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

Brad Knowles brad at stop.mail-abuse.org
Tue Feb 8 22:41:16 UTC 2005

At 5:09 PM -0500 2005-02-08, Tom Smith wrote:

>  As it turns out, this happened to have been using V4.1.1. The 4.2.0
>  build on this system was defective and had been moved out of the way.

	As Dr. Mills has said, the code has been significantly improved 
since then.  Even the current ntp-dev is significantly improved over 
4.2.0-REL.  Try building from the latest snapshot tarball.  That's 
what Steve and I have been doing our most recent testing with.

>  So, without DNS in the way, 4.2.0 gets it from a 50:1 difference to a
>  10:1 difference. A big improvement, to be sure, but this is only with
>  3 servers (none of them the local clock, of course).

	To the best of my knowledge, all of our collected experience so 
far is detailed at 
<https://ntp.isc.org/bin/view/Support/StartingNTP4>, with the issues 
regarding sensitivity to startup delays in section

	In short, with a reasonable number of servers (i.e., six to 
nine), with a reasonable /etc/ntp.conf (making use of iburst, "tos 
minclock 4 minsane 4", etc...), using NTP servers by hostname instead 
of IP address, making sure that the NTP servers are "good" as well as 
close by (within 20-50ms delay), a good nameserver running on the 
local machine, etc... you should be able to see startup times on the 
order of fifteen seconds.  I think that's perfectly reasonable for 
virtually all situations, including high-value financial businesses 
(including those which would lose $50 million per second of downtime) 
and military applications.

	However, if you absolutely positively have to get that down 
further, then listing six servers by IP address and not name, you 
should be able to see startup times around eleven seconds.  Removing 
the "tos minclock 4 minsane 4" and using four servers listed by IP 
address, you should be able to get down to seven seconds.

	If you must cut that down further, you can make use of "tos 
maxdist 16", and get it all the way down to three seconds, but 
further reductions in the number of servers won't help you, nor will 
anything else we've tried.  However, if you do that, then I believe 
that you will get what you deserve.

	Also note that the lowest minpoll the code allows is four.  If 
you try to go below that, it silently limits you to this floor.

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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