[Pw_forum] Further reduce disk IO for ph.x

[Paolo Giannozzi]

On Fri, 2012-12-07 at 17:25 +1100, Yao Yao wrote:

> For these disk IO, is it writing output files or swapping out data to save RAM? 

the code writes to file in order to save both RAM and CPU time.
In particular, it keeps in memory at a given time only data
(\psi(k), \psi(k+q), \delta V(q)\psi(k), \delta\psi(k)) for 
a single k-point. It also saves some intermediate results.

> Since the total output file size is small compared to the disk IO bandwidth, I 
> guess the answer is the latter one, right? If yes, where is the data swap file 
> located? 

there are many files, not a single one. They are all written into a
subdirectory of "outdir"/"prefix".save

> if it's possible to avoid swapping in the expense of more RAM consumption,
> that's also OK for me.

it is definitely possible, but it requires some programming work. The
code "as is" uses a lot of disk space. Note however that modern
operating systems tend to keep files into RAM as much as possible,
so if you have enough RAM. it may not make such a big difference
whether you keep things into RAM or if you write them to file.

P.
-- 
Paolo Giannozzi, IOM-Democritos and University of Udine, Italy