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