[rrd-developers] Unexpected behaviour of PREDICT and PREDICTSIGMA

Steve Shipway s.shipway at auckland.ac.nz
Sat Apr 26 23:27:27 CEST 2014

Thanks for the clarification... I'm still not sure that I think the 0-shift should be included when using a negative count but that's likely personal preference and, as you say, the explicit list is always available anyway.

Maybe it should also allow the use of 0 to mean a single 0-shift -- ie
to be the same as 

Of course, currently with the negative setup  not including n, this is also the same as
for any value of s, which is a bit pointless and the main reason I felt something was amiss.

I have a paragraph written up on this behaviour for the RRDTool book I'm working on (yes! it lives! like a zombie it pulls itself from the 2-year-old grave...) so I'll email this to Tobi for inclusion in the online manual if he wants.

As for a predict_median and predict_percentile -- as you say, they could be very CPU-expensive, and also if you're going that far it's likely that you'd just set up a few HW RRAs to use instead.  Still, it never hurts to have more tools in your toolbox...

If we're adding new operations to the RPN, my choice would be for ROL and ROR (rotate top 3 stack items) and some date calculations (given an epoch time, extract day of week, hour of day, etc in local timezone).  Maybe I'll download the latest dev snapshot and see what I can do.


Steve Shipway
University of Auckland ITS
UNIX Systems Design Lead
s.shipway at auckland.ac.nz
Ph: +64 9 373 7599 ext 86487

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