On 10/12/06, Ihno Krumreich <[EMAIL PROTECTED]> wrote:
> IIRC the NTP mechanism was to review adjusting the change of the drift > every 2 seconds or so. Even though this is very little work, it does > make VM think the guest is busy and keeps it in queue. Asking the snmp > agent every minute for some MIBs (when the guest did real work) is an > order of maginute less annoying for VM. No. Thats not true. When starting NTP looks quite often (dont remember how often). The more the reference clock and the local clock are in sync the less NTP checks the state. This goes down to a check every few hours. So after a while NTP should not be visible at all from a VM point of view.
I'm not used to that response on my posts, so let me clarify. The model behind the ntp support in the kernel is a based on 4 different parameters: * constant base offset * frequency difference (skew) * aging of the oscillator (drift) * variation in the skew (jitter) The correction needed to steer the system clock to keep up with true time is used to determine these parameters. The offset is done by yanking the clock initially. The skew is done by a small extra that is added with each jiffy count. The drift is implemented by a change of the skew that is made every 2 seconds. The jitter is not corrected but recognized by the algorithms to avoid too drastic changes to the drift. My experience is limited, but all zSeries clocks that I have measured were a weeny bit too slow (some skew of ~ 1 ppm) but stable (no measureable drift). So provided you correct the initial offset (Operator Mickey Mouse clock) with an ntpd -q the worst that happens is that the (daily) ntpd -q will 'yank' the clock forward a little (say 100 ms per day or so). This is good enough for a lot of applications. The variation introduced by the dispatch queue in CP is way larger than what you want to correct. This dispatch delay will be depending on the system overall load. These long term variations as observed by ntpd running in a Linux virtual machine do not match the jitter model very well. The net effect is that running ntpd against some set of reference clocks makes your clock less stable than the zSeries clock. Rob -- Rob van der Heij Velocity Software, Inc http://velocitysoftware.com/ ---------------------------------------------------------------------- For LINUX-390 subscribe / signoff / archive access instructions, send email to [EMAIL PROTECTED] with the message: INFO LINUX-390 or visit http://www.marist.edu/htbin/wlvindex?LINUX-390
