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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. 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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  1. Backward Compatibility WG “Where all the cool kids hang out”

  2. 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, …)

  3. 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 231 relevant • Datasets getting larger • Disk IO getting larger • Coalescing off-node msgs.

  4. 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! ✖ ✖ ✖ ✖

  5. 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_LARGE • Create a system for API versioning • Update all functions to use MPI_Count • Make new duplicate functions with MPI_Count, MPI_Rank, MPI_Size, … • E.g., MPI_SEND_EX ✖ ✖ ✔ Might be ok…? Rip the band-aid off! Preserves backward Compatibility  ✔

  6. MPI Backwards Compatibility WG “Count on us to find a solution”

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