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The MPI C++ bindings: what happened, and why?

Jeff Hammond at Argonne tells me that there’s some confusion in the user community about MPI and C++.

Let me see if I can clear up some of the issues.

In this blog entry, I’ll describe what has happened to the C++ bindings over time (up to and including their removal in MPI-3), and why.  In a second blog entry, I’ll describe what this means to real-world C++ MPI applications.

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Negative MPI tags: Just. Say. No.

A user on the Open MPI mailing list recently asked about using negative tags for point-to-point MPI sending/receiving operations.  He was using bit-mapped tags, and needed just one more bit.

Although this may seem like an innocent, easy-to-fulfill feature request, there are several reasons why we can’t allow the use of negative tags.

First and foremost, the MPI standard disallows negative tags.  It is explicitly mentioned in MPI-3 section 3.2.3 that tag values must be non-negative integers.

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MPI-3.0 has passed!

We’re here at the MPI Forum in Vienna where the Forum has just unanimously voted to accept the MPI-3.0 document.

Woo hoo!!

This document caps a 4-year effort that started in January of 2008.  MPI-3.0 clarifies many pending MPI-2.2 issues and adds some significant new user-level features to the standard:
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MPI_Ibarrier: Crazy?

Most people’s reactions to hearing about the new MPI-3 non-blocking “barrier” collective think: huh?

Why on earth would you have a non-blocking barrier?  The whole point of a barrier is to synchronize — how does it make sense not to block while waiting?

The key is re-phrasing that previous question: why would you block while waiting?

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Process Affinity in OMPI v1.7 (part 2)

In my last post, I described the Simple mode of Open MPI v1.7’s process affinity system.

The Simple mode is actually quite flexible, and we anticipate that it will meet most users’ needs. However, some users will need more flexibility. That’s what the Expert mode is for.

Before jumping in to the Expert mode, though, let me describe two more features of the revamped v1.7 affinity system.

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