One-shot-oriented event timers management

Alexander Motin mav at
Mon Aug 30 10:07:47 UTC 2010

Gary Jennejohn wrote:
> Hmm.  I applied your patches and am now running the new kernel.  But I
> only installed the new kernel and didn't do make buildworld installworld.
> Mu systat -vm 1 doesn't look anything like yours.  I'm seeing about 2300
> interrupts per second and most of those are coming from the hpet timers:
> 1122 hpet0:t0
> 1124 hpet0:t1

It means 1000Hz of hardclock (hz) events mixed with 127Hz of statclock
(stathz) events. HPET timer here works in one-shot mode handling it.

> So, what else did you do to reduce interrupts so much?
> Ah, I think I see it now.  My desktop has only C1 enabled.  Is that it?
> Unfortunately, it appears that only C1 is supported :(

Yes, as I have said, at this moment empty ticks skipped only while CPU
is in C2/C3 states. In C1 state there is no way to handle lost events on
wake up. While it may be not very dangerous, it is not very good.

Alexander Motin

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