On 20/11/2014 09:10, Miroslav Lichvar wrote:
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@raspberrypi ~ $ cat /proc/interrupts | grep -i time
3: 4351879 ARMCTRL BCM2708 Timer Tick
pi@raspberrypi ~ $ sleep 10
pi@raspberrypi ~ $ cat /proc/interrupts | grep -i time
3: 4353699 ARMCTRL BCM2708 Timer Tick
pi@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,
I have not been able to compile the current kernel. With the previous
tests removing the "Tickless system" option restored non-zero jitter values.
http://bugs.ntp.org/show_bug.cgi?id=2314
I don't have a copy of the kernel config file. I guess that
/proc/interrupts is a count of interrupts? I didn't realise that. Let
me check.....
Running the sleep 10 sequence from a command procedure gives a
difference of 1055, so I guess that's 105.5 interrupts per second. Does
sound like 100 Hz, yes.
Running the command while another terminal was running "cat /dev/urandom
> /dev/null" resulted in 1063 interrupts, so 106.3 Hz.
Does that mean I'm tickless or not?
--
Cheers,
David
Web: http://www.satsignal.eu
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