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
