Re: setting NTP time sync interval on vxWorks

[Ralph Lange <ralph.lange@gmx.de>]

Hi Yuliang,

On 06/07/2015 04:14, Zhang Yuliang wrote:
> > Why do you want to change the sync interval?
> Originally, i want to improve the accuracy of the vxWorks internal clock by shorter the sync interval.
> Now, based on your reply ,it think maybe it will not impove much.

Accuracy is a beast. ;-)

On vxWorks, the resolution of the EPICS timestamp is the resolution of 
the system ticks. Look at the code for NTPTimeGetCurrent(): it checks 
ticksSince = if ticks have passed since the last clock read time stamp 
(skipping ticks if the system clock was > 1 tick faster than the NTP 
clock at the last synchronization), and then adds ticksSince * tick 
period to the clock read time stamp.

No matter what you do, this will limit the accuracy to the resolution of 
+/- 1 tick period = +/- 16.67ms (for the default 60Hz ticks).

Any deviation of the tick clock will be in addition to that. The 
deviation correction is done in ticks, too. No matter how often you 
sync, the correction will happen when the error has grown to one tick, 
pulling down accuracy by another tick period.
The deviation over one sync rate adds on top. In case of your 60.7Hz 
tick clock that runs 7.2ms per minute too fast, the worst case is one 
tick plus one minute (sync rate) of deviation, i.e. at 23.8ms off NTP time.
Add the worst case resolution error of 16.67ms to get a maximum 
difference between system and NTP time stamps of 40.4ms.

Shortening the NTP sync interval will reduce the deviation error towards 
its minimum of 16.67ms, but can't do anything against the resolution 
error of 16.67ms.

Obviously, the only effective way of improving the accuracy is pushing 
up the vxWorks system tick rate. But be careful: This increases the 
interrupt load which some smaller systems / slower CPUs might not be 
able to cope with. Also I have seen code that hard codes the tick rate 
instead of using sysClkRateGet().

Most newer CPUs have hardware on board (e.g. usec precision programmable 
32bit counters) that would allow a much more precise time stamp without 
increasing the interrupt load. If you give up portability across CPU 
types, you can write an EPICS General Time Provider that uses such 
hardware to obtain high accuracy time stamping for your records.

[Or use Linux that has a 1kHz tick rate and a real NTP daemon (that is 
bending the system clock), which will get you into the 1ms accuracy range.]

> Another question:
>   If the vxWorks clock rate is 60 Hz and measured clock rate is 60.7 Hz, and suppose the networks is ok(ntp client can be synced every time per minute). Is the max deviation less than one tick(16.67ms)?

See above: the worst case for the difference between system and NTP 
stamps is always = 2 * tick period + max deviation over one sync interval

Cheers,
~Ralph


> > -----原始邮件-----
> > 发件人: "Ralph Lange" <ralph.lange@gmx.de>
> > 发送时间: 2015年7月3日 星期五
> > 收件人: "EPICS Tech-Talk" <tech-talk@aps.anl.gov>
> > 抄送: "Zhang Yuliang" <zhangyl@ihep.ac.cn>
> > 主题: Re: setting NTP time sync interval on vxWorks
> >
> > Hi Yuliang,
> >
> > The NTP sync interval is a compile time constant, thus it cannot be
> > changed at run time.
> >
> > One question, though.
> > Your time report shows that the measured tick rate is 60.007 Hz, which
> > is a difference of about 2 us per tick, 120 us per second, 7.2 ms per
> > minute. Time resolution is in ticks, i.e. 16.67 ms. That means the NTP
> > sync will skip a single tick in the counter every two or three minutes.
> > Why do you want to change the sync interval?
> > Setting it longer will make the situation worse: more than one tick will
> > be skipped at larger intervals.
> > Setting it shorter will increase network traffic without changing the
> > situation much: a deviation of 7.2 ms per minute will always be
> > corrected by skipping one tick after between two and three minutes.
> >
> > Cheers,
> > ~Ralph
> >
> >
> > On 03/07/2015 10:36, Zhang Yuliang wrote:
> > > Hi all,
> > >
> > > I want to know how to set the NTP time sync interval on vxWorks. The output of "NTPTime_Report 1" shows the default interval is 60 seconds:
> > >
> > > timingMain>NTPTime_Report 1
> > > NTP driver is synchronized with server
> > > Syncronization interval = 60.0 seconds
> > > Last synchronized at 2015-07-03 16:06:19.062927
> > > OS tick rate = 60 Hz (nominal)
> > > Measured tick rate = 60.007 Hz
> > > NTP Server = 10.1.201.31
> > > value = 0 = 0x0
> > >
> > > I can modify the variable "NTPTimeSyncInterval" in file $(EPICS_BASE)\src\libCom\osi\osiNTPTime.c to change the sync interval. Is there a command can be used in ioc shell?
> > >
> > > Best regards,
> > > Zhang Yuliang