Messages in this thread | | | Date | Mon, 25 Jul 2005 17:05:17 -0400 | From | Bill Davidsen <> | Subject | Re: Variable ticks |
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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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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