[ntp:questions] ntpd on embedded risc

Unruh unruh-spam at physics.ubc.ca
Wed Feb 18 16:06:27 UTC 2009


Steve Kostecke <kostecke at ntp.org> writes:

>On 2009-02-18, Unruh <unruh-spam at physics.ubc.ca> wrote:

>> cnm3332 at gmail.com (Christopher Mire) writes:
>>
>>>I have a small embedded linux machine. Moxa UC-7112 Plus that I want
>>>to use as NTP server. http://www.moxa.com/product/UC-7110-LX.htm Its
>>>has MOXA ART ARM9 32-bit 192 MHz processor CPU.
>>
>>>Here are statistics I collected after using it a bit. This is using
>>>gpsd 2.33 to collect NMEA, PPS.
>>
>> "a bit" means what? Remember that ntpd takes 1 hour to cut the error
>> by half. Thus unless you ran this for more than 10 hours, these
>> offsets mean nothing.

>Poppycock.

>A snapshot is merely a snapshot regardless of when it is taken.

But a snapshot taken before the system has settled down to its long term
behaviour is especially useless. The way ntp works is that intially it
takes a while to settle down to its long term behaviour. With PPS control
it usually settles down to usec offset but it takes of order of 10 hours to
do so. Thus snapshots taken long before that are especially useless. If
after 10 hours the system still has msec offsets from a PPS then there is
something seriously wrong.


>Long term performance is what's important. It may be evaluated through
>the use of multiple snapshots (over a suitable period) or by summarizing
>the statistics files.

Long term performance with completely rediculous short term performance (
17ms from a PPS control) means nothing. 

IF his system has been running for 10 hours or more and he has 17ms
offsets, his system is broken. If it has been running for only 1 hour then
that kind of offset is not at all unusual.

And a single snapshot is all I need to tell me that.



>> [---=| TOFU protection by t-prot: 53 lines snipped |=---]

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>-- 
>Steve Kostecke <kostecke at ntp.org>
>NTP Public Services Project - http://support.ntp.org/




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