[ntp:questions] Re: Getting good NTP tracking

Eino-Ville Talvala quantumet at gmail.com
Mon Jun 26 21:18:00 UTC 2006


Richard B. Gilbert wrote:
> Eino-Ville Talvala wrote:
> 
>> Ok,
>>
>> I've been messing with this for a few weeks, and haven't yet managed 
>> to get NTP to where I'd like to be (and where I think it should be 
>> able to be).
>>
>> I'm on an academic network, which has both a university-wide stratum 2 
>> server (actually, a pool of a few servers), and a departmental stratum 
>> 3 server.  I'm trying to set up a pair of machines that I want 
>> synchronized to each other at < 5ms consistently.
>>
>> Given that the delays between the machines and the servers are on the 
>> order of 0.3 ms, I'd expect to be able to maintain offsets at less 
>> than 5 ms consistently.  However, while my average offset values are 
>> usually in that range, I'm seeing RMS offsets on the range of 10-30 
>> ms, with daily peaks around 90 ms for some servers.
>>
>> Both machines run Centos 4.2, and I've now disabled (temporarily) both 
>> the firewall and SELinux protection for ntpd in an attempt to figure 
>> out the problem.  Here is the ntp.conf file for one machine (with 
>> commented out bits removed, and anonymized)
>>
>> ---------------------------------------------
>>
>> restrict default nomodify notrap
>> restrict 127.0.0.1
>>
>> server <UNIVERSITY>
>> server <DEPARTMENT>
>> peer <SECOND MACHINE>
>>
>> # Let's throw in servers from the public pool
>>
>> server 0.north-america.pool.ntp.org
>> server 1.north-america.pool.ntp.org
>> server 2.north-america.pool.ntp.org
>>
>>
>> driftfile /var/lib/ntp/drift
>>
>> statistics loopstats
>> statsdir /var/log/ntp/
>> filegen peerstats file peers type day link enable
>> filegen loopstats file loops type day link enable
>>
>>
>> -------------------------------------------------
>>
>> And here's a bit of peerstats (from before I added in the public pool 
>> servers):
>>
>> peers.20060620
>>        ident     cnt     mean     rms      max     delay     dist     
>> disp
>> ========================================================================== 
>>
>> <UNIVERSITY>     132   -4.089   90.922  986.193    4.340  939.038   
>> 30.380
>> <DEPARTMENT>      137   18.467   27.483  114.025    1.615  939.420   
>> 32.474
>> <SECOND MACHINE>     137   24.696   29.110   59.226    1.236  938.365 
>> 25.751
>> peers.20060621
>>        ident     cnt     mean     rms      max     delay     dist     
>> disp
>> ========================================================================== 
>>
>> <UNIVERSITY>      84    5.482   21.357   89.828    7.645   45.069   
>> 14.830
>> <DEPARTMENT>       85   10.523   10.672   34.443    4.559   23.647   
>> 14.829
>> <SECOND MACHINE>      84    5.883    5.845   18.113    1.458   31.992 
>> 20.572
>> peers.20060622
>>        ident     cnt     mean     rms      max     delay     dist     
>> disp
>> ========================================================================== 
>>
>> <UNIVERSITY>      84   -2.083  115.705  931.227    7.427   46.999   
>> 14.830
>> <DEPARTMENT>       84    4.506    7.546   41.621    2.652   39.808   
>> 14.828
>> <SECOND MACHINE>      85    1.590    3.878   14.613    1.384   32.507 
>> 18.043
>>
>> ----------------------------------------------------
>> And here's the driftfile:
>>
>> -96.459
> 
> I'm a little puzzled by the low values of "cnt".   With 84-85 samples 
> per day, it looks as if your system is polling the servers at the 
> maximum poll interval of 1024 seconds.  Did you, by any chance, tamper 
> with the default values of MINPOLL and MAXPOLL?  It's generally a poor 
> idea.  Ntpd will adjust the poll interval upwards and downwards as 
> conditions change and as limited by MINPOLL and MAXPOLL.  The defaults 
> allow for the conditions usually found.  By setting MINPOLL to 10, you 
> force the longer polling interval even when ntpd needs a shorter 
> interval to achieve good synchronization!
> 

I've certainly not intentionally changed it anywhere - if that's an 
ntp.conf setting, I haven't changed it.

I should clarify a bit - I just added the 3 public pool servers and 
turned off SELinux protection for ntpd this morning, so I don't yet know 
if those changes may fix my problem.




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