On Nov 21, 2005, at 9:14 AM, Angel Tsankov wrote:
> In fact, the issue of swapping was the first that came to my mind as
> well. However, I also doubt that this is the reason.
> It seems to me that other activities in the system could contribute to
> the longer execution times when both copies of the program execeutee
> on the same node. In this case both CPU are busy executing not only
> the CG solver but other applications as well. In the other case one of
> the CPUs in each node executes the CG solver while the other CPU is
> free to execute other applications.
> I've noticed, however, that the activity of other applications is very
> low. Moreover, the solver has been executed 5 times for each matrix
> size (and each type of communication) and the times are almost the
> same for each of the 5 runs. This way I make sure the times are
> accurate.
> Another possible reason is that ther is that more communications are
> overlapped with computations in the case of larger systems.
> Of course, other ideas are welcome.
These are all definite possibilities. Unfortunately, it's something
that only you, the application developer, can reasonably answer.
Caching effects can also have a big impact (I think that was mentioned
in a prior mail).
> And finally, I just wonder if communication time depends on the amount
> of data to be transferred when shared memory is used.
Yes, message size definitely affects communication time, regardless of
the communication media (i.e., even with shared memory).
--
{+} Jeff Squyres
{+} The Open MPI Project
{+} http://www.open-mpi.org/
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