Most probably your GPU was the bottleneck over london. Not only Vram usage, as said above, but geneal computing power of the GPU. (Needs to shade all the buildings etc) Thus, the CPUs did not need to compute to their full power, as the GPU could not accept new instructions. (The maximum prerendered frames setting in your nvidia control panel affects this too, as this is the amount of frames the GPU renders ahead, if the GPU is stilly busy and does not accept the CPU date at the moment). When leaving london, the GPU usage decreased and now the CPUs could be used more, as the GPU was accepting new data faster then over london.
The fact that you see increased usage of the second GPU (not in SLI) when the first one saturated might be the nvidia driver using the 2nd GPU for the windows background processes.
|