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The New MPI-3 Remote Memory Access (One Sided) Interface

February 29, 2012 at 11:34 pm PST

Today we feature a deep-dive guest post from Torsten Hoefler, the Performance Modeling and Simulation lead of the Blue Waters project at NCSA, and Pavan Balaji, computer scientist in the Mathematics and Computer Science (MCS) Division at the Argonne National Laboratory (ANL), and as a fellow of the Computation Institute at the University of Chicago.

Despite MPI’s vast success in bringing portable message passing to scientists on a wide variety of platforms, MPI has been labeled as a communication model that only supports “two-sided” and “global” communication. The MPI-1 standard, which was released in 1994, provided functionality for performing two-sided and group or collective communication. The MPI-2 standard, released in 1997, added support for one-sided communication or remote memory access (RMA) capabilities, among other things. However, users have been slow to adopt such capabilities because of a number of reasons, the primary ones being: (1) the model was too strict for several application behavior patterns, and (2) there were several missing features in the MPI-2 RMA standard. Bonachea and Duell put together a more-or-less comprehensive list of areas where MPI-2 RMA falls behind. A number of alternate programming models, including Global Arrays, UPC and CAF have gained popularity filling this gap.

That’s where MPI-3 comes in.

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MPI-3 Fortran bindings prototype now available

February 27, 2012 at 5:00 am PST

At long last, Craig Rasmussen (from Los Alamos National Laboratory) and I are ready to publish our prototype implementation of the MPI-3 Fortran bindings.  The new MPI-3 Fortran bindings are coming up for their second vote at the upcoming MPI Forum meeting in Chicago; this public release satisfies the “must implement all new proposed behavior” requirement for proposals to get in MPI-3.

The good stuff:

Please download and give this implementation a whirl! We’d love to hear your feedback.

So let’s dive a little deeper into the details…

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MPI-3 small new function: MPI_GET_LIBRARY_VERSION

February 25, 2012 at 8:16 am PST

There’s been some discussion of Big New Features in MPI-3 recently (and more are coming!).  But today, I want to talk about a little new feature.  Something small, but still useful.

I’m talking about a new function named MPI_GET_LIBRARY_VERSION.

Its only purpose in life is to shed some light on to the implementation that you’re using.  It returns a simple string, and is intended to be human-readable (vs. being machine-parseable). The format of the string is not mandated, but it is assumed that MPI implementations will include reasonably detailed information about their specific version.

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More MPI-3 newness: const

October 16, 2011 at 5:00 am PST

Way back in the MPI-2.2 timeframe, a proposal was introduced the add the C keyword “const” to all relevant MPI API parameters.  The proposal was discussed at great length.  The main idea was twofold:

  • Provide a stronger semantic statement about which parameter contents MPI could change, and which it should not.  This mainly applies to user choice buffers (e.g., the choice buffer argument in MPI_SEND).
  • Be more friendly to languages that use const(-like constructs) more than C.  The original proposal was actually from Microsoft, whose goal was to provide higher quality C# MPI bindings.

Additionally, the (not deprecated at the time) official MPI C++ bindings have had const since the mid-1990s — so why not include them in the C bindings?

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New things in MPI-3: MPI_Count

October 8, 2011 at 5:00 am PST

The count parameter exists in many MPI API functions: MPI_SEND, MPI_RECV, MPI_TYPE_CREATE_STRUCT, etc.  In conjunction with the datatype parameter, the count parameter is often used to effectively represent the size of a message.  As a concrete example, the language-neutral prototype for MPI_SEND is:

MPI_SEND(buf, count, datatype, dest, tag, comm)

The buf parameter specifies where the message is in the sender’s memory, and the count and datatype arguments indicate its layout (and therefore size).

Since MPI-1, the count parameter has been an integer (int in C, INTEGER in Fortran).  This meant that the largest count you could express in a single function call was 231, or about 2 billion.  Since MPI-1 was introduced in 1994, machines — particularly commodity machines used in parallel computing environments — have grown.  2 billion began to seem like a fairly arbitrary, and sometimes distasteful, limitation.

The MPI Forum just recently passed ticket #265, formally introducing the MPI_Count datatype to alleviate the 2B limitation.

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