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From: Greg Lindahl (lindahl_at_[hidden])
Date: 2005-10-12 15:35:50


On Wed, Oct 12, 2005 at 07:06:54PM +0100, Ashley Pittman wrote:

> As it turns out I'm in a position to measure this fairly easily, our MPI
> sits on top of a library called libelan, this does all the tag matching
> at a very low level, all MPI does is convert the communicator into a bit
> pattern, calculate the length from the type and count and convert from
> lrank to grank, passing the call on.

As you point out later, that's a lot more than what needs to be
measured.

> Regardless of the numbers this is a *high performance* industry and
> doing this would be a step in the wrong direction.

If the hit is tiny and the benefit is large, the high performance
industry will gladly adopt it. I have yet to see, for example, anyone
rewrite their MPI code from f77 into C in order to avoid the current
overhead for the f77 MPI interface. In an ideal world, an MPI ABI will
have the same f77 overhead as today, and C overhead similar to f77.

In any case, accurately measuring this overhead and then discussing it
is better than discussing it without accurately measuring.

-- greg