Do optimizations for same_size, same_disp_unit in RMA

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Do optimizations for same_size, same_disp_unit in RMA #2188

Closed mpichbot closed 3 years ago

mpichbot commented 8 years ago

Originally by jczhang on 2014-10-14 17:11:54 -0500

MPI-3.0 defines info key same_size for MPI_Win_create and friends. MPI-3.1 will define same_disp_unit as well. These two hints are useful in optimizations. For example, in exascale computing, we can save a lot of memory by avoiding storing size, disp_unit for other ranks, if we know they are the same on all ranks.

See the original MPI ticket here: https://svn.mpi-forum.org/trac/mpi-forum-web/ticket/369 See discussion of the hints in an old ticket here: https://trac.mpich.org/projects/mpich/ticket/2138

hzhou commented 3 years ago

@minsii I believe we have this optimization in ch4 already, right?

I am closing this for now. Reopen if necessary.

minsii commented 3 years ago

MPIDIG does not have such a storage. OFI already eliminates O(P) MPIDI_OFI_WIN(win).winfo based on runtime check, no matter the user hints are set or not. For UCX, looks like O(P) MPIDI_UCX_WIN_INFO(win, i).disp still exists and no optimization is applied. But, I would consider the memory optimization is minor and is not worth to optimize by paying the cost of more complex code.

In summary, agreed to close the issue:-)