Re: [Boost-users] hybrid parallelism

Subject: Re: [Boost-users] hybrid parallelism
From: David Abrahams (dave_at_[hidden])
Date: 2010-11-04 20:46:28

At Thu, 4 Nov 2010 16:34:19 +0100,
Matthias Troyer wrote:
> 
> On Nov 4, 2010, at 3:14, Brian Budge <brian.budge_at_[hidden]> 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