[Q-e-developers] QE I/O default

[Paolo Giannozzi]

Dear Ye

> I noticed yesterday that the wf_collect is set true as default. Probably you
> already had a lot of discussion on your side

not really: having realized that I seldom get useful answers when I
ask opinions about this or that, I no longer ask, just do this or that
and wait for complaints.

> Do we have confident that wf_collect will not add significant of time on
> large simulations?

At this stage, performances are not my major concern: correctness and
maintainability are. The I/O of QE has become a mess beyond control,
it needs to be simplified and documented, the old stuff must disappear
ASAP. Note that the possibility to write "distributed" files is still
there (at least for PW; for CP it can be easily re-added),

> Is the performance good on both lustre and GPFS file system?

No idea.

> I didn't have much experience with the recent added hdf5 feature.

Nor have I (by the way, it is not yet working with new I/O)

> Does the WF collect use  parallel collective I/O? or like the old fashion
> collect the WF on the master  and write by it.

For the time being: old fashion. Of course, everybody is welcome to
implement with "the latest and the greatest", but first of all, we
need something working and that can be modified without hours of
reverse engineering.

> Is the performance good? Measured bandwidth?

I hope it will be, but I leave to other people measuring bandwidths.

> On the machines I use, the GPFS has 8 aggregators by default and PIO
> performance is better than creating individual files. The lustre does the
> opposite and has only 1 OST by default and thus write sequentially with PIO.
> Writing individual files becomes faster. Of course you can tune both of
> them, just very tricky.
>
> Do QE still create the file per MPI rank from the beginning? 4k empty file
> is a bit slow to create and pain to 'ls'. When I do DFT+U basically the
> number of files doubles or triples I don't remember exactly.

This story of QE opening too many files is becoming an urban legend
like "QE is 10 times slower than VASP". The default setting, since
some time, is that QE doesn't open anything while running.

> PS: In the past, I had the experience that QE was not able to ready its own
> collected WF when the record (using IOTK) is very large >100GB.

this doesn't look like a QE problem, though, unless it is a iotk
problem. IOTK is no longer used to write binaries by the way.

Paolo

> Not collecting the WF was the preferred way for me. It should not be a problem
> with hdf5 since the dataset is per band and much smaller.
>
> Thanks,
> Ye
> ===================
> Ye Luo, Ph.D.
> Leadership Computing Facility
> Argonne National Laboratory
>
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>



-- 
Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222