MPI_WTIME计时器在OpenMPI 2.0.2中运行速度约2倍

Question

在将OpenMPI从1.8.4更新为2.0.2之后，我使用mpi_wtime（）跑了错误的时间测量。使用版本1.8.4，结果与OMP_Get_wtime（）计时器返回的结果相同，现在MPI_WTIME运行速度约2倍。

什么会导致这种行为？

我的示例代码：

#include <omp.h>
#include <mpi.h>
#include <stdio.h>
#include <stdlib.h>
int some_work(int rank, int tid){
  int count = 10000;
  int arr[count];
  for( int i=0; i<count; i++)
    arr[i] = i + tid + rank;
  for( int val=0; val<4000000; val++)
    for(int i=0; i<count-1; i++)
      arr[i] = arr[i+1];
  return arr[0];
}

int main (int argc, char *argv[]) {
  MPI_Init(NULL, NULL);
  int rank, size;
  MPI_Comm_size(MPI_COMM_WORLD, &size);
  MPI_Comm_rank(MPI_COMM_WORLD, &rank);
  if (rank == 0)
    printf("there are %d mpi processesn", size);
  MPI_Barrier(MPI_COMM_WORLD);
  double omp_time1 = omp_get_wtime();
  double mpi_time1 = MPI_Wtime();
  #pragma omp parallel 
  {
    int tid = omp_get_thread_num();
    if ( tid == 0 ) {
      int nthreads = omp_get_num_threads();
      printf("There are %d threads for process %dn", nthreads, rank);
      int result = some_work(rank, tid);
      printf("result for process %d thread %d is %dn", rank, tid, result);
    }
  }
  MPI_Barrier(MPI_COMM_WORLD);
  double mpi_time2 = MPI_Wtime();
  double omp_time2 = omp_get_wtime();
  printf("process %d omp time: %fn", rank, omp_time2 - omp_time1);
  printf("process %d mpi time: %fn", rank,  mpi_time2 - mpi_time1);
  printf("process %d ratio: %fn", rank, (mpi_time2 - mpi_time1)/(omp_time2 - omp_time1) );
  MPI_Finalize();
  return EXIT_SUCCESS;
}

编译

g++ -O3 src/example_main.cpp -o bin/example -fopenmp -I/usr/mpi/gcc/openmpi-2.0.2/include -L /usr/mpi/gcc/openmpi-2.0.2/lib -lmpi

和运行

salloc -N2 -n2 mpirun --map-by ppr:1:node:pe=16 bin/example 

给出

之类的东西

there are 2 mpi processes
There are 16 threads for process 0
There are 16 threads for process 1
result for process 1 thread 0 is 10000
result for process 0 thread 0 is 9999
process 1 omp time: 5.066794
process 1 mpi time: 10.098752
process 1 ratio: 1.993125
process 0 omp time: 5.066816
process 0 mpi time: 8.772390
process 0 ratio: 1.731342

我首先写的比率并不一致，但仍然足够大。

OpenMPI 1.8.4的结果还可以：

g++ -O3 src/example_main.cpp -o bin/example -fopenmp -I/usr/mpi/gcc/openmpi-1.8.4/include -L /usr/mpi/gcc/openmpi-1.8.4/lib -lmpi -lmpi_cxx

给出

result for process 0 thread 0 is 9999
result for process 1 thread 0 is 10000
process 0 omp time: 4.655244
process 0 mpi time: 4.655232
process 0 ratio: 0.999997
process 1 omp time: 4.655335
process 1 mpi time: 4.655321
process 1 ratio: 0.999997

Answer 1

我的群集上有类似的行为（与您的OpenMPI版本相同，2.0.2），问题是CPU频率的默认调速器，即"保守"。将调速器设置为"绩效"后，MPI_WTIME（）的输出与正确的时间对齐（在我的情况下，"时间"的输出）。看来，对于某些较旧的Xeon处理器（例如Xeon E5620），当使用过于激进的策略以进行动态频率调整时，某些时钟功能会偏向 - 相同的OpenMPI版本不会在同一集群中的较新Xeon上遇到此问题。

Answer 2

也许mpi_wtime（）本身可能是一个昂贵的操作？如果您避免在OpenMP时间的一部分中测量MPI_WTIME（）消耗的时间，结果是否会变得更加一致？例如：

double mpi_time1 = MPI_Wtime();
double omp_time1 = omp_get_wtime();
/* do something */
double omp_time2 = omp_get_wtime();
double mpi_time2 = MPI_Wtime();