Barry Smith <[email protected]> writes: > There is still something wonky here, whether it is the MPI implementation > or how PETSc handles the assembly. Without any values that need to be > communicated it is unacceptably that these calls take so long. If we > understood __exactly__ why the performance suddenly drops so dramatically we > could perhaps fix it. I do not understand why.
I guess it's worth timing. If they don't have MPI_Reduce_scatter_block then it falls back to a big MPI_Allreduce. After that, it's all point-to-point messaging that shouldn't suck and there actually shouldn't be anything to send or receive anyway. The BTS implementation should be much smarter and literally reduces to a barrier in this case.
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