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

Alex Peyser peyser.alex at
Tue Jun 16 14:51:17 CDT 2009

On Tuesday 16 June 2009 03:29:47 pm Satish Balay wrote:
> 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
> application.
> If using MATDENSE - a much higher percentage of time will be in blas.
> Satish
> > Ah, another layer of administration-code may now be required to properly
> > allocate jobs.
> >
> > Alex

Unfortunately, I'm writing a language. I don't know the application apriori. I 
have my current application -- which I can benchmark and is dense. But I 
would hate to predetermine the situation for future problems. Recompiling 
BLAS (and everything depending on it) on a per-use basis isn't a terribly 
attractive solution -- but that's a future problem.

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