[ntp:questions] Change poll interval at runtime?
unruh at invalid.ca
Sun Feb 26 14:23:02 UTC 2012
On 2012-02-26, Ron Frazier (NTP) <timekeepingntplist at c3energy.com> wrote:
> On 2/25/2012 6:42 PM, unruh wrote:
>> On 2012-02-25, Richard B. Gilbert<rgilbert88 at comcast.net> wrote:
>>> On 2/25/2012 1:20 AM, A C wrote:
>>>> On 2/24/2012 21:26, A C wrote:
>>>>> Is it possible to change the polling interval of one or more associated
>>>>> servers at runtime? It seems like I should be able to run:
>>>>> ntpq -c "writevar&associd hpoll=N" or is it ppoll?
>>>> Actually, I should have been more specific and say change the minimum
>>>> polling interval. In other words, be able to adjust the conf file's
>>>> minpoll flag at runtime instead of restarting.
>>> What problem are you trying to solve?
>>> NTPD does a pretty good job of adjusting itself most of the time.
>>> Short poll intervals are useful when correcting large errors.
>>> Long poll intervals allow NTPD to make small corrections very accurately.
>> IF the drift is very stable, long poll intervals allow ntpd to make
>> small corrections to the drift rate accurately. They in general make the
>> offsets larger and mean that the system cannot respond to changes (eg
>> temp changes due to the coputer being used) rapidly. This is especially
>> true of ntpd because it throws away 7/8 of the measurements in the clock
>> filter, meaning that the actuall poll time is about 8 times longer than
>> the set poll interval. (poll 10 then has an effective poll of 8000 sec)
>> Anyway, that is an aside. I agree with you that I do not understand what
>> problem he is trying to solve.
> I'll admit to not having read every sentence in this thread, but I have
> a question about what was just said. Even if the clock algorithm
> discards 7/8 of the measurements, as stated, isn't ntpd sitll hitting
> the remote servers at whatever the minpoll value is set to, or the
> default minpoll value if not set. So, if the internet servers are set
> to a minpoll value of 8 (approximately 4 minutes), aren't those remote
> servers actually getting polled every 4 minutes, regardless of what ntpd
> does with the data.
Which is why ntpd is very profligate of data. Not that hitting a remote
server once every 4 min is much of a load-- that is about 1b/s on a link
that can co 100000000b/s. Mind you if 100000000 machines are all trying
to do that, things get serious.
More information about the questions