Detecting CPU throttling on over temperature

Daniel O'Connor doconnor at
Wed Sep 9 07:43:39 UTC 2009

On Wed, 9 Sep 2009, Robert Noland wrote:
> On Wed, 2009-09-09 at 10:20 +0930, Daniel O'Connor wrote:
> > On Wed, 9 Sep 2009, Ian Smith wrote:
> > >  > > Does anyone know if it is possible to determine if this is
> > >  > > the case? ie is there a way to be informed if throttling has
> > >  > > occurred?
> > >
> > > Might be easier to hack powerd.c as an existing pretty
> > > lightweight way of monitoring CPU freq (to log or signal on
> > > detected freq lowered by throttling, say?) even if you don't
> > > need/want it to actually vary freq according to load, eg setting
> > > idle/busy shift factors to 'never/always'?
> >
> > Hmm, that could work.
> >
> > It seems odd to me that there is no direct way the BIOS can notify
> > the OS it's throttling the CPU though.
> Some BIOS can and do send an ACPI event when the proc gets hot.  In
> my experience, this was not a good thing though.  The BIOS that I
> remember dealing with this on would continuously send the alarms, so
> while TCC would kick in and throttle the CPU, the event processing
> kept it at 100% utilization until it was powered off to cool.  I have

This system seems to stall for a few seconds and then come back, I 
haven't see any messages about it in dmesg though.

> also been able to determine that TCC had kicked in by looking at the
> cpu frequency via sysctl and comparing that to the max frequency
> reported for the proc.

Yeah, although I couldn't run ps when the CPU was stalled so I'm not 
sure if I'd catch it or not :)

> If the BIOS sent the alarm, but throttled the rate it wouldn't have
> been so bad.  Not that I had any active fan control on that box to do
> anything about it really, but TCC might have actually worked if it
> wasn't flooding the acpi event processor.

Having the BIOS or CPU do it automatically is sensible since it's a time 
critical task.. Some basic notification would be nice though. It 
boggles my mind how difficult it is to do such basic things sometimes..

Daniel O'Connor software and network engineer
for Genesis Software -
"The nice thing about standards is that there
are so many of them to choose from."
  -- Andrew Tanenbaum
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