Backward compatibility wg
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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

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Backward Compatibility WG

“Where all the cool kids hang out”


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?

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)

  • 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)

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

“Count on us to find a solution”


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