[ntp:questions] Re: ntpd, boot time, and hot plugging

Tom Smith smith at cag.lkg.hp.com
Sun Feb 6 11:51:12 UTC 2005

Brad Knowles wrote:
>     At this point, all I'll say is that there are a lot of factors 
> involved, and if you try to set up the situation so as to be as 
> comparable as possible, ntpdate does not fare well.

Maybe you missed the data showing identical conditions and
a greater than 50:1 difference between the 2? One is 2 notes back.
The note you replied to. There are 2 or 3 other previous posts with
detailed data showing the same ting.

>  Moreover, ntpdate 
> has some nasty failure modes (which have been described by others) if 
> you don't give it enough servers to check against and/or if some of them 
> are down.

Including a down server. ntpd has the same problems if you don't
give it enough servers if they're down, after all.

>     I know what you want to use it for.
>     You want a guaranteed less-than-one-second "good enough" answer for 
> doing any necessary large-scale changes to the clock, afterwards you can 
> start up various somewhat time-sensitive applications while the system 
> can start getting into the detailed long-term clock maintenance "in the 
> background".

Exactly. And ntpd itself is one of tose time-sensitive applications
that happens sometimes to react badly to starting in the wrong place
(yielding drift rates pinned at +-500).

>     Problem is, the things you can do in order to get the upper limit 
> down below one second are the same sorts of things which tend to give 
> you really nasty failure modes.

Yes, welll, perhaps. I'll take the demonstrated and often seen failure
modes of not doing it over the theoretical ones, though.

>     I am not at all convinced that you can have your cake and eat it, 
> too -- Past illusions of being able to do so in the past with ntpdate 
> not withstanding.

I guess it's all a matter of experience. I assure you I have very
few illusions left.

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