[Beowulf] wall clock time for mpi_allreduce?

[Charlie Peck]

On Sep 10, 2010, at 10:46 PM, xingqiu yuan wrote:

> Hi
> 
> I found that use of mpi_allreduce to calculate the global maximum and
> minimum takes very long time, any better alternatives to calculate the
> global maximum/minimum values?

If only the rank 0 process needs to know the global max and min you can use MPI_Reduce() rather than MPI_Reduceall() which will substantially reduce the communication time.  The difference is that with MPI_Reduce() the result of the reduction is not communicated back to the other ranks.

charlie