[Pw_forum] -nimage -npool -ntg -ndiag

[Eduardo Ariel Menendez Proupin]

Hi, I found in the new, or maybe not so new, users guide, that 1000 atoms or
so can be calculated, and new ways to paralelize.

The example in the manual is

 mpirun -np 4096 ./pw.x -nimage 8 -npool 2 -ntg 8 -ndiag 144 -input myinput.in

I have played a bit, but not with a massive computer, and I have found that
the default options are always better than my unexpert  choices.

So, I would like to see some hints, in addition to what  is reproduced below
(from the users guide), about the good choices of -ntg and -ndiag. Maybe
some examples is enough to understand it.


>From the users guide:

This execute the PWscf code on 4096 processors, to simulate a system with 8
images, each of which is distributed across 512 processors. K-points are
distributed across 2 pools of 256 processors each, 3D FFT is performed using
8 task groups (64 processors each, so the 3D real-space grid is cut into 64
slices), and the diagonalization of the subspace Hamiltonian is distributed
to a square grid of 144 processors (12x12).

Default values are: -nimage 1 -npool 1 -ntg 1 ; ndiag is chosen by the code
as the fastest n^2 (n integer) that fits into the size of each pool.

*Massively parallel calculations*: For very large jobs (i.e. O(1000) atoms
or so) or for very long jobs to be run on massively parallel machines (e.g.
IBM BlueGene) it is crucial to use in an effective way both the "task group"
and the "ortho group" parallelization. Without a judicious choice of
parameters, large jobs will find a stumbling block in either memory or CPU
requirements. In particular, the "ortho group" parallelization is used in
the diagonalization of matrices in the subspace of Kohn-Sham states (whose
dimension is as a strict minumum equal to the number of occupied states).
These are stored as block-distributed matrixes (distributed across
processors) and diagonalized using custom-taylored diagonalization
algorithms that work on block-distributed matrixes.
Thanks

-- 
Eduardo Menendez
Departamento de Fisica
Facultad de Ciencias
Universidad de Chile
Phone: (56)(2)9787439
URL: http://fisica.ciencias.uchile.cl/~emenendez
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