[Pw_forum] PW very sensitive to number of processors and pools

[Christopher O'Brien]

I am trying to do benchmarks and determine how best to parallelize pw.x for scf calculations using a toy system consisting of a single Cu unit cell. However it seems as if there are 'magic' combinations of processors and pools that determine if a run will work. Is there a pattern to choosing these parameters? 

As an example, I used ewcut=35Ry, ecutrho=350Ry and a monkhorst-pack k-points grid. I have an older beowulf cluster with 2 single-core procs per node and 1GB RAM each.
As an example of the sensitivity two cases are shown:
                  kpoints    nprocs  npool nr3  nr3s  npp  npps
Working  10x10x10  20          10        30    20     15     12
Working  20x20x20  32          2          30    20     2        2
Failing     30x30x30  20          2          30    20     4        3
Failing     30x30x30  64          8          30    20     4        3
As you can tell I am randomly guessing here. Even the 10x10x10 case took a lot of guessing to get it to work.

All of the crashes occur in one of two places: 1.) During writing of the k-points, or 2.) while writing the band energies at each k-point.
Case 1)
>         k(  282) = (   0.7833333  -0.0500000   0.8166667), wk =   0.0008889
>         k(  283) = (   0.7500000  -0.0166667   0.7833333), wk =   0.0008889
> p1_8011:  p4_error: net_recv read:  probable EOF on socket: 1
> p7_25354:  p4_error: net_recv read:  probable EOF on socket: 1
> p19_22825:  p4_error: net_recv read:  probable EOF on socket: 1
Case 2)
>           k = 0.4500-0.4500 1.3500 (   271 PWs)   bands (ev):
> 
>      7.6764   9.3915   9.6220  10.1803  10.9132  12.6419  16.5014  30.5475
>     33.0769  33.8058


Thanks in advance.

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Christopher J. O'Brien
cjobrien at ncsu.edu

Ph.D. Candidate
Computational Materials Group
Department of Materials Science & Engineering
North Carolina State University
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