cx_lowest and CPU usage

[Kevin Oberman]

> From: Bengt Ahlgren <bengta at sics.se>
> Date: Thu, 14 Feb 2008 18:14:33 +0100
> Sender: owner-freebsd-acpi at freebsd.org
> 
> Andriy Gapon <avg at icyb.net.ua> writes:
> 
> > on 11/02/2008 23:41 Andriy Gapon said the following:
> >> on 01/02/2008 17:37 Andriy Gapon said the following:
> >>>> Andriy Gapon wrote:
> >>>>> Report for 7.0-RC1 on quite old hardware: 440BX-based motherboard,
> >>>>> 450Mhz Pentium III (Katmai).
> > [snip]
> >>>>> There is a weird thing: if I change cx_lowest to C2 when the machine is
> >>>>> completely idle, top shows that CPU usage for interrupts immediately
> >>>>> jumps to almost 20%. Change cx_lowest to C1, CPU usage drops back to
> >>>>> almost 0%.
> >>>>> Is this normal ?
> > [snip]
> >
> > I mis-reported the issue. Actually the above behavior occurs if I
> > throttle CPU 50% (via acpi throttling) and I am not concerned about this
> > at all.
> >
> > C2 has even stranger effects.
> > On almost idle system, with cx_lowest=C1, top reports about 0-2% user,
> > 0% nice, 0-2% system, 1-2% interrupt, 94-98% idle.
> > After changing cx_lowest to C2, I see the following: 0-2% user, 0% nice,
> > 0-2% system, 94-98% interrupt, 1-2% idle.
> 
> I see a similar effect on my TP with Pentium-M when it is in C3 or C4,
> but it's more in the order of 4% when in C3 and some 10-15% in C4.  I
> think that the additional time accounted to interrupts is due to the
> time it takes to wake the CPU up from the particular Cx-state.  My C3
> takes 85 (us?? or cycles???):
> 
> [root at P142 ~]# sysctl dev.cpu.0.cx_supported
> dev.cpu.0.cx_supported: C1/1 C2/1 C3/85
> 
> [...]
> 
> > Just in case, here's a little bit of sysctl output:
> > dev.cpu.0.freq: 448
> > dev.cpu.0.freq_levels: 448/-1 224/-1
> > dev.cpu.0.cx_supported: C1/0 C2/90
> > dev.cpu.0.cx_lowest: C2
> > dev.cpu.0.cx_usage: 1.71% 98.28%
> 
> With this slow CPU, a wakeup time of 90 from C2 could very well result
> in this much interupt time.  It just barely manages to wake up,
> execute the clock interrupt and go to sleep again before the next
> clock interrupt.  What if you reduce HZ?

Possible dumb question. Do you (either of you) have USB drivers in your
kernel or loaded?

This is just a shot in the dark, but I have seen weird things when I
have USB drivers loaded and usually don't load them on my laptop
until/unless I need them. At very least, USB kills the battery on my
T43 a LOT faster.
-- 
R. Kevin Oberman, Network Engineer
Energy Sciences Network (ESnet)
Ernest O. Lawrence Berkeley National Laboratory (Berkeley Lab)
E-mail: oberman at es.net			Phone: +1 510 486-8634
Key fingerprint:059B 2DDF 031C 9BA3 14A4  EADA 927D EBB3 987B 3751
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