[ntp:questions] Query about NTP accuracy
unruh-spam at physics.ubc.ca
Mon May 25 20:35:58 UTC 2009
Hans =?iso-8859-1?Q?J=F8rgen?= Jakobsen <hjj at wheel.dk> writes:
>On Fri, 22 May 2009 23:47:35 GMT, Unruh wrote:
>> Note also that it can depend on the network harware. When we ran 100Mb/s
>> switches, the delay time was astonishingly regular and repeatable. Now
>> that we have switched to Gb switches it is not. It much worse. There
>> seems to be a one way "quantization" It used to be that the round trip
>> time from the machines to the ntp stratum 1 source was consistantly
>> 140-160usec. Now it varies, with one level still at the 140usec, but
>> other round trip times of 280 or even longer (on the same machine).
>> And they seem to be one
>> way delays ( the worst kind). Ie, you really need to select the switches
>> between your stratum 1 sources and your machines carefully to get the
>> best behaviour.
>There could be interrupt coalescing going on in one or more network cards.
Same network cards. Just different switches between the computers.
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