[ntp:questions] Re: synchronization

Ron Croonenberg ronc at depauw.edu
Tue Mar 14 18:24:57 UTC 2006


I am wondering if the "outgoing" connection (the one going off campus) 
is all that good.  Is there a way to check that (if you're not one of 
the networking people downstairs ....)

Ron


Richard B. Gilbert wrote:
> Ron Croonenberg wrote:
> 
>> Richard B. Gilbert wrote:
>>
>>>> noquery
>>>
>>>
>>>
>>>
>>> noquery, on a line by itself is invalid!!!!  If it were valid, you 
>>> would be shooting yourself in the foot because "noquery" means "do 
>>> not respond to querys" which means your server couldn't serve time 
>>> anyway!!!!
>>
>>
>>
>> Well..  I took the "default" config file and it was in there..  so I 
>> left it there.
>>
>>>> ============================================================================== 
>>>>
>>>> +time-b.nist.gov .ACTS.           1 u   43   64  377   36.473  
>>>> 3598.51 33.187
>>>> *tick.usnogps.na .USNO.           1 u   50   64  377  107.202  
>>>> 3594.40 31.645
>>>> +NAVOBS1.MIT.EDU .PSC.            1 u   43   64  377   36.762  
>>>> 3596.60 32.328
>>>> ********************************************************
>>>
>>>
>>>
>>>
>>> This is a pretty dismal looking ntpq banner!!  The offsets say that 
>>> your clock is off by more than three seconds.  At the maximum slew 
>>> rate of 500 parts per million, it will take several hours to bring 
>>> your clock into synchronization.
>>
>>
>>
>> yes I know..  that's why I noticed there to be a problem
>>
>>> How are you starting ntpd?  What options are you using?  If you use 
>>> the -g option, ntpd should set the clock unconditionally at startup; 
>>> e.g. it should query the servers to find out what time it is and then 
>>> set your clock to that time.
>>
>>
>>
>> I start it with : 'service ntpd start"
>> That results into : ntpd -U ntp -p /var/run/ntpd.pid
>>
>>> Next, the three servers you are using appear poorly chosen.   You 
>>> should not be using stratum 1 servers unless you will be serving time 
>>> to several hundred clients!  All the public stratum 1 servers are 
>>> generally loaded to the breaking point and should be avoided if 
>>> possible.  The figures for round trip delay are quite high!  107 
>>> milliseconds is downright unreasonable!  36 is not very good either.  
>>> The highest delay I have is 19 milliseonds.   Look for servers closer 
>>> to you; e.g. with shorter round trip delays.
>>
>>
>>
>> Ok..  so how do I find different servers ?
> 
> 
> http://ntp.isc.org/bin/view/Servers/WebHome
> has lists of all public stratum 1 and stratum 2 servers.  Look for 
> servers near you in geographical space and test them for nearness in net 
> space (low round trip delays).  The last time I looked, there were about 
> 200 stratum 1 and 200 stratum 2 servers listed.
> 
> Also see http://ntp.isc.org/bin/view/Servers/NTPPoolServers
> You have little choice of pool servers; you can ask for one in "Europe" 
> or "US", etc. but if you are in New York, you may be assigned a pool 
> server in California.  The pool is intended to supplement, not replace 
> your choice of servers.
> 
> As others have pointed out; I forgot to bash Linux for its handling of 
> clock interrupts and to suggest a hardware problem with your client's 
> clock.
> 
> Some versions of Linux default to updating the clock at rates of 250, 
> 1000, or 2000 HZ which makes it likely that device drivers will mask or 
> disable the clock interrupt long enough to lose more than one clock 
> interrupt.  Setting the update frequency to 100 Hz has been known to 
> work wonders.
> 
> ntpd cannot correct a frequency error greater than 500 PPM which 
> translates to about 43 seconds per day of clock drift.  It's rare for a 
> computer clock to be that far off but it has been known to happen.  Most 
>  clocks fall in the +/- 50 PPM range.  If your system, without ntpd 
> running, gains or loses more than 43 seconds per day, ntpd can't do a 
> thing for it.  Possible fixes include replacing the mother board or 
> tinkering with the tick rate via the adjtimex command.




More information about the questions mailing list