[ntp:questions] PPS jitter in Linux

DaveB g8kbv at uko2.co.uk
Wed Apr 11 09:37:47 UTC 2012

In article <jldmke$5ff$1 at dont-email.me>, Null at BlackList.Anitech-
Systems.invalid says...
> Mike S wrote:
> > Chris Albertson wrote:
> >> Mike S<mikes at flatsurface.com>  wrote:
> > I've played around with different cpufreq setting, thinking
> >>  it might be related to the processor speed during an IRQ,
> >>  but that seems to have minimal impact
> >> (performance vs. conservative vs. ondemand).
> >>
> >> I think your CPU goes into some power saving mode when
> >>  there is no load and takes some number of micro seconds
> >>  to wake up after an interrupt.
> >> Keeping a load on it prevents the sleep or power save
> >>  mode and hence the need to "wake up."
> >
> > Then explain why setting the cpufreq governor to "performance"
> >  (which locks it to the highest frequency (
> > http://www.mjmwired.net/kernel/Documentation/cpu-freq/governors.txt )
> >  has no significant effect.
> _Exactly_ which CPU are you using?
> FYI, There are some modern CPUs that ignore OS, BIOS, ...
>  under certain conditions, and will power and/or frequency
>  manage the cores for thermal management.

Later Intel chips in general do that, cycle stealing/skipping to keep 
the heat down in desperation when the cooler fall's off!  I had it 
happen on a desktop, a plastic clip failed during an office move.   IT 
decided to scrap the machine as unrepariable (they didnt even take the 
covers off.)  Needless to say, its still working fine some years later 
after the cooler was cleaned and refitted.

There are Y'tube videos from others showing it's effects on a Windows 
machine (slows to a real crawl, but does keep working.)

It is said some AMD chips will burn up under the same situation.  I have 
no experience of that.

I have however noticed that in general Linux causes a given PC to 
consume much more power than when running any recent Windows.  As found 
by monitoring the DC power consumption of a laptop, without the battery 

Ubuntu 11.10 Linux is very much more "hungry" than Windows XP pro SP3 on 
the same machine.  (An Acer Aspire 3610 from memory.)  The resulting 
heat from the CPU cooler, is also good to keep the coffee warm.

And that's just sitting at the desktop some minutes after booting, with 
no app's running, and no other activity (that I know of.)


Dave B.

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