David Vengerov wrote: > Mark Haywood wrote: > >> David Vengerov wrote: >> >>> 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. >> >> >> Actually, for x86 this already defined for you. CPUs cannot >> necessarily be changed to an arbitrary frequency. Usually, there are a >> limited number of frequencies that are supported and those frequencies >> are exported to the OS via the ACPI _PSS objects. > > Yes, there are only several possible frequencies that can be chosen, but > which one of them should be chosen? It is not optimal to keep stepping > through them after a certain jump in the CPU utilization, and a jump > between frequencies might be more appropriate. >
An excessive concern w/ optimality is unwarranted, I think. As long as the system continues to adaptive search for an optimum power state and avoids unnecessary oscillations or poor steady-state selection of power levels, whether a new power level is reached in 1 or 2 or 3 steps is probably insignificant in terms of net power savings. - Bart -- Bart Smaalders Solaris Kernel Performance barts at cyber.eng.sun.com http://blogs.sun.com/barts
