Merging arrays from all ranks using MPI -


i have array of same length on ranks (lets assume 10). values in array contain rank of processor. example ...

proc 1: [1 0 0 0 0 1 0 0 0 1]  proc 2: [0 2 2 0 0 0 0 2 2 0]  proc 3: [0 0 0 3 3 0 3 0 0 0]

now efficient way (using mpi-2) processors end following array

[1 2 2 3 3 1 3 2 2 1]

which can thought of sum of arrays (distributed on ranks). performance important want fast on 1k+ cores.

this doable mpi_allreduce() mpi_sum or mpi_max operator. see documentation of mpi_allreduce. supposed implemented in 1 of best possible ways given architecture.

int arr_a[len], arr_b[len]; ... // fill in arr_a mpi_allreduce(arr_a, arr_b, len, mpi_int, mpi_sum, mpi_comm_world); // result in arr_b

or if short on memory use in-place operation hurt performance:

mpi_allreduce(mpi_in_place, arr_a, len, mpi_int, mpi_sum, mpi_comm_world);

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