Messages in this thread | | | Date | Tue, 27 May 2008 20:50:33 +0530 | From | Vaidyanathan Srinivasan <> | Subject | Re: [RFC PATCH v1 0/3] Scaled statistics using APERF/MPERF in x86 |
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* Arjan van de Ven <arjan@infradead.org> [2008-05-27 07:19:00]:
> \> > > > > that's a case where it really makes sense; it's the case where the > > > thing that controls the cpu P-state actually learns about how much > > > work was done to reevaluate what the cpu frequency should be going > > > forward. Eg it's a case of comparing actual frequency (APERF/MPERF) > > > to see what's useful to set next. > > > IDA makes this all needed due to the dynamic nature of the concept > > > of "frequency". > > > > Scaled statistics relative to maximum CPU capacity is just a method of > > exposing the actual CPU utilisation of applications independent of CPU > > frequency changes. > > > > Reason behind the metric is same as the above fact that you have > > mentioned. The CPU frequency governors cannot make decisions only > > based on idle time ratio. It needs to know current utilisation (used > > cycles) relative to maximum capacity so that the frequency can be > > changed to next higher level. > > > > Higher level management software that wants to control CPU capacity > > externally will need similar information. > > > I entirely understand that desire.
Good :)
> But you're not giving it that information! > The patch is giving it a really poor approximation, an approximation > that will get worse and worse in upcoming cpu generations.
I agree that power capping and acceleration makes the metric approximate. But was are trying to be as accurate and meaningful as APERF/MPERF ratio is in the processor hardware.
Can I state the problem like this: The metric is as accurate and meaningful as APERF/MEPRF ratio, but the interpretation of the metric is subject to the knowledge of power constraint or acceleration currently in effect.
--Vaidy
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