[ntp:questions] How do I prevent sudden system time jumps.

Chuck Swiger cswiger at mac.com
Wed Jul 13 22:59:32 UTC 2011


On Jul 13, 2011, at 2:44 PM, Hal Murray wrote:
>> OK.  I asked since a timewarp of 200ms is a bit surprising for real HW,
>> but is something to be expected if you were running in a VM.
> 
> It's easy to get a time-warp of 200 ms on a DSL link.  Just download
> a huge file, say a CD.  The queuing delay on the input to the DSL
> link turns into asymmetrical delays.  I've seen delays up to 3.5 seconds.


Having some other peers on the LAN helps, as does ntpd's history of observed RTTs to try and notice if there is excessive delays on a particular sample.

That said, people experiencing 3.5s of delay might have machines with the TCP send and receive windows (ie, net.inet.tcp.recvspace & sendspace or similar sysctls) cranked up too large for the bandwidth * delay cross-product of their link.  I've seen these set to absurd values (ie, 16MB), when a sane value for a DSL link is on the order of 10 - 30 KB, and even gigabit ethernet would be happy with a setting of around 1MB.

  http://en.wikipedia.org/wiki/Bandwidth-delay_product

If possible, enabling QoS to prioritize NTP traffic ought to help also.

Regards,
-- 
-Chuck




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