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Subject: [OMPI users] Should order of implementation matter for MPI_reduce?
From: Andrew J Gomes (a-gomes_at_[hidden])
Date: 2013-03-04 17:16:17


I have the following code which does not give the expected result:
if (mpi_rank!=0)
{ MPI_Reduce(&intensity,&intensity ,1,MPI_DOUBLE,MPI_SUM,0,MPI_COMM_WORLD);
MPI_Reduce(&intensity2,&intensity2 ,1,MPI_DOUBLE,MPI_SUM,0,MPI_COMM_WORLD); }
else
{
MPI_Reduce(MPI_IN_PLACE,&intensity2 ,1,MPI_DOUBLE,MPI_SUM,0,MPI_COMM_WORLD);
MPI_Reduce(MPI_IN_PLACE,&intensity ,1,MPI_DOUBLE,MPI_SUM,0,MPI_COMM_WORLD);}

However if I change the the order of the else block (reduction of intensity variable before intensity2 variable) to :
else
{
MPI_Reduce(MPI_IN_PLACE,&intensity ,1,MPI_DOUBLE,MPI_SUM,0,MPI_COMM_WORLD);
MPI_Reduce(MPI_IN_PLACE,&intensity2 ,1,MPI_DOUBLE,MPI_SUM,0,MPI_COMM_WORLD);}

I get the expected result. This surprised me. Is there a reason the order of the else block should match the order of the if block? Is there a more robust way of implementing this type of code. Thank you

Andrew