[ntp:questions] Support for "tickless" systems

Miroslav Lichvar mlichvar at redhat.com
Thu Nov 20 09:10:25 UTC 2014


On Thu, Nov 20, 2014 at 07:27:47AM +0000, David Taylor wrote:
> On 19/11/2014 11:56, Miroslav Lichvar wrote:
> >Can you try 3.17 or later and see if it's fixed? Also, it would be
> >interesting to know if adding nohz=off to the kernel command line
> >instead of recompiling works as a workaround too.
> 
> I found the right file (thanks, Rob, yes there are more options as you say)
> and tried setting nohz=off but it made no difference - jitter still reported
> as zero.

Interesting. When you tested the kernel compiled without CONFIG_NO_HZ,
where ntpd reported non-zero jitter, was that the only difference
compared to the original kernel which reported zero jitter?

> How would I tell whether the nohz=off was actually accepted or not, i.e. how
> to determine whether the kernel is tickless or not?

I'm not sure if there is any reliable way to tell that from
user-space, beside parsing the kernel command line.

> pi at raspberrypi ~ $ cat /proc/interrupts | grep -i time
>   3:    4351879   ARMCTRL  BCM2708 Timer Tick
> pi at raspberrypi ~ $ sleep 10
> pi at raspberrypi ~ $ cat /proc/interrupts | grep -i time
>   3:    4353699   ARMCTRL  BCM2708 Timer Tick
> pi at raspberrypi ~ $
> 
> I don't know how to interpret the difference of 1820 in those two numbers.
> The first two commands were typed by hand, by the way, the third with an
> up-arrow recall.

That's between 100 and 250 Hz, so the kernel could be compiled with
CONFIG_HZ=100. Do you see that in the kernel config file? Does the
interrupt rate change significantly when you load the CPU, e.g. by
running "cat /dev/urandom > /dev/null" ?

-- 
Miroslav Lichvar


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