[gmx-users] GPU gets faster with more molecules in system

[Christian Mötzing]

Hi,

I compiled mdrun-gpu and tried some waterbox systems with different
atoms counts.

atoms  | GPU    | CPU
2.400  | 1.015s | 774s
4.800  | 1.225s | 1.202s
9.600  | 1.142s | 1.353s
19.200 | 2.984s | 2.812s

Why does the system with 9.600 atoms finish faster than the one with
4.800? I tripple checked the simualtions and even GROMACs tells me that
the atom count in the system is like above. So I think no mistaken
there. A diff of md.log only shows differences in output values for each
step.

Is there any explanation for this behaviour?

Christian