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

[Raffaello Bianco]

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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