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

David L. Mills mills at udel.edu
Wed Feb 9 00:43:14 UTC 2005


Brad,

Let me be sure this is your intent. Your minclock 4 says you must have 
at least 4 severs pass the maxdist threshold on the way down before 
setting the clock. Against my better judgement the default minclock is 
1, so ordinarily the clock is set by the first server found, and it 
might be a falseticker. I am happier with 4, because that's the 
Byzantine minimum that reliably kicks out a single ralseticker. Your 
minsane 4 requires the clustering algorithm to stop tossing out outlyers 
when 4 remain. Also good Byzantine. But, if you did that, it would be a 
good idea to use a couple more servers, say a total of 6, so the 
clusting algorithm could improve the quality. However, with minclock 4, 
if for some reason less than that number of servers were actually 
available, the clock would never be set.

Is that what you had in mind? I have no problem with it, but I do want 
to make sure the model is clearly understood.

Dave

Brad Knowles wrote:

> 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 6.1.4.4.
> 
>     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.
> 



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