The simple question seems to be: Can MPI_BYTE be used safely on this communicator? If even one datatype requires translation then the answer is "no". How about just calling the per communicator attribute something like "MPI_BYTE_SAFE" and leave other debates about the meaning of heterogeneous aside? There is clearly room to add complexity but is there a real need? It seems to me that communicator attribute "MPI_BYTE_SAFE" provides 99% of the value and is easy to do. Dick Treumann - MPI Team/TCEM IBM Systems & Technology Group Dept 0lva / MS P963 -- 2455 South Road -- Poughkeepsie, NY 12601 Tele (845) 433-7846 Fax (845) 433-8363 mpi-22-bounces_at_[hidden] wrote on 03/03/2008 01:24:11 PM:

On Mar 3, 2008, at 12:54 PM, Dries Kimpe wrote:

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Narrowing the scope to "whether translation functionality is required" is good. But note that whether the translation functionality is used may be specific to a given (communicator,peer,datatype) tuple.

You're right that this depends on the communicator and on the peer;

...and the datatype.

...

It would not be uncommon to have one 'different (memory representation wise)' node in the communicator. This could be a login node, a visualisation node, ...

So, from a performance point of view (and also for the mpi implementation internally), it's actually better to record this information for (communicator, source, dest). However, my main reason for this proposal was to enable an application that doesn't care about datatypes (might be more common than you'd think, looking at some of the mpi-subset ideas) to abort execution if it detects that data conversion is needed when communicating to its peers.

My point is that some datatypes may require translation while others may not. It's not always just endian differences -- sometimes there's floating point representation differences, or size differences (e.g., sizeof(int) != sizeof(int)).

-- Jeff Squyres Cisco Systems

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