i have a kthread which runs alone on one core from a multi-core CPU. This kthread disables all IRQs for that core, runs a loop as fast as possible and measures the maximum loop duration with the help of the TSC. The whole ACPI stuff is disabled (no frequency scaling, no power saving, etc.).
My problem is, that the maximum loop duration apparently depends on the gpu.
- When the system is used normal (a little bit office, Internet and programming stuff / not really busy) then the maximum loop duration is around 5 us :-(
- The same situation, but with a stressed CPU (the other three cores are 100% busy) leads to a maximum loop duration of approximately 1 us :-|
- But when the GPU is switching into idle mode (turning-off the screen), then the maximum loop duration is going down to less than 300 ns :-)
Why is that? And how can i influence this behavior? I thought the CPU and the RAM are directly connected. I recognized, that the maximum loop duration becomes better on a system with a external graphic card for the first situation. For the second and third case i couldn't see a difference. I also tested AMD and Intel systems without success - always the same :-(
I'm fine with the second case. But is it possible to achieve that without stressing the CPU additionally?
Many thanks in advance!
Billy
