Question

当我使用MPI_Wtime（）记录计算时间时，它返回几个奇怪的数字，这似乎不正确。我的代码非常简单：

#include <stdio.h>
#include "mpi.h"
int main(int argc,char *argv[])
{
  int i,sum,total;
  int size,rank,start,end,sumGlobal;
  double t1,t2;
  total = 10000;
  t1 = MPI_Wtime();
  MPI_Init(&argc,&argv);
  MPI_Comm_size(MPI_COMM_WORLD,&size);
  MPI_Comm_rank(MPI_COMM_WORLD,&rank);
  start = rank*(total/size)+1;
  if (rank==(size-1))
  {
    end = total;
  }
  else
  {
    end = start+(total/size)-1;
  }
  sumGlobal = 0;
  sum = 0;
  for (i=start;i<=end;i++)
  {
    sum += i;
  }
  MPI_Reduce(&sum,&sumGlobal,1,MPI_INT,MPI_SUM,0,MPI_COMM_WORLD);
  t2 = MPI_Wtime();
  printf("rank = %d,sum = %d,sumGlobal = %d,time=%f\n",rank,sum,sumGlobal,t2-t1);
  MPI_Finalize();
  return 0;
}

运行结果是（使用16个核心）：

rank = 0,sum = 195625,sumGlobal = 50005000,time = 1520044462.143606
rank = 1,sum = 586250,sumGlobal = 0,time = 1520044462.143662
rank = 3,sum = 1367500,sumGlobal = 0,time = 1520044462.143665
rank = 4,sum = 1758125,sumGlobal = 0,time = 1520044462.143610
rank = 5,sum = 2148750,sumGlobal = 0,time = 1520044462.143651
rank = 6,sum = 2539375,sumGlobal = 0,time = 1520044462.143608
rank = 7,sum = 2930000,sumGlobal = 0,time = 1520044462.143604
rank = 9,sum = 3711250,sumGlobal = 0,time = 1520044462.143647
rank = 10,sum = 4101875,sumGlobal = 0,time = 1520044462.143655
rank = 11,sum = 4492500,sumGlobal = 0,time = 1520044462.143606
rank = 12,sum = 4883125,sumGlobal = 0,time = 1520044462.143614
rank = 13,sum = 5273750,sumGlobal = 0,time = 1520044462.143672
rank = 14,sum = 5664375,sumGlobal = 0,time = 1520044462.143643
rank = 15,sum = 6055000,sumGlobal = 0,time = 1520044462.143605
rank = 2,sum = 976875,sumGlobal = 0,time = 1520044462.143665
rank = 8,sum = 3320625,sumGlobal = 0,time = 1520044462.143652

正如您所看到的，“时间”值非常奇怪。那么如何解决呢？谢谢。

Answer 1

您的程序与MPI标准不一致：

您无法在 MPI_Wtime()之前使用MPI_Init() （也不能在MPI_Finalize() fwiw之后使用。

移动t1初始化应解决问题。

MPI_Wtime（）返回奇怪的数字

1 个答案: