[Pw_forum] Further reduce disk IO for ph.x

[Yao Yao]

Dear All,

I have been working with a lot of phononic calculations with ph.x. It worked 
very well. However, ph.x do a lot of writing. On a 48-core compute node, it 
takes ~35MB/s disk IO band width (estimated from network traffic of that node, 
which is without local disk). I tried "reduce_dio" option, which reduce the 
rate to ~20MB/s.

However this is still not enough for me. Our cluster has ~40 such nodes. If I 
run more than ~7 jobs concurrently, my jobs will deplete the disk IO 
bandwidth. Then everything will be very slow. A simple "ls" command will take 
~10 seconds to return.

I'm seeking a way to further reduce disk IO for ph.x. I don't need recover 
though I have limited run duration per job, because I can continue with 
"start_q" and "last_q", which is very nice.

For these disk IO, is it writing output files or swapping out data to save RAM? 
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? Can I customise the path? I want to put it in /dev/shm, which is a 
48GB RAM disk (I have far more RAM than QE needs). On the other hand, if it's 
possible to avoid swapping in the expense of more RAM consumption, that's also 
OK for me.

I guess putting the whole folder in /dev/shm may work but that's probably my 
last option because it requires copy back and forth.

Thank you very much.

Regards,
Yao

-- 
Yao Yao
PhD. Candidate
3rd Generation Strand
ARC Photovoltaics Centre of Excellence
University of New South Wales
Sydney, NSW Australia