linux-kernel - Re: [RFC] maximum latency tracking infrastructure

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Date:	Thu, 24 Aug 2006 23:57:24 +0200
From:	Arjan van de Ven <arjan@...ux.intel.com>
To:	dwalker@...sta.com
Cc:	linux-kernel@...r.kernel.org, len.brown@...el.com
Subject: Re: [RFC] maximum latency tracking infrastructure

On Thu, 2006-08-24 at 14:52 -0700, Daniel Walker wrote:
> On Thu, 2006-08-24 at 19:41 +0200, Arjan van de Ven wrote:
> > Subject: [RFC] maximum latency tracking infrastructure
> > From: Arjan van de Ven <arjan@...ux.intel.com>
> > 
> > The patch below adds infrastructure to track "maximum allowable latency" for power
> > saving policies.
> > 
> > The reason for adding this infrastructure is that power management in the
> > idle loop needs to make a tradeoff between latency and power savings (deeper
> > power save modes have a longer latency to running code again). 
> > The code that today makes this tradeoff just does a rather simple algorithm;
> 
> I was just thinking that it might be cleaner to register a structure
> instead of tracking identifiers to usecs. You might get a speed up on
> some of the operations, like unregister.

it makes things a lot more complex for both the user and the
infrastructure though, and I doubt it's going to be a performance gain;
you need to walk all registered items anyway to decide the new minimum
value if you unregister one for example.

> Another thing I was thinking about is that this seems somewhat contrary
> to the idea of using dynamic tick (assuming it was in mainline) to
> heuristically pick a power state. Do you have any thoughts on how you
> would combine the two?

Actually it's designed in part FOR this case!
So how that will work (thought experiment, I don't have the code yet)

In idle, determine the time the next scheduled event is.
Then given that time go over the C-states and pick the deepest C-state
that
1) satisfies the requested latency
2) has a latency that is a small enough fraction of the total time

(2 is needed to not pick a 1 msec-latency C state for a 1ms idle, that
won't save you power most likely, so you need to have enough time in
"real" idle)

so when you know your latency requirements, you now can pick a DEEPER
sleepstate than you could before (or at least the right one)... dynticks
needs this more than anything :)

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