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Re: A simple cpufreq(9)
On Thu, Sep 29, 2011 at 03:36:03PM -0500, David Young wrote:
> What's the difference in power savings between changing C-state and
> changing frequency? Do the power savings from every change in C-state
> dominate the savings from any change in frequency?
Depends on the machine. But generally on x86, C-states appear to be now the
dominant form. But these go side by side. To cut the corners short: the
general (hardware) idea is that while few CPUs are in a deep C-state (i.e.
idle), a group of other CPUs can enter a high-performance P-state. The net
result should be increase of performance, despite of the power management.
But obviously for instance ARM may do this all differenly, using only
> It seems that ultimately we need an API for telling a power-savings goal
> and constraints (latency, throughput, battery life, the screen isn't too
> dark to read) for the system to meet. Do you hope for someone to build
> that into the kernel on top of cpufreq(9)?
Not really; as I've written before, my opinion is that most of this should
be in the user space. The CPU PM is an exception for obvious reasons. There
could be a more involved API for cpu_idle(9) though (cf ).
 The Linux cpuidle-subsystem; http://lwn.net/Articles/384146/
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