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Large Counts in MPI

Large Counts in MPI. A Brief History. How it all started. HP-MPI users request 64-bit counts ( nat’l labs) HP takes issue up in Forum, Ticket #14, September 2008 Proposes new extended functions for inclusion in MPI 2.2 ( MPI_SendL , etc) Covers pt2pt, collective, datatypes , rma

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Large Counts in MPI

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  1. Large Counts in MPI A Brief History

  2. How it all started • HP-MPI users request 64-bit counts (nat’l labs) • HP takes issue up in Forum, Ticket #14, September 2008 • Proposes new extended functions for inclusion in MPI 2.2 (MPI_SendL, etc) • Covers pt2pt, collective, datatypes, rma • Uses MPI_Aint for count type

  3. Forum Feedback • Too big a change for 2.2, delay to 3.0 • Define new MPI_Count type instead of using MPI_Aint • Leads to ticket #117, Feb 2009

  4. Ticket #117 • Modifies existing APIs • Covers pt2pt, collective, datatypes, rma • Uses MPI_Count for count type

  5. Forum Feedback • Lots of thrashing • Does anyone really needMPI_Count? • Does new functionality use MPI_Count? • What’s the impact on Fortran? • Is the MPI_Count version of all APIs too big of an API explosion? • HP sells HP-MPI, ticket lays idle for a while

  6. Ticket #220 • Focus shifts from communication to MPI-IO • Ticket #117 resurrected, changed in scope to just: • MPI_GET_COUNT_L • MPI_GET_ELEMENTS_L • MPI_STATUS_SET_ELEMENTS_L • Ticket #220 introduces _L version of MPI-IO routines. • Back to _L suffix!

  7. Forum Feedback • Discussion about suffixing rules • What happens when the next suffix is introduced? • i.e. timeout parameter • Generally rejected by forum as being too big of a change for too little benefit • Dave Gives up.

  8. Current Proposal Rip off the band-aid!! • Define MPI_Count as >= int • Define as int for back compatibility • Apply it to all APIs • No suffixing • Requires two libs • back compat • long counts • Can be handled in single codebase • Implementers decide which to ship/support

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