MPI Working Group

Point to Point Communication

Current Topics

  • Streams
    • Active discussions in teleconferences
  • Allocate receive
    • On hold until San Jose meeting
  • Receive reduce
    • On hold until San Jose meeting

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What is an MPI Stream?

  • From single sender to single receiver only
    • Joined by an existing communicator
  • Ordered and reliable
  • Sender can send any amount(s) of data
  • Receiver can receive any amount(s) of data
    • (up to what is available)

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Discussion Issues with MPI Streams

  • Data-types as the unit of transmission?
    • Should normal message boundaries be ignored?
  • Flow-control/buffering?
    • e.g. receiver consistently slower than sender
  • Allow buffer underrun or block receiver?
    • e.g. receiver wants 33 integers but only 16 available
  • What benefit(s) does an MPI Stream have over normal point-to-point?

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Allocate Receive

  • MPI_Arecv: the receive buffer is an output argument instead of an input argument, and the implementation allocates that memory internally.
  • Allows implementation to allocate memory for the size of the message, eliminates buffering overhead when message size is not known a priori.
  • Allows copy-free implementation of unexpected messages using an eager-like protocol.

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Receive Reduce/Accumulate

  • MPI_Recv_{reduce,accumulate}: the incoming data is reduced/accumulated onto the receive buffer.
  • Matches a common application pattern during boundary element exchange and allows implementation to minimize buffering in this case and potentially do more efficiently.
  • Useful for creating user-defined, potentially dynamic reduction trees, without graph communicators.
  • May allow for more efficient implementation of some forms of active-messages.

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Meeting details

  • Teleconference calls
    • Fortnightly on Monday at 11:00 central US
    • Next on 5th January 2015
  • Email list:
    • mpiwg-p2p@lists.mpi-forum.org
  • Face-to-face meetings

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MPI P2P WG Update for SC14 - Google Slides