[Pw_forum] PW.x homogeneous electric field berry phase calculation in trigonal cell

[Lorenzo Paulatto]

On Tuesday, February 7, 2017 6:47:18 PM CET Louis Fry-Bouriaux wrote:
>     Interesting, I will try that, I will add that I tried the calculation
> with lelfield=.true. and all efield_cart values to zero and it gives me the
> same error. I also reduced the number of auto k points to speed up testing
> (auto: 6 2 2 0 0 0/ nppstr=6, which takes ~160s). I may take a look at the
> code maybe there is something that can be done if I identify what is going
> on :/

Hello,
I managed to find the original emails from 2012 about the issue; it used to be 
much worst but Paolo Giannozzi wrote a quick fix that does make the issue much 
lighter. 

Still. the case gdir=3 (or pdir=3, if you are doing polarization) is much 
faster than the other cases, and there is not easy solution. The reason is 
that the code needs to build planes of G-vectors that are orthogonal to the 
direction of the k-points string. 

The bottleneck is a single call:
CALL mp_sum(aux_g(:), intra_bgrp_comm )
which collects a wavefunctions over all the CPUs.

This call becomes slower and slower when more CPUs are involved. It used to be 
unbearably slow, now it is just slow. 

There is no obvious solution, as the parallelisation is hard-coded along the z 
direction. Try to stick to gdir=3 if you can.

hth

-- 
Dr. Lorenzo Paulatto
IdR @ IMPMC -- CNRS & Université Paris 6
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