[ntp:questions] Re: Advice on sync clock between cluster of linux v2.6 to +-1us

Nick spam at nospam.blueyonder.co.uk
Wed Jan 12 20:12:12 UTC 2005


Thank you Brad, Richard and Helmut for your prompt replies.

Originally I was thinking a connection setup similar to would suffice:

External clock Reference --> Master Node --> Other computers nodes via 
Ethernet

I did not realise that inherent delays as Richard mention in Ethernet 
would be the best achievable accuracy. Maybe a setup below would be more 
appropriate:

External Reference --> All computer nodes using a signal booster/splitter

Even still, I doubt this would guarantee the accuracy between the nodes 
of +- 1 microsecond.

Helmut - When I mentioned 'cluster', I meant a group (< 10) of computers 
connected by Ethernet.

I have been given this more thought today and suspect I am approaching 
this incorrectly.

The problem resolves event timing in a test rig. Each computer node are 
part of the test rig and basically act as an I/O interface to the 
system-under-test. Each node would be given a list of event instructions 
and the time stamp when the event should be triggered. This time stamp 
would be relative to the start of the test and hence my original 
statement that "actual time is not required".

Thinking aloud - Another solution could be using the processor local 
clock. I believe all 386 processors have them and assume their drift 
would not be excessive. (Anyone have statistics on this?) Each node 
loads their instructions and wait for the master node to send a start 
signal. The master node could send a Ethernet broadcast 'start' packet 
(assuming the network infrastructure sends broadcast data to all ports 
at the same time). On receipt, each client node records their local 
clock time stamp as the "start time" and immediately run the test. All 
the events in the test would be triggered relative to this "start time". 
I wouldn't like to say how accurate this is but willing to take opinions 
or better solutions.

All the best,
Nick



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