[QE-users] time consuming band structure calculation for a supercell

[Paolo Giannozzi]

There are a few tests here:
   https://gitlab.com/max-centre/benchmarks/-/tree/master/Quantum_Espresso
but there is much ongoing work on diagonalization algorithms.

Paolo

On Mon, Dec 14, 2020 at 4:08 PM Michal Krompiec <michal.krompiec at gmail.com>
wrote:

> Dear Lorenzo,
> Speaking of ppcg, is there any published (or otherwise public) benchmark
> of ppcg vs Davidson and/or cg? For which cases can ppcg be expected to be
> faster?
> Best regards,
> Michal Krompiec, Merck KGaA
>
> On Mon, Dec 14, 2020 at 2:12 PM Lorenzo Paulatto <paulatz at gmail.com>
> wrote:
>
>> p.p.s I mean diagonalization='ppcg'
>>
>> On 2020-12-14 15:10, Lorenzo Paulatto wrote:
>> > p.s. If you can use a newer version of QE that does calculation="ppcg"
>> > I found it to be much (i.e. 6x) faster in this case
>> >
>> > cheers
>> >
>> > On 2020-12-14 14:50, Lorenzo Paulatto wrote:
>> >> Hello,
>> >>
>> >> I've had a look at the output, and a part for the cutoff which
>> >> appears a bit too high (you are probably safe with 50/400Ry of
>> >> ecutwfc/ecutrho) I only see to small problems:
>> >>
>> >> 1. the scf calculation is using 6 pools with 10 k-points, which means
>> >> that 4 pools have twice as much work to do as the others. In the
>> >> ideal case, the number of pools should be a divisor of the number of
>> >> k-points (i.e. 2, 5 or 10 in your case). Also, it is recommended that
>> >> the number of CPUs in a pool are a divisor of the number of CPUs on
>> >> each computing node, to avoid too much inter-node communication. In
>> >> your case, the best choice with 72 CPUs (on two nodes?) could be 2
>> >> pools. You may gain a bit of time, but this is not going to change a
>> >> lot. You should consider using more CPUs if you have the budget. For
>> >> example, 10 pools of 12 or 18 CPUs each.
>> >>
>> >> 2. The bands calculation runs on 12 CPUs and has a single k-point,
>> >> while each pool of the SCF one has up to 2 k-points. We would expect
>> >> that the bands calculation take about half as an scf step, i.e. about
>> >> 50 seconds. However, the bands calculation has some trouble
>> >> diagonalizing the Hamiltonian, you see it writes:
>> >>
>> >>      ethr =  2.76E-12,  avg # of iterations =120.0
>> >>
>> >> while typically the very last scf diagonalization is
>> >>
>> >>      ethr =  2.98E-12,  avg # of iterations =  3.3
>> >>
>> >> This is because, the scf calculation can start with a very good guess
>> >> good the wavefunction, while the bands calculation does not. It is
>> >> still faster than doing the entire scf procedure, but just by a
>> >> factor ~2.3
>> >>
>> >> Fortunately, you do not usually need the eigenvalues to a precision
>> >> of 10e-12. You can set the threshold by hand using the keyword
>> >> diago_thr_init, I guess 1.d-6 should be tight enough. However, double
>> >> check what you get in output, because I am half-suspecting that it
>> >> may be over-written by the value in the restart file
>> >>
>> >> cheers
>> >>
>> _______________________________________________
>> 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
>
> _______________________________________________
> 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



-- 
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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.quantum-espresso.org/pipermail/users/attachments/20201215/aaf3fc10/attachment.html>