Re: strange behaviour of ntp peerstats entries.

Unruh,

It would seem self evident from the equations that minimizing the delay variance truly does minimize the offset variance. Further evidence of that is in the raw versus filtered offset graphs in the architecture briefings. If nothing else, the filter reduces the variance by some 10 dB. More to the point, emphasis added, the wedge scattergrams show just how good the filter can be. It selects points near the apex of the wedge, the others don't matter. You might argue the particular clock filter algorithm could be improved, but the mission in any case is to select the points at or near the apex.

While the authors might not have realized it, the filter method you describe is identical to Cristian's Probabilistic Clock Synchronization (PCS) methiod described in the literature some years back. The idea is to discard the outlyer delays beyond a decreasing threshold. In other words, the tighter the threshold, the more outlyers are tossed out, so you strike a balance. I argued then and now that it is better to select the best from among the samples rather than to selectively discard the outlyers.

There may be merit in an arugment that says the points along the limbs of the wedge are being ignored. In principle, these points can be found using a slective filter that searches for an offset/delay ration of 0.5, which in fact is what the huff-n'-puff filter does. To do this effectively you need to know the baseline propagation delay, which is also what the huff-n'-puff filter does. Experiments doing this with symmetric delays, as agains the asymmetric delays the huff-n'-puff filter was designed for were inconclusive.

Dave

Unruh wrote:

snip

Oh yes. popcorn suppression is important. I agree. But the filter goes well
beyond that. My eaction is that on the one hand people keep saying how
important net load is, and that one does not want to use poll intervals
that are much smaller than 8 or 10, and on the other hand, throwing away
80-90% of the data collected. Remin ds me of the story of Saul, king of the
Israelites, whose army was besieged, and he mentioned that he was thirsty.
A few of his soldiers risked everything to get through the enemy lines and
bring him water. He was so impressed that he poured it all out on the
ground, in tribute to their courage. I have always found that story an
incredible insult to the bravery instead.

The procedure does drastically reduce the variance of the delay, but does
not much for the variance of the offset, which is of coure what is
important. Just to bring up chrony again, it uses both a suppression where
round trips greater than say 1.5 of min are discarded, and data is weighted
by some power of the invere of the delay.

snip
.

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