[DFTB-Plus-User] Possible bug in DFTB+ with MPI

[David Furman]

 Hi all,
I have a question concerning the running of DFTB+ with MPI. There seems to
be a problem when running more than one DFTB+ instance (with MPI
parallelization) on the same system.

When I run 4 instances (separate folders) of dftb+ with 2 cores each (i.e.
8 cores in total):
mpirun.openmpi -np 2 dftb+, each %CPU usage drops to 50%.

This is the 'top' output:
========================================================
 35426 user    20   0  423384  40404  18248 R  50.5  0.0   0:37.69 dftb+
 35434 user    20   0  423080  39720  18248 R  50.2  0.0   0:05.70 dftb+
 35396 user    20   0  422044  38660  17644 R  49.8  0.0   1:21.35 dftb+
 35402 user    20   0  423384  40460  18304 R  49.8  0.0   1:09.54 dftb+
 35403 user    20   0  422044  38872  17848 R  49.8  0.0   1:16.15 dftb+
 35427 user    20   0  422044  39048  18032 R  49.8  0.0   0:32.93 dftb+
 35435 user    20   0  421732  38164  17716 R  49.8  0.0   0:09.07 dftb+
 35395 user    20   0  423384  40360  18204 R  49.5  0.0   1:13.18 dftb+
=========================================================

Whereas, when I run one instance with 8 cores, the efficiency is 100% as
expected:

=========================================================
 35837 user    20   0  441868  35196  19112 R 100.3  0.0   0:04.90 dftb+
 35828 user    20   0  442580  35676  19120 R 100.0  0.0   0:04.83 dftb+
 35829 user    20   0  442544  36024  19432 R 100.0  0.0   0:04.91 dftb+
 35830 user    20   0  441912  35356  19360 R 100.0  0.0   0:04.91 dftb+
 35831 user    20   0  441688  35048  19304 R 100.0  0.0   0:04.90 dftb+
 35833 user    20   0  441904  35656  19500 R 100.0  0.0   0:04.91 dftb+
 35841 user    20   0  441516  34824  19076 R 100.0  0.0   0:04.89 dftb+
 35843 user    20   0  441224  34456  18868 R  99.7  0.0   0:04.89 dftb+
=========================================================

I run both cases with OMP_NUM_THREADS=1.
with open-mpi 1.10.2 and gcc 5.4.0.

Could anyone give a hint about what is wrong?
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