Can PETSc detect the number of CPUs on each computer node?

Satish Balay balay at
Tue Jun 16 14:29:47 CDT 2009

On Tue, 16 Jun 2009, Alex Peyser wrote:

> On Tuesday 16 June 2009 02:29:14 pm Matthew Knepley wrote:
> >
> > This is a common misconception. In fact, most time is spent in MatVec or
> > BLAS1, neither of which benefit from MT BLAS.

> Interesting. At least my misconception is common. 
> That makes things tricky with ATLAS, since the number of threads is a 
> compile-time constant. I can't imagine it would be a good idea to have an 8x 
> BLAS running 8xs simultaneously -- unless the mpi jobs were all 
> unsynchronized. It may be only 10-20% of the time, but that's still a large 
> overlap of conflicting threads degrading performance.
> I'll have to do some benchmarks. Is the 10-20% number still true for fairly 
> dense matrices?

Its just a number I pulled out of a hat [for sparse matrix
solves]. -log_summary would be the correct thing for a given

If using MATDENSE - a much higher percentage of time will be in blas.


> Ah, another layer of administration-code may now be required to properly 
> allocate jobs.
> Alex

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