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

Brad Knowles brad at stop.mail-abuse.org
Sun Feb 6 05:59:36 UTC 2005

At 4:53 AM +0000 2005-02-06, Tom Smith wrote:

>  Maybe a large part of your ntpdate time was printing out the messages.

	Could be.  I doubt that could make it go from 0.7 seconds to 14 
seconds, but it might have added a bit.

>  There was some question earlier about whether DNS delays might
>  explain the lengthy time for ntpd. So I performed the same experiment
>  on the same system, which is its own DNS server, caching the
>  names first, and doing ntpdate before ntpd to make doubly sure.
>  There is no meaningful difference.

	I ran ntpdate and ntpd multiple times myself, so as to make sure 
that DNS caching was not an issue.  I also didn't muck about with 
minpoll or maxpoll, although I did use iburst.  Still, my ntpd 
execution wasn't very much longer than my ntpdate (which was slower 
than your full one), and my ntpd startup was considerably faster than 

	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.  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.

	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".

	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.

	If you're *that* sensitive to time on startup, then you're 
probably also sensitive to nasty failure modes.

	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.

Brad Knowles, <brad at stop.mail-abuse.org>

"Those who would give up essential Liberty, to purchase a little
temporary Safety, deserve neither Liberty nor Safety."

     -- Benjamin Franklin (1706-1790), reply of the Pennsylvania
     Assembly to the Governor, November 11, 1755

   SAGE member since 1995.  See <http://www.sage.org/> for more info.

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