[ntp:questions] ntpd and database servers

unruh unruh at wormhole.physics.ubc.ca
Fri Jan 22 01:38:44 UTC 2010


On 2010-01-21, Brian Utterback <brian.utterback at sun.com> wrote:
> unruh wrote:
>> But the timestamps are not lies. they are far closer to the real time
>> than is that serverd by ntpd, and it is time that ntp serves, not slew
>> rate. ntpd wanders off into the wilderness ( I have seen it go off by
>> 20ms, when its steady state error is 2 usec) with its slewing that those
>> times are far more lies that the 20usec off but with a slew rate of
>> 700PPM that you might occasionally with chrony. Remember at the fastest
>> ntp queries once every 16sec. By that time the slew is long finished on
>> chrony ( but barely started on ntpd). Ie, I simply do not believe what
>> you are saying here. 
>
> Well, clearly a step is no different from the client's POV than a fast
> slew that completes entirely between polls. But a sufficiently long
> slew is going to result in downstream clients observing that their
> clocks are much too fast or much too slow and make adjustments to
> their own drift rates. The longer the slew goes on, the longer the

Lets say that the clock is out by 10 min. then it is true that ntp with
step that 10 min on startup ( or quit) and get itself to the right time.
chrony, without the initslewstep will slew away that 10 min over the the
next 100 min. Thus anyone else who queries chrony during that time will
get a bad time, out by many minutes. On the other hand one can also
easily tell chrony to step if it is out by that amount. the more
critical situation is if the clock is out by say .1 sec. chrony will get
rid of that in the next second. ntpd will over the next few hours to
days (depending on the poll) slowly alter its internal rate to finally
drift that time offset to zero. along the way it might actually go the
wrong way for a while and exceed the "step threshold", step to 0, and
then have to correct the totally wrong slew rate left over from the
initial attemp to slew the correction. 

> clients will be adjusting their own drifts until eventually they
> client's drift corrections are going to match the servers fast slew.
> Then when the server gets to where it is going, the slew will stop and
> the clients will continue right past that point. Then the clients will
> have to spend the same amount of time getting their drift corrections
> back to normal, at which point they will have overshot and have to get
> back.

This is certainly possible if chrony is trying to correct a huge error.
And since under the fast slew chrony is running at about 100000PPM off
the correct rate the ntp clocks will exceed the ntp limit of 500PPM, and
get clamped to that rate for a long time, which will help mitigate their
trying to follow the high chrony rate. 


>
> Perhaps if everybody used chrony it would adjust so quickly that it
> wouldn't matter. And I do agree ntpd in slewalways mode would be even
> worse or at least more susceptible, since the likelihood of a slew
> lasting longer than a poll interval will be greater. But I do know
> that the drift cap is needed to make the proofs in Das Buch work out.
>
> Now, it may be that the edge cases that ntpd handles better than
> chrony never happen in real life; I don't know. But I do know that
> within the parameters of proofs ntpd is well behaved. I don't think we
> have the same assurances about chrony.

I agree that chrony has not received the same attention to the theory of
operation that ntp has. As Hassler said, maybe someone has a grad
student who needs a masters thesis topic. 


>
> This is OT, but I do think that ntpd could be greatly improved and
> could learn a lot from the approach crony uses. But I don't think
> chrony is appropriate in all cases either nor is it perfect yet. I


> have pointed out some clear flaws in ntpd that would be relatively
> easy to fix, but was rebuffed. I do intend to address the myself
> someday (assuming Dr. Mills will allow the changes in ntp_proto.c) but
> haven't had the time yet.

If you have some pointers about clear flaws in chrony, please point them
out. At present the next release ( which includes refclock support) is
very close, and bug reports are needed. chrony-dev at chrony.tuxfamily.org

What is ntpd doing about the 2038 bug by the way?

>
> Brian Utterback




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