[mpich-discuss] How expensive is MPI_Win_create() compared to memcpy()?

[Jed Brown]

To present a "nice" API, I have a choice between trying to reuse windows
internally, which will usually involve a local memcpy(), and creating new
windows to address user's memory (after checking that it is suitably
aligned). I foresee caching and reusing windows for small operations and
creating new windows for sufficiently large operations. In the MPICH2
implementation, I see an MPI_Allgather() with 3 MPI_Aint for ch3 and with a
32-byte struct for DCMF.

1. Is the implementation likely to always perform this synchronizing and
not strictly memory-scalable MPI_Allgather()?

2. How expensive should I consider this operation to be? Are there
micro-benchmark results scaling out to 10k-100k cores somewhere?

I'll end up implementing both versions, but it would be nice to know how
urgent it is likely to be and have a guess for where to place the threshold.
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