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!
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