[Pw_forum] density files

[Silviu Zilberman]

Paolo Giannozzi wrote:
> On Wednesday 08 February 2006 12:18, Silviu Zilberman wrote:
>
>   
>> I have been running calculations on Lemieux which is an alpha cluster
>> super computer in Pittsburgh. For some reason that is still mysterious
>> to me, writing these density files on the scratch space took very long
>> time, ~30 (!) minutes for a 68MB file.
>>     
>
> maybe you should rename your machine "Lepire" :-)
>
> The charge density file should be written only when the wavefunctions
> are written, every "isave" steps and at the end of the run. If it is written
> at each time step, this is definitely wrong. 
>   
The charge density is written only every isave steps, and I set it to a 
very large number to avoid this time-consuming i/o. But even if I write 
it just once at the end of the calculation, it would still require ~90 
minutes for 3 files, which is completely crazy, given that the maximal 
time allocated per job is 12 hours on this supercomputer.
> The charge density in a parallel calculation is collected to a single node
> and written from there. Since it is not wise to collect it into a single
> array, each slice from each processor is collected and written. Maybe 
> this algorithm is not optimal (maybe it is even "pessimal"). You should 
> try to understand where exactly the machine spends all this time and
> why
>   
I may do it, but for the time being, these files are not very useful for 
me. I can change the code to respect again the disk_io parameter and 
avoid writing these files all together. However I would like to know 
first if there was some reasoning behind dumping these files by default 
without user control over it.

Thanks, Silviu.