| [chrony-dev] System time offset bias |
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- To: chrony-dev@xxxxxxxxxxxxxxxxxxxx
- Subject: [chrony-dev] System time offset bias
- From: Bryan Christianson <bryan@xxxxxxxxxxxxx>
- Date: Thu, 20 Aug 2015 20:25:33 +1200
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Even with all the improvements made to fixing the local offset, I'm still seeing a bias of about 1 usec (which is an order of magnitude better than where we started)
I presume this is the cumulative effects of function call overheads (both local to chronyd and system calls) made after we have measured time stamps with gettimeofday().
I've been looking at using a bit of negative feedback and calculating the mean of offset_register as measured by get_offset_correction() and applying a small percentage of this mean offset to the predicted error in start_adjust(). The results are looking promising.
Because get_offset_correction() is being called at random intervals, I don't think its correct to calculate the mean with a simple exponential moving average, but that a weighted rolling average taken over say 60 samples might be more appropriate. Please correct me if I'm wrong because an exponential mean is far simpler.
I'll put a patch up later this evening for you too look at.
Regards
Bryan
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