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

[Lorenzo Paulatto]

Hello Raffaello,
the first thing you should do, is be sure that your are writing to some 
high performance filesystem. This is often called scratch in computing 
centers, to be sure you should ask the people responsible.

cheers

On 9/30/20 4:04 PM, Raffaello Bianco 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
> 
> 
> 
> _______________________________________________
> Quantum ESPRESSO is supported by MaX (www.max-centre.eu)
> users mailing list users at lists.quantum-espresso.org
> https://lists.quantum-espresso.org/mailman/listinfo/users
> 

-- 
Lorenzo Paulatto - Paris