[Pw_forum] Trying to find reasonable parallelization parameters

[J. J. Ramsey]

----- Original Message ----
> From: Lorenzo Paulatto <paulatto at sissa.it>
> To: Axel Kohlmeyer <akohlmey at cmm.chem.upenn.edu>; PWSCF Forum <pw_forum at pwscf.org>
> Sent: Friday, January 23, 2009 3:45:39 AM
> Subject: Re: [Pw_forum] Trying to find reasonable parallelization parameters
> 
> On Thu, 22 Jan 2009 19:37:00 +0100, Axel Kohlmeyer 
> wrote:
> > very simple. -npools can be maximally the actual number of k-points that
> > you are using, and that should be exploited to the maximum.
> 
> True, but the maximum may not be the number of kpoints; using the parallel 
> davidson algorithm (default) the hamiltonian matrix will be distributed and 
> diagonalized on the larger perfect square number of processor *inside* each 
> pool.

*Each* pool? I had gotten the impression from the documentation (<http://www.quantum-espresso.org/wiki/index.php/Running_on_parallel_machines#Understanding_Parallelism_in_Quantum-Espresso>) that if, say, I had 36 CPU cores and 4 pools, the diagonalization would by default take place over only 9 cores, not 9 cores for each pool.

(FWIW, I also understand that the diagonalization takes place on a subset of the processors performing the FFTs, so it's not like I have some cores doing diagonalization and a completely different set of cores handling the plane wave parallelization. Not sure if that's right, though.)

BTW, thanks for the tip on diago_david_ndim. I may give that a try.