[Pool] time reset

Hal Murray hmurray at megapathdsl.net
Thu Mar 24 20:44:22 UTC 2011

>  What could be going on with my NTP session that is causing the time to
> reset a every every hour or more?  I've rebooted my box, deleted the drift
> file. Ran ntpdate to another server and then started ntpd, and still I keep
> getting these resets. 

We should make a Wiki page on this.

There are several things I can think of:

1) Some other program is setting the time.  (perhaps a cron job)

2) Your local clock is broken.  There are several ways this can happen.
  a) Power saving can changes the CPU speed
  b) The kernel timekeeping software could be broken.
  c) Ages ago, you could get this when a lot of disk activity caused lost 
interrupts.  I haven't seen that on PCs in a while, but it might come back on 
embedded systems.

3) Your network connection is variable and unsymmetrical.  You can easily get 
problems if you do a big download over a DSL connection.  ntpd assumes your 
network delays are symmetric.  If your system gets calibrated when the 
traffic is low (and symmetric) and you start a big download (or upload) 
without much traffic in the other direction, queuing delays can cause a big 
enough time shift to confuse ntpd.  On my DSL line, I see up to 3 second 
delays.  If this is the problem, tinker huffpuff might help.

Lots of interesting info at http://www.bufferbloat.net/  (but not directly 
ntp related)

These are my opinions, not necessarily my employer's.  I hate spam.

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