Date: Thu, 14 Feb 2008 18:14:33 +0100 From: Bengt Ahlgren <bengta@sics.se> To: Andriy Gapon <avg@icyb.net.ua> Cc: freebsd-acpi@freebsd.org Subject: Re: cx_lowest and CPU usage Message-ID: <uh7bq6jcveu.fsf@P142.sics.se> In-Reply-To: <47B4103A.6090902@icyb.net.ua> (Andriy Gapon's message of "Thu\, 14 Feb 2008 11\:56\:10 %2B0200") References: <479F0ED4.9030709@icyb.net.ua> <479F62D9.6080703@root.org> <47A33CCB.3090902@icyb.net.ua> <47B0C10F.6000109@icyb.net.ua> <47B4103A.6090902@icyb.net.ua>
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Andriy Gapon <avg@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@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? Bengt
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