[Pw_forum] Why does Ewald sum calculation need so much RAM

[Thomas Gruber]

> Message-ID: <1342541146.16360.64.camel at fe12lx.fisica.uniud.it>
> are we talking about RAM, or about CPU, or both, and in which 
> subroutine exactly?
To save CPU time it gets a RAM problem because this is the limiting factor.
> it was done in this way because the Ewald part has never been a 
> serious problem for anybody until now.
It looks like it is in my case.
> sure there is, but you need to locate exactly where too much cpu time 
> is spent and where too much RAM is required. Then, we can discuss 
> about possible solutions. P.
I attached a graphic of the RAM usage for an "-npool 8" calculation and 
added the output file named after the time stamp from the graphic. I 
can't tell you more than it takes almost three days to print the line " 
PHONON       :     2d   18h22m CPU        2d   21h55m WALL" and it uses 
4 times more RAM than in the scf iterations afterward. Unfortunately the 
scaling is not very good:
>   With 16 cores it takes "3d
> 6h" and on 32 cores it takes "2d 16h" and I have only 5 days on that
> cluster for one job.
What is the program doing during this time and can I do it only once and 
give it as input for the other splited phonon calculations?
I tried to use the "-npool" flag, but I can't use it because my RAM is 
to "small" (64GB+, some nodes have more). To save time I like to put 
more representations in one job, but because of this time demanding step 
I get over the time limit of 5 days. So i have to repeat this 
calculations 450 times instead of once.

Thanks in advance.

-- 
Thomas Gruber
Institut für Theoretische Physik
TU Bergakademie Freiberg
Leipziger Str. 23, 09599 Freiberg, Germany
Tel: +49 (0)3731 392006
Email:thomas.gruber at physik.tu-freiberg.de

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