[mvapich-discuss] How to achieve a “single global reduction” in a typical C++ code?

[Daniel WEI]

Dear List,

I have heard a lot about "single global reduction", but I still don't
understand how it is realized in a typical C++ code, could anyone shed me
some lights on how it is actually done?

Let's use the following code as an example, could anyone show me how it
could be modified, so to have only one reduce? Thanks

double s1, s2;
void sum( double& data ) {
  double s = 0.0;
  for ( j=jmin; j<=jmax; j++ ) {
    s += data[j];
  }
  MPI_Allreduce(&s,&s1,1,MPI_SUM,..) ;}
void sumsq( double& data ) {
  double s = 0.0;
  for ( j=jmin; j<=jmax; j++ ) {
    s += data[j] * data[j];
  }
  MPI_Allreduce(&s,&s2,1,MPI_SUM,..) ;}
for( i=0; i<N; i++ ) {
  sum(a[i]);
  sumsq(a[i]);
  var[i] = (s2 - s1*s1/N)/(N-1);}





Zhigang Wei
----------------------
*University of Notre Dame*
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