[QE-users] pw.x -nt X, optimization at GGA and Hybrid calculations

[Lucas Nicolás Lodeiro Moraga]

Hi all!

I am performing a scaling test with QE (versions 6.1 and 6.3) for a big
sistem >2000 electrons with GGA/USPP. I found that when I use more than one
node the use of -nt is very usefull to improve the calculation time. For
this system nt=number of nodes is the best choise in most cases... but
sometimes nt = number of nodes + X, with X = 1,2,3,4... (no more than 4) is
better. As an example, if i use 440 processors, with 44 processors nodes,
the best nt is 11.
Also, I read that the -nt is usefull when the FFT 3D grid < n° processors,
but in some cases when FFT 3D grid > n° processors, -nt 2 or 3 improves the
calculation too.
Can someone help me with this topic, give me more information and
internalize me in this topic?

On the other hand, I try this with hybrid functional in a simple
semiconductor sistem with ~50 atoms with HSE/NCPP. With 6.1 version the
calculation CRASH when I use -nt 2, the error message is:
     iteration #  1     ecut=    80.00 Ry     beta=0.30
     Davidson diagonalization with overlap
     ethr =  1.00E-02,  avg # of iterations =  8.0

     negative rho (up, down):  1.101E-01 0.000E+00


 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
     Error in routine electrons (1):
     charge is wrong: smearing is needed
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

     stopping ...

When I use smearing:
     iteration #  1     ecut=    80.00 Ry     beta=0.30
     Davidson diagonalization with overlap
     ethr =  1.00E-02,  avg # of iterations =  8.0

     negative rho (up, down):  1.081E-01 0.000E+00

     total cpu time spent up to now is       24.5 secs

     WARNING: integrated charge=   312.31515712, expected=   320.00000000

 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
     Error in routine electrons (1):
     charge is wrong
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

     stopping ...

It is very weird, and with 6.3 version the calculation runs without
problem, but take more time, and the speedup is very very low in comparison
with one node calculation.
There is a bug in 6.1 version? Is it a bug in the 6.1 compilation?
Somebody knows some method to accelerate the hybrid calculations to be able
to occupy more nodes?

Regards

Lucas Lodeiro
Universidad de Chile
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