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
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