[mpich-discuss] Best way to do "Allgather to subcommunicator"?

[Jed Brown]

Given an original communicator of size P and S subcomms each of size P/S, I
effectively want to do an "Allgather" with the result distributed over each
of the S subcomms. Some options include

1. point-to-point
2. Allgather, then each process keeps the part that is most relevant to
them (limited by memory)
3. Creating subcomms for each stratum in the S subcomms, Gather to leader,
then Bcast along strata

Are there better ways to do this? The simpler "Gather to subcomm" operation
is also useful.

Background: this redundant redistribution is one approach to relieving
coarse grid bottlenecks in multigrid. Making it faster would definitely
have a tangible impact on solver performance.
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