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What should the poll be for the shared memory driver (type 28)?
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David Taylor
Jun 17, 2013, 10:24:42 AM6/17/13
to
What should the poll be for the shared memory driver (type 28)? This
Web page:
http://www.eecis.udel.edu/~mills/ntp/html/drivers/driver28.html
suggests that the driver internally has a poll interval of 1 second,
another document I've seen suggests setting:
server 127.127.28.1 minpoll 1 prefer
and yet in the ntpq -p billboard the poll is shown as 8 seconds.
Loopstats are reported at 8 second intervals, and the poll interval is
shown as 3 (i.e. 8 seconds).
I'm confused! What should be the correct setting in the ntp.conf file?
BTW: this comes about from Folkert van Heusden who is using a user-mode
program to make NTP on a Raspberry Pi more precise, i.e. providing PPS
support without having to modify the kernel. See:
http://www.febo.com/pipermail/time-nuts/2013-June/077484.html
--
Thanks,
David
Web: http://www.satsignal.eu
Web page:
http://www.eecis.udel.edu/~mills/ntp/html/drivers/driver28.html
suggests that the driver internally has a poll interval of 1 second,
another document I've seen suggests setting:
server 127.127.28.1 minpoll 1 prefer
and yet in the ntpq -p billboard the poll is shown as 8 seconds.
Loopstats are reported at 8 second intervals, and the poll interval is
shown as 3 (i.e. 8 seconds).
I'm confused! What should be the correct setting in the ntp.conf file?
BTW: this comes about from Folkert van Heusden who is using a user-mode
program to make NTP on a Raspberry Pi more precise, i.e. providing PPS
support without having to modify the kernel. See:
http://www.febo.com/pipermail/time-nuts/2013-June/077484.html
--
Thanks,
David
Web: http://www.satsignal.eu
David Lord
Jun 17, 2013, 12:12:25 PM6/17/13
to
David Taylor wrote:
> What should the poll be for the shared memory driver (type 28)? This
> Web page:
>
> http://www.eecis.udel.edu/~mills/ntp/html/drivers/driver28.html
>
> suggests that the driver internally has a poll interval of 1 second,
> another document I've seen suggests setting:
>
> server 127.127.28.1 minpoll 1 prefer
>
> and yet in the ntpq -p billboard the poll is shown as 8 seconds.
> Loopstats are reported at 8 second intervals, and the poll interval is
> shown as 3 (i.e. 8 seconds).
>
> I'm confused! What should be the correct setting in the ntp.conf file?
I've used type-28 + radioclkd2 for many years with MSF and for a
> What should the poll be for the shared memory driver (type 28)? This
> Web page:
>
> http://www.eecis.udel.edu/~mills/ntp/html/drivers/driver28.html
>
> suggests that the driver internally has a poll interval of 1 second,
> another document I've seen suggests setting:
>
> server 127.127.28.1 minpoll 1 prefer
>
> and yet in the ntpq -p billboard the poll is shown as 8 seconds.
> Loopstats are reported at 8 second intervals, and the poll interval is
> shown as 3 (i.e. 8 seconds).
>
> I'm confused! What should be the correct setting in the ntp.conf file?
while with DCF. I've never set minpoll/maxpoll but it has always
defaulted to 6, ie 64 sec. DCF has a phase shift second marker
but is too distant for that to useful. When DCF reception was
good the timekeeping was better than from msf (+/- 0.1 ms vs 1ms).
As you mentioned above, type 28 refclock pages indicate the driver
is polled by ntp at 1 second intervals (as from ntpd-4.2.5p138).
David
Steve Kostecke
Jun 17, 2013, 12:36:49 PM6/17/13
to
On 2013-06-17, David Taylor <david-...@blueyonder.co.uk.invalid> wrote:
> another document I've seen suggests setting:
>
> server 127.127.28.1 minpoll 1 prefer
>
> and yet in the ntpq -p billboard the poll is shown as 8 seconds.
> Loopstats are reported at 8 second intervals, and the poll interval is
> shown as 3 (i.e. 8 seconds).
grepping the source tree is very informative...
> another document I've seen suggests setting:
>
> server 127.127.28.1 minpoll 1 prefer
>
> and yet in the ntpq -p billboard the poll is shown as 8 seconds.
> Loopstats are reported at 8 second intervals, and the poll interval is
> shown as 3 (i.e. 8 seconds).
According to ./ntpd/ntp_peer.c:
/*
* It is an error to set minpoll less than NTP_MINPOLL or to
* set maxpoll greater than NTP_MAXPOLL. However, minpoll is
* clamped not greater than NTP_MAXPOLL and maxpoll is clamped
* not less than NTP_MINPOLL without complaint. Finally,
* minpoll is clamped not greater than maxpoll.
*/
And in ./include/ntp.h we find:
:#define NTP_MINPOLL 3 /* log2 min poll interval (8 s) */
--
Steve Kostecke <kost...@ntp.org>
NTP Public Services Project - http://support.ntp.org/
David Taylor
Jun 17, 2013, 2:02:09 PM6/17/13
to
On 17/06/2013 17:36, Steve Kostecke wrote:
[]
than my knowledge of "C"! In refclock_shm.c, it does say that the
"peek" routine is called every second, so if the type 28 driver has an
internal poll of one second, does it matter what min/max poll is set in
the ntp.conf file? Does it even need to be set at all?
I'll try setting it to 4, as a test, and see whether anything changes.
Sorry if I'm asking very basic questions, but if I'm going to write a
small Web page about this I would rather the contents be accurate and,
if possible, best practice.
--
Cheers,
David
Web: http://www.satsignal.eu
[]
> grepping the source tree is very informative...
>
> According to ./ntpd/ntp_peer.c:
>
> /*
> * It is an error to set minpoll less than NTP_MINPOLL or to
> * set maxpoll greater than NTP_MAXPOLL. However, minpoll is
> * clamped not greater than NTP_MAXPOLL and maxpoll is clamped
> * not less than NTP_MINPOLL without complaint. Finally,
> * minpoll is clamped not greater than maxpoll.
> */
>
> And in ./include/ntp.h we find:
>
> :#define NTP_MINPOLL 3 /* log2 min poll interval (8 s) */
Thanks, Steve. My knowledge of the source tree is even more limited
>
> According to ./ntpd/ntp_peer.c:
>
> /*
> * It is an error to set minpoll less than NTP_MINPOLL or to
> * set maxpoll greater than NTP_MAXPOLL. However, minpoll is
> * clamped not greater than NTP_MAXPOLL and maxpoll is clamped
> * not less than NTP_MINPOLL without complaint. Finally,
> * minpoll is clamped not greater than maxpoll.
> */
>
> And in ./include/ntp.h we find:
>
> :#define NTP_MINPOLL 3 /* log2 min poll interval (8 s) */
than my knowledge of "C"! In refclock_shm.c, it does say that the
"peek" routine is called every second, so if the type 28 driver has an
internal poll of one second, does it matter what min/max poll is set in
the ntp.conf file? Does it even need to be set at all?
I'll try setting it to 4, as a test, and see whether anything changes.
Sorry if I'm asking very basic questions, but if I'm going to write a
small Web page about this I would rather the contents be accurate and,
if possible, best practice.
--
Cheers,
David
Web: http://www.satsignal.eu
David Taylor
Jun 17, 2013, 2:11:47 PM6/17/13
to
On 17/06/2013 17:12, David Lord wrote:
[]
match the normal PPS/kernel method (and reduce the size of the log
file). Will watch the jitter and see whether it changes. The
GPS-derived PPS signal should be within 0.1 us, making the PC the
limiting factor.
[]
> I've used type-28 + radioclkd2 for many years with MSF and for a
> while with DCF. I've never set minpoll/maxpoll but it has always
> defaulted to 6, ie 64 sec. DCF has a phase shift second marker
> but is too distant for that to useful. When DCF reception was
> good the timekeeping was better than from msf (+/- 0.1 ms vs 1ms).
>
> As you mentioned above, type 28 refclock pages indicate the driver
> is polled by ntp at 1 second intervals (as from ntpd-4.2.5p138).
>
>
> David
Thanks, David. I'm trying a poll of 4 now (16 seconds) as that will
> while with DCF. I've never set minpoll/maxpoll but it has always
> defaulted to 6, ie 64 sec. DCF has a phase shift second marker
> but is too distant for that to useful. When DCF reception was
> good the timekeeping was better than from msf (+/- 0.1 ms vs 1ms).
>
> As you mentioned above, type 28 refclock pages indicate the driver
> is polled by ntp at 1 second intervals (as from ntpd-4.2.5p138).
>
>
> David
match the normal PPS/kernel method (and reduce the size of the log
file). Will watch the jitter and see whether it changes. The
GPS-derived PPS signal should be within 0.1 us, making the PC the
limiting factor.
Steve Kostecke
Jun 17, 2013, 6:05:40 PM6/17/13
to
On 2013-06-17, David Taylor <david-...@blueyonder.co.uk> wrote:
> On 17/06/2013 17:36, Steve Kostecke wrote: []
>
>> grepping the source tree is very informative...
[snip: relevant quotes from the NTP Reference Implementation source]
> On 17/06/2013 17:36, Steve Kostecke wrote: []
>
>> grepping the source tree is very informative...
> Thanks, Steve. My knowledge of the source tree is even more limited
> than my knowledge of "C"!
for the items I quoted.
> In refclock_shm.c, it does say that the
> "peek" routine is called every second, so if the type 28 driver has an
> internal poll of one second, does it matter what min/max poll is set
> in the ntp.conf file? Does it even need to be set at all?
> Sorry if I'm asking very basic questions, but if I'm going to write a
> small Web page about this I would rather the contents be accurate and,
> if possible, best practice.
Contributing (i.e. embedding and not merely linking) this
> small Web page about this I would rather the contents be accurate and,
> if possible, best practice.
article/topic/page to the Community Supported Documentation at
http://support.ntp.org/support is the best way to make this information
useful to the NTP Community.
Harlan Stenn
Jun 17, 2013, 7:00:23 PM6/17/13
to
It does more good to get NTP-related eyeballs at ntp.org than at
3rd-party sites.
H
David Taylor
Jun 18, 2013, 2:39:30 AM6/18/13
to
On 17/06/2013 23:05, Steve Kostecke wrote:
[]
If someone wants to rewrite my existing Web information in the format
required there, they are welcome, although they will need to keep track
of any changes. I find the editor not easy to use, and prefer to have
master copies on my information on my own Web server, and on multiple
hard disks here.
[]
> Contributing (i.e. embedding and not merely linking) this
> article/topic/page to the Community Supported Documentation at
> http://support.ntp.org/support is the best way to make this information
> useful to the NTP Community.
I've added a link to: http://support.ntp.org/bin/view/Support/ConfiguringNTP
> article/topic/page to the Community Supported Documentation at
> http://support.ntp.org/support is the best way to make this information
> useful to the NTP Community.
If someone wants to rewrite my existing Web information in the format
required there, they are welcome, although they will need to keep track
of any changes. I find the editor not easy to use, and prefer to have
master copies on my information on my own Web server, and on multiple
hard disks here.
Miroslav Lichvar
Jun 18, 2013, 3:41:17 AM6/18/13
to
On Mon, Jun 17, 2013 at 07:02:09PM +0100, David Taylor wrote:
> Thanks, Steve. My knowledge of the source tree is even more limited
> than my knowledge of "C"! In refclock_shm.c, it does say that the
> "peek" routine is called every second, so if the type 28 driver has
> an internal poll of one second, does it matter what min/max poll is
> set in the ntp.conf file? Does it even need to be set at all?
IIRC, the one second interval is used only to collect the SHM samples
> Thanks, Steve. My knowledge of the source tree is even more limited
> than my knowledge of "C"! In refclock_shm.c, it does say that the
> "peek" routine is called every second, so if the type 28 driver has
> an internal poll of one second, does it matter what min/max poll is
> set in the ntp.conf file? Does it even need to be set at all?
and store them in a buffer. In the minpoll/maxpoll interval the
collected samples are processed in a median filter and one final
sample is used to update the clock. This improves the jitter.
> I'll try setting it to 4, as a test, and see whether anything changes.
affects also the jitter as there will be a different number of samples
in the filter.
--
Miroslav Lichvar
David Taylor
Jun 18, 2013, 5:07:47 AM6/18/13
to
On 18/06/2013 08:41, Miroslav Lichvar wrote:
[]
>> I'll try setting it to 4, as a test, and see whether anything changes.
>
> The difference with NTP sources is that changing the poll interval
> affects also the jitter as there will be a different number of samples
> in the filter.
I've put a couple of graphs here:
http://www.satsignal.eu/ntp/Raspberry-Pi-NTP.html#user-mode
which show a drop of RMS offset from around 5 microseconds to 4
microseconds and a drop in averaged jitter from just below 7 to just
above 5 microseconds, when the minpoll was changed from 3 to 4.
[]
> IIRC, the one second interval is used only to collect the SHM samples
> and store them in a buffer. In the minpoll/maxpoll interval the
> collected samples are processed in a median filter and one final
> sample is used to update the clock. This improves the jitter.
Yes, that makes sense.
> and store them in a buffer. In the minpoll/maxpoll interval the
> collected samples are processed in a median filter and one final
> sample is used to update the clock. This improves the jitter.
>> I'll try setting it to 4, as a test, and see whether anything changes.
>
> The difference with NTP sources is that changing the poll interval
> affects also the jitter as there will be a different number of samples
> in the filter.
http://www.satsignal.eu/ntp/Raspberry-Pi-NTP.html#user-mode
which show a drop of RMS offset from around 5 microseconds to 4
microseconds and a drop in averaged jitter from just below 7 to just
above 5 microseconds, when the minpoll was changed from 3 to 4.
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