Step or Sloping Graphs

Even though the backend of Rosenberg NMS uses rrdtool RRD files, the front end graphs are created using jqPlot.  The idea is to have a set of templates for the different types of graphs and just apply them to the various data sets.  It makes things a lot simpler for new graphs because you just select which one you want; unless you want something a lot different which would involve a new graph template.

In any case, anyone that looks enough at the standard rrdtool graphs will know they are a series of steps. While it depends on the RRA, usually they are 5 minute steps, so a graph showing an increasing rate might show 5 minutes of 2 Mbps and then the next 5 minutes of 11 Mbps. jqPlot graphs as I’ve currently got them draws a line between two data points, so there would be a sloping line starting at 2.5 minutes (half of the first 5 minute interval) and 2Mbps sloping up to 11 Mbps at 7.5 minutes.

At first I thought this was wrong and spent some time attempting to “fix” the graph by making it look like a rrdtool graph more. Someway through that process I stopped and wondered, what IS the right way? The answer like a lot of other things, is “it depends”.

For a graph showing a rate, such as the output bits per second on an interface, the way this is done is at regular intervals a counter is measured. So if at time 0 the counter is 140 and time 60 the counter is 200 and finally at time 120 the counter is at 800 there has been an average rate of 1 [(200-140)/60] and 100 [(800-200)/60)]. rrdtool would show a horizontal bar at 1 bps and then another horizontal bar at 100 at the next time interval. jqPlot would show a sloping line going from 30,1 up to 90,100.

Two graphs looking very different from the same data, what gives?  Each graph is right, or rather is showing an estimation of different things, hence their differences.

rrdtool is showing the average rate for that time period. It is in some ways accurate because leaving aside missed wrap-arounds and resets that many bits did pass through that interface for the time specified.  However often the graphs are interpreted incorrectly as the real rate and not an average.  We can be reasonably sure for a rate that it would not be 1 for exactly a minute and then immediately jump to 100 for another minute.  This isn’t rrdtool’s fault, it is just how the graphs can be interpreted.

jqplot will show more “realistic” graphs, with a curve sloping up. However this too makes assumptions that the rate increase is linear which often it is not. It just gives the illusion that the graph knows more about the data than it really does.

In the end, both graphs are at the same time accurate and misleading. It’s important when looking at any graph in general (not just these two types) that you understand its limits and assumptions.  To give one example of the problems that can be missed, traffic interfaces may have microbursts (large amount of traffic in short amount of time) which, due to the averaging that goes on in graphing are invisible to graphs and give an incorrect account of what is going on.

 

 

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