Re: Variable ticks

[Bill Davidsen <davidsen@tmr.com>]

Brown, Len wrote:
>  
>
> > I was thinking about variable tick times, and I can think of three 
> > classes of action needing CPU attention.
> > - device interrupts, which occur at no predictable time but would pull 
> > the CPU out of a HLT or low power state.
> > - process sleeps of various kinds, which have a known time of 
> > occurence.
> > - polled devices...
> >
> > Question one, are there other actions to consider?
>
>
> Yes.
> Speaking for ACPI C3 state, note that DMA also
> wakes up the CPU -- even if there was no device interrupt.
> (aka, "the trouble with USB")

Trouble? Why would USB do DMA unless there was a device activity?
> > Question two, what about those polled devices?
>
>
> it is a real challenge to save power under such conditions,
> unless you can throttle the polling rate such that
> the processor can actually enters idle while polling
> is underway.
>
>
> > I've been asked to give a high level overview of the recent discussion 
> > for a meeting, and while I want to keep it at the level of "slower 
> > clock, fewer interrupts" and "faster clock, better sleep 
> > resolution," I 
> > don't want to leave out any important issues, or be asked a question 
> > (like how to handle polling devices) when I have no idea what 
> > people are thinking in an area.
>
>
> From a power management point of view, what is important
> is what we do when idle.  ie. on my laptop, from a power
> savings point of view, I wouldn't care
> much if HZ=1000 all the time if HZ=0 when in idle.

That could hurt the polling performance, all right. ;-)
> Outside of idle, the tick rate is much less important to
> power savings, unless the change in tick rate was significant
> enough to change the load enough that we'd want to change the
> target non-idle MHz of the processor.

Isn't that more or less what on demand does?

Thanks for the feedback, I can probably just say that DMA wakes the CPU 
from C3 and let it ride, I don't want to skip it, but neither do I need 
to go into detail.

--
   -bill davidsen (davidsen@tmr.com)
"The secret to procrastination is to put things off until the
 last possible moment - but no longer"  -me
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