  double   *init,tmp[100] = {0};

  start = clock();
  s1 = clock();
  cudaMalloc((void **)&init,100*sizeof(double));
  s2 = clock();
  s3 = clock();
  printf("free time =%lf malloc time =%lf copy time =%lf\n",((double) (s1 - start)) / CLOCKS_PER_SEC,((double) (s2 - s1)) / CLOCKS_PER_SEC,((double) (s3 - s2)) / CLOCKS_PER_SEC);

  return 0;

More information about the petsc-dev mailing list