Different simulations depending on number of CPU?

Submitted by Baric on Wed, 03/29/2017 - 17:32

Hi everybody,
I could not find anything about this in the forum yet, so maybe I missed it. But I have experienced something weird lately.
When I run the same simulation on 1 or 2 CPU I get different results. And by different I mean slightly different in the trajectory and the forces.
You can check this with the tutorial movingMeshGran. It does not matter how often you run it on a single core, the result is always the same (checked with diff on the dump files, no difference). As soon as I start a parallel run they differ. And not only this, 2 CPU differ from 4 CPU usw..

Example:
In the movingMeshGran, I get 1355 particles left at timestep 30000, while I have 1326 particles left on 2 CPU. So it is not just some last few digits but real differences in the trajectory.

I have also tried using mpirun -np 1 and no mpirun, which does not make any difference.

What can be the reason for this? Does anyone know?

Regards
Valentin

Forums:

Daniel Queteschiner | Thu, 03/30/2017 - 09:01

Particle insertion depends on the number of processors used, for example, due to optimization reasons. This is expected behaviour.