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Backward Compatibility WG. “Where all the cool kids hang out”. The Big Issue: Counts Larger Than 2 31. Counts are expressed as “ int ” / “INTEGER” Usually limited to 2 31 Propose a new type: MPI_Count Can be larger than an int / INTEGER “Mixed sentiments” within the Forum

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backward compatibility wg

Backward Compatibility WG

“Where all the cool kids hang out”

the big issue counts larger than 2 31
The Big Issue:Counts Larger Than 231
  • Counts are expressed as “int” / “INTEGER”
    • Usually limited to 231
  • Propose a new type: MPI_Count
    • Can be larger than an int / INTEGER
  • “Mixed sentiments” within the Forum
    • Is it useful? Do we need it? …oy!

MPI_SEND(void *buf, int count, …)

MPI_SEND(void *buf, MPI_Count count, …)

do we need mpi count
Do we need MPI_Count?

YES

NO

Very few users

Affects many, many MPI API functions

Potential incompatibilities

E.g., mixing int and MPI_Count in the same application

  • Some users have asked for it
  • Trivially send large msgs.
    • No need to make a datatype
  • POSIX went to size_t
    • Why not MPI?
  • Think about the future:
    • Bigger RAM makes 231relevant
    • Datasets getting larger
    • Disk IO getting larger
    • Coalescing off-node msgs.
ok so how to do it 1 of 2
Ok, so how to do it? (1 of 2)
  • Use MPI_Count only for new MPI-3 routines
  • Change C bindings
    • Rely on C auto-promotion
  • Only fix MPI IO functions
    • Where MPI_BYTE is used
  • New, duplicate functions
    • E.g., MPI_SEND_LARGE

Inconsistent,

confusing to users

Bad for Fortran, bad

for C OUT params

Inconsistent,

confusing to users

What about sizes,

tags, ranks, …oy!

ok so how to do it 2 of 2
Ok, so how to do it? (2 of 2)

Forum has hated

every proposal

Technically makes

current codes invalid

  • Fully support large datatypes
    • E.g., MPI_GET_COUNT_LONG
  • Create a system for API versioning
  • Update all functions to use MPI_Count
  • Make new duplicate functions with MPI_Count, MPI_Tag, MPI_Size, …
    • E.g., MPI_SEND_EX

Might be ok…?

Rip the band-aid off!

Preserves backward

Compatibility 

mpi backwards compatibility wg

MPI Backwards Compatibility WG

“Count on us to find a solution”

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