Jeff Squyres wrote:
WHAT: Have Fortran MPI_SEND/MPI_RECV directly call the corresponding
PML functions instead of the C MPI_Send/MPI_Recv
WHY: Slightly optimize the blocking send/receive in Fortran (i.e.,
remove a function call)
WHERE: ompi/mpi/f77/*.c -- possibly add an --enable switch to
configure to enable/disable this behavior
WHEN: For OMPI v1.4
TIMEOUT: Tuesday teleconf, 17 Feb 2009
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Taking some inspiration from NEC MPI, it might be useful to remove an
extra function call from some common Fortran MPI functions (I'm
specifically proposing MPI_SEND/MPI_RECV, but others could be done as
well). Specifically, instead of having the Fortran MPI_SEND/MPI_RECV
call the C versions of MPI_Send/MPI_Recv, they could just do [almost]
exactly the same thing as the C versions: error checking on the MPI
parameters and calling the PML back-end functions.
The net performance win for this is likely very small. However, this
idea has been on my to-do list for forever, so I thought I'd ask if
people cared/objected.
Benefit
- Remove a function call from the critical performance path; possibly
save a little latency
The only "benefit" is "possibly a little"? This is not at all
compelling. Is the hoped-for benefit measurable? I assume a pingpong
latency test over shared memory is the only hope you have of observing
any benefit. Have you attempted to measure this, or is this benefit
only conjecture?
Drawback
- Duplicate some code (but this code rarely/never changes)
It's still code bloat.
- May violate MPI profiling libraries that assume that the Fortran
MPI API functions call the C MPI API functions
I'm not real familiar with the issues here, but this strikes me as a
serious drawback.
Granted, on the NEC platform, function calls are *VERY* expensive --
so having their Fortran MPI API functions directly call their
back-end functions makes much more sense than calling the C API
functions. On the OS's and platforms that OMPI supports, we'll
likely see a much smaller benefit (indeed, its effects may only be
visible over shared memory -- if at all). But it may be worthwhile
just in the "it's the right thing to do" category.
Thoughts?
