Thanks, Bart. It seems then that there two types of policies that can be deployed in a system. The first type of policy decreases CPU clock frequency if the CPU utilization drops below 100% and increases the frequency as the CPU utilization rises. The interesting question with this policy is what frequency f should be used (as a fraction of the maximum) when a certain CPU utilization is observed. As Bart pointed out, different workloads will respond differently to decreases in CPU frequency (the CPU utilization may rise proportionately or it may not rise at all). The best way to approach this problem, I think, is to ask the user to specify (or choose from several options) a utility curve describing the accepted performance degradation vs. a decrease in the consumed power. Then, the application feedback can be used to tune the policy that sets CPU frequency based on observed CPU utilization so as to maximize the user utility. What do you think about this approach? Is this something you would like to experiment with?
The second type of policy decides whether the load on several CPUs should be "compacted" into fewer CPUs so as to create some idle CPUs that can be kept running at the minimum frequency. This decision can be made based on the current and recent utilization of the CPUs, their run queue lengths, etc. The ultimate choice between policies of this type should be made based on the application feedback and on the performance vs. power utility curve specified, so that the policy that maximizes the final utility should be chosen. Do you think this approach is also worth evaluating? David Bart Smaalders wrote: > David Vengerov wrote: > >> Mark, >> Thank you, Mark, for the link describing the Intel Enhanced Intel >> SpeedStep Technology. I noticed that they suggest for the kernel to >> poll each CPU at regular time intervals, asking about its >> utilization. Then, they plan to adjust the CPU frequency based on its >> utilization. >> >> Do you guys think that the CPU frequency should be changed in >> response to changes in CPU utilization? > > > Sure - if the CPU is 100% busy and then becomes idle, reducing CPU > frequency seems like a reasonable plan. Of course, if the only > thing the user cares about is response latency to infrequent events, > this policy won't work well. But as a default, it's a pretty good > choice. > >> If so, should we expect the CPU utilization to rise as the CPU >> frequency is decreased and vice versa? > > > Depends on workload. If code is memory bound on a non-threaded CPU > when it runs, CPU freq. will have little effect on either throughput > or CPU utilization. > > On the other hand, if we're computing Pi periodically, I would > expect utilization to go up as freq. goes down. > > - Bart > >
