[ntp:questions] Support for "tickless" systems

David Taylor david-taylor at blueyonder.co.uk.invalid
Thu Nov 20 07:27:47 UTC 2014


On 19/11/2014 11:56, Miroslav Lichvar wrote:
> On Wed, Nov 19, 2014 at 10:09:42AM +0000, David Taylor wrote:
>> In bug 2314, I reported that the jitter was always reported as 0 soon after
>> NTP had started, and this was traced to the Linux in use on the Raspberry Pi
>> being tickless.  Recompiling the kernel without the tickless option was a
>> work-round, but is it possible to get jitter values with a tickless system?
>
> There was a problem with clock stability in the tickless mode on idle
> systems, which should be fixed or at least significantly improved in
> 3.17. I'm not sure how it could cause the jitter to be reported as
> zero though.
>
> 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.

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 found these 
diagnostics:

pi at raspberrypi ~ $ dmesg | grep -i nohz
[    0.000000] Kernel command line: dma.dmachans=0x7f35 
bcm2708_fb.fbwidth=592 bcm2708_fb.fbheight=448 bcm2708.boardrev=0x10 
bcm2708.serial=0x15e75d6b smsc95xx.macaddr=B8:27:EB:E7:5D:6B 
bcm2708_fb.fbswap=1 bcm2708.disk_led_gpio=47 
bcm2708.disk_led_active_low=0 sdhci-bcm2708.emmc_clock_freq=250000000 
vc_mem.mem_base=0x1ec00000 vc_mem.mem_size=0x20000000 
dwc_otg.lpm_enable=0 console=tty1 root=/dev/mmcblk0p6 rootfstype=ext4 
elevator=deadline rootwait bcm2708.pps_gpio_enable 
bcm2708.pps_gpio_pin=18  nohz=off

which just reflects the edit I made to cmdline.txt, and:

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.

-- 
Thanks,
David
Web: http://www.satsignal.eu



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