Re: [Boost-users] hybrid parallelism

At Thu, 4 Nov 2010 16:34:19 +0100, Matthias Troyer wrote:

On Nov 4, 2010, at 3:14, Brian Budge wrote:

...

Do these tasks share a lot of data? If they are really lightwieght memory-wise, heavy computationally, and don't require fine-grained communication with each other, I'd go with David's suggestion, as it will be easier to write, and the performance won't be much different.

If you use a lot of memory, need fine-grained chatter between tasks, or the tasks are pretty cheap, threads may be (much) better.

Brian

I second this opinion for several reasons

First, mixing MPI with multithreading can be hard since many MPI implementations are not thread safe. Be sure to only let the master thread use MPI.

Good point; I hadn't thought of that.

Secondly, it adds another level of complexity. Starting M*N MPI processes is much easier, unless you waste too much memory that way.

That was mostly what I had in mind.

Third, we have just recently benchmarked several multithreaded LAPACK routines in the Intel, AMD and other lapack libraries and compared them to the MPI based routines in SCALAPACK. Surprisingly the MPI implementations outperformed the multithreaded ones by a large margin.

Was that done running the MPI processes together on one machine? -- Dave Abrahams BoostPro Computing http://www.boostpro.com