RE: [linux-pm] [PATCH] cpuidle: avoid singing capacitors



-----Original Message-----
From: Pierre Ossman [mailto:drzeus-list@xxxxxxxxx]
Sent: Tuesday, March 04, 2008 9:19 AM
To: Pallipadi, Venkatesh
Cc: Dave Jones; Andi Kleen; Alan Stern; LKML; Adam Belay; Lee
Revell; linux-pm@xxxxxxxxxxxxxxxxxxxxxxxxxx; Pavel Machek
Subject: Re: [linux-pm] [PATCH] cpuidle: avoid singing capacitors

On Tue, 4 Mar 2008 07:14:23 +0100
Pierre Ossman <drzeus-list@xxxxxxxxx> wrote:


Many devices today are of a less than stellar quality, and singing
capacitors are a common problem. A high-pitch noise is
created, caused
by power fluctuations as the processor enters and leaves
deep sleep at
a high frequency.

Instead of just disabling the deep sleep (which wastes power), this
patch allows you to reduces the number of times it is entered so that
the frequency can be kept inaudible.

Signed-off-by: Pierre Ossman <drzeus@xxxxxxxxx>
--

Hold off on this. It turns out it still has major problems.
The menu algorithm now and then gets really bad at predicting
sleep times, completely killing this avoidance scheme.

(On that subject, does anyone except Adam understand that
algorithm? Some comments wouldn't hurt...)

Prediction is based on cumulative time till "non expected wakeup". So,
prediction will come into play only when there are very short wakeups
due to "unexpected wakeups".

So for now, I'm back to thinking that measuring the interval
between deep sleeps is the better approach. I could use some
ideas for a good clock source though. I haven't dug much
deeper than jiffies when it comes to kernel timekeeping.

I think best solution is to use get_last_residency that is already
there. If the last residency or expected_us is very low, you can avoid
deep idle states. That way you don't have to depend on jiffies being
updated at the time you are checking it.

Thanks,
Venki
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