[ESPResSo-devel] [bug #44133] Temperature in LB_GPU is too high

[Georg Rempfer]

Follow-up Comment #6, bug #44133 (project espressomd):

(7) If there are no particles in the system, the global seed is not propagated
to the GPU, which then uses uninitialized memory as seed (according to
Dominic).


@Axel: By using thread_seed=global_seed+thread_index, subsequent runs whose
global_seed doesn't differ by number_of_threads or more produce correlated
temperatures. The reason is that the simulation uses the same random numbers
to create the noise, just at shifted positions in the grid. The shift doesn't
matter so much, since the temperature uses the summed kinetic energies. The
only difference between the two systems (considering temperature) comes from
the first few threads, that actually use new random numbers, as well as the
different spacial relations of the "noises". If this does indeed explain what
we see, then the drifting apart of two runs with two very similar seeds (+6)
takes a very long time, though.

Of course this is not a problem in principle, one just needs to use
sufficiently different seeds, but seeding with the plain pid is common (as in
the test case), that's why I thought it should be changed. Dominic uses PID^k
with a large k from the TCL level.  Something like that would already work.

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