Re: So, what's the status on the recent patches here?

Preece Scott-PREECE
2006-08-30T13:39:44Z

> [mailto:linux-pm-bounces@lists.osdl.org] On Behalf Of Matthew Locke

> Latency is a good example of how mixing sleep states with
> operating points doesn't quite work. Latency for switching
> to a new operating
> point is very a much a function of the current operating point. I
> think latency for sleep states is the same every time. Also
> which direction are you capturing latency for, suspend or
> resume? If your power manager is making decisions based on
> latency, I could imagine that it needs to know the latency
> for going into and out of that state.
---

Well, you could also argue that the ability to express latency is
a good argument for having an approach that does mingle frequency-based,

voltage-based, and sleep-state Ops, though not necessarily exactly
what oppoint does! That is, if the policy manager is to make good
decisions about whether to shift to a different frequency or sleep
at one of n possible sleep levels, it would exactly want to be able
to consider the alternatives in terms of the expected time and power
cost of each possible transition and whether the expected time to
remain in that state would be sufficient to recoup those costs.

That is, you'd like a big state-transition table that has a tuple of
costs associated with each possible transition from each possible state.

The utility of the expected times, of course, depends on whether you
are able to infer anything about how long an idle (or low-work)
period is likely to persist. I think this is a fertile area for
experimentation, though we haven't had time to do it, yet. And,
while it's superficially easier to take on for embedded devices
that have limited numbers of states, the notion of a system manager
that watches work patterns and their corresponding load levels and
idle times is not particularly far-fetched.

On the other hand, getting infrastructure in place is more
important, today! But, that said, if user-space interfaces
are forever, you would like to present a user-space interface
that can be easily extended to support such notions in the future,
without breaking compatibility.

Scott