Hi, I came across a problem in experiments that makes me wondering if there is any optimization of collective calls, such as MPI_Allreduce, for 2^n number of ranks? We did some experiments on Argonne Vesta system to measure the time of MPI_Allreduce calls using 511, 512 and 513 processes. (one process per node). In each run, the synthetic benchmark first did some computation and then called MPI_Allreduce 30 times, for total 100 loops. We measured the total time spent on communication. We found that 512-process run gives the best performance. The time for 511, 512 and 513 processes are 0.1492, 0.1449 and 0.1547 seconds respectively. 512-proc outperforms 511-proc by 3.7%, and 513-proc by 6.7%. The mpich version we used is as follows. $ mpichversion MPICH Version: 3.1.2 MPICH Release date: Mon Jul 21 16:00:21 CDT 2014 MPICH Device: pamid MPICH configure: --prefix=/home/fujita/soft/mpich-3.1.2 --host=powerpc64-bgq-linux --with-device=pamid --with-file-system=gpfs:BGQ --disable-wrapper-rpath MPICH CC: powerpc64-bgq-linux-gcc -O2 MPICH CXX: powerpc64-bgq-linux-g++ -O2 MPICH F77: powerpc64-bgq-linux-gfortran -O2 MPICH FC: powerpc64-bgq-linux-gfortran -O2 Thanks! Best, Aiman _______________________________________________ discuss mailing list [email protected] To manage subscription options or unsubscribe: https://lists.mpich.org/mailman/listinfo/discuss