[QE-users] Huge difference between Wall time and CPU time in electron-phonon calculation

[Michal Krompiec]

Dear Rafaello,
Are you using a local (preferably SSD-based) scratch drive, or a very fast
parallel file system?
Best wishes,
Michal Krompiec
Merck KGaA

On Wed, 30 Sep 2020 at 15:05, Raffaello Bianco <
raffaello.bianco.it at gmail.com> wrote:

> Dear QE users and developers,
>
>  I am doing an electron-phonon coupling calculation in this way (I am
> using QE v 6.6).
> First, I have done an scf calculation. Then, I have done a phonon
> calculation where I have printed the dvscf files, with
>
>    fildvscf         = 'dvscf'
>
> Subsequently, I have done the electron-phonon calculation changing the
> k-mesh grid, with
>
>     trans            = .false.
>     electron_phonon  = 'simple'
>
> The calculation ends correctly, but for some q points I have noticed a
> huge difference between
> CPU and Wall time, like
>
> PHONON       :     15h55m CPU   3d18h56m WALL
>
> From the report at the end of the output, the I/O davcio routine seems to
> be the
> "guilty":
>
>
>     General routines
>      davcio       :    107.89s CPU 263856.08s WALL (  520331 calls)
>
>      Parallel routines
>
>       Electron-phonon coupling
>      elphon       :  41730.55s CPU 309708.87s WALL (       1 calls)
>      elphel       :  41671.20s CPU 309625.04s WALL (      60 calls)
>
>       General routines
>      davcio       :    107.89s CPU 263856.08s WALL (  520331 calls)
>
>  This calculation was done with 10 processors and npool = 10, if I use 40
> processors and npool = 10 it is worse (as can probably be expected due to
> the higher number of I / O operations). I have looked at the documentation
> but I am not very familiar with these things, thus I still have several
> doubts. Any suggestions on tests to do or how to improve performance, or at
> least comments to clarify the problem, would be greatly appreciated.
>
> Thank you for your time,
>
> Best,
> Raffaello Bianco
>
>
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