Re: Question about min/max poll and an interesting plot showing maxpoll's effect
- From: John Pettitt <jpp@xxxxxxxxxxxxx>
- Date: Thu, 20 Apr 2006 15:11:21 -0700
Richard B. Gilbert wrote:
Ntpd is supposed to set it's own polling interval; something to do with
the "Allen Variance". My feeble understanding translates into English
as "short poll intervals allow ntpd to correct large errors quickly and
long poll intervals allow ntpd to correct small errors accurately".
Anyway, I'm a little surprised that shorter is better. I have two
machines as clients of my stratum 1 (GPS) server. All three are Sun
workstations; the server is an Ultra 10 440 MHz, one of the clients is
also an Ultra 10 440 MHz the other is an Ultra 5 360 MHz. All have been
running for several days. The Ultra 5 is polling at 64 second intervals
with an estimated error of 11056 us while the Ultra 10 is polling at 256
second intervals with an estimated error of 500 us.
The proper poll interval appears to be highly dependent on the quality
of the local clock.
My experience is that shorter is better with commodity hardware that is subject to rapid thermal
cycling - my home machines are in the basement which also houses the furnace - I can see the furnace
12 min on/off cycle in the frequency offset graphs. Equally when periodic compute intensive jobs
run (one of my S1's is also a file/backup server) the resultant thermal shock to the machine pushes
it way off - if I let ntpd poll long (or omit flag 3 on refclocks) it spends all it's time chasing
the thermal cycle and never quite makes it. Will a poll of 64 (or less on my own servers) it
catches it before it gets 250us off and corrects. My co-lo machines which have a much more even
load and are in an cabinet in a machine room show a dinural cycle that ntp copes with pretty well
even at a poll interval of 1024.
John
.
- References:
- Question about min/max poll and an interesting plot showing maxpoll's effect
- From: John Ackermann N8UR
- Re: Question about min/max poll and an interesting plot showing maxpoll's effect
- From: Richard B. Gilbert
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