[ntp:questions] Support for "tickless" systems
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? I would like to re-open this bug if NTP can be made to report jitter correctly on a tickless Linux. -- Cheers, David Web: http://www.satsignal.eu ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Support for "tickless" systems
On Wed, Nov 19, 2014 at 10:09:42AM +, 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. -- Miroslav Lichvar ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Support for "tickless" systems
3.17 what? Miroslav Lichvar wrote: > On Wed, Nov 19, 2014 at 10:09:42AM +, 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. > > -- > Miroslav Lichvar ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Support for "tickless" systems
On 19/11/2014 11:56, Miroslav Lichvar wrote: On Wed, Nov 19, 2014 at 10:09:42AM +, 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. The latest Linux for the Raspberry Pi is 3.12.32+, I'm afraid. Would the command-line you speak of be the one containing - for example - console=tty1 rootfstype=ext4 -- Thanks, David Web: http://www.satsignal.eu ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Support for "tickless" systems
David Taylor wrote: > On 19/11/2014 11:56, Miroslav Lichvar wrote: >> On Wed, Nov 19, 2014 at 10:09:42AM +, 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. > > The latest Linux for the Raspberry Pi is 3.12.32+, I'm afraid. > > Would the command-line you speak of be the one containing - for example > - console=tty1 rootfstype=ext4 Yes, but normally there are a lot more options than that. You can edit /boot/cmdline.txt and reboot. But be careful that you don't put garbage there and make the boot fail. When that still happens you can put the card in another system (laptop, PC via USB adapter) and edit it again. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Support for "tickless" systems
On 19/11/2014 11:56, Miroslav Lichvar wrote: On Wed, Nov 19, 2014 at 10:09:42AM +, 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 [0.00] 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=25000 vc_mem.mem_base=0x1ec0 vc_mem.mem_size=0x2000 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 ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
