[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] statistical time calibration
Hello, Roger pointer me to a FreeBSD commit [1] introducing such there. While we don't start at 2000ms (but rather at 50), this still looked interesting enough to take a closer look. I think I've mostly understood the idea and implementation now, with the exception of three things: 1) When deciding whether to increment "passes", both variance values have an arbitrary value of 4 added to them. There's a sentence about this in the earlier (big) comment, but it lacks any justification as to the chosen value. What's worse, variance is not a plain number, but a quantity in the same units as the base values. Since typically both clocks will run at very difference frequencies, using the same (constant) value here has much more of an effect on the lower frequency clock's value than on the higher frequency one's. 2) The second of the "important formulas" is nothing I could recall or was able to look up. All I could find are somewhat similar, but still sufficiently different ones. Perhaps my "introductory statistics" have meanwhile been too long ago ... (In this context I'd like to also mention that it took me quite a while to prove to myself that the degenerate case of, in particular, the first iteration wouldn't lead to an early exit from the function.) 3) At the bottom of the loop there is some delaying logic, leading to later data points coming in closer succession than earlier ones. I'm afraid I don't understand the "theoretical risk of aliasing", and hence I'm seeing more risks than benefits from this construct. Beyond that there are implementation aspects that I'm not happy with, like aforementioned delay loop not dealing with a TSC which did start from a large "negative" value, and which hence would eventually wrap. Nor is the SMI (or other long latency events) aspect being taken care of. But any such concern could of course be dealt with as we port over this logic, if we decided we want to go that route. My main concern is with the goal of reaching accuracy of 1PPM, and the loop ending only after a full second (if I got that right) if that accuracy cannot be reached. Afaict there's no guarantee that 1PPM is reachable. My recent observations suggest that with HPET that's feasible (but only barely), but with PMTMR it might be more like 3 or more. The other slight concern I have, as previously voiced on IRC, is the use of floating point here. Jan [1] https://cgit.freebsd.org/src/commit/?id=c2705ceaeb09d8579661097fd358ffb5defb5624
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