update frequency for stats in /proc/net/dev

A number of years ago I had written a comprehensive system monitoring tool called collectl, which among other things allowed me to monitor network traffic in real-time display as well as log the data to a file. Furthermore, that file can be generated in a format suitable for plotting with gnuplot. As it turned out, I would very frequently see spikes of 200MB/sec on my 1Gb link. A colleague noticed the reason was because the network counters were being updated every 0.9765 seconds and this was causing the problem. I don't know how long this problem existed but it was certainly there in 2.4 kernels. As it turns out, my tool is capable of monitoring with a fractional frequency and I have been able to get good data in spite of this problem. However, I've since noticed that now the stats are updated once a second but that also means when I process the data at 0.9765 I get the wrong numbers again. Clearly one answer is to just update the counters more frequently but I suspect that is not being done for reasons of performance.

Anyhow, I just wanted to let people know that ALL tools that monitor once a second on older counters will get the wrong numbers and tools that correct for the wrong number by using fractional intervals (and I suspect mine is the only one that does) but run on newer kernels will also get the wrong numbers. In any event, if anyone is interested in trying out collectl - it monitors a LOT more than just networks - you can snag a copy of from http://collectl.sourceforge.net/ if you'd like to take if for a drive. The website has a lot of output examples to give you a better idea what it can do. I even included a writeup about the odd network performance observations at http://collectl.sourceforge.net/NetworkStats.html

-mark

* Mark Seger <[email protected]> 2007-12-18 08:37

I've solved this problem by using netlink to read the interface counters ten times per second and maintain an own counter from which I calculate the rate exactly once per second/minute/hour. The rate per second may still be inaccurate to some degree, therefore I keep a history of 2-5 rates and take them into account to smoothen the result. This works fairly well with _all_ operating systems.

I guess I'm not entirely sure what you're saying with respect to 10 times/sec. Is this once very .1 secs or 10 times in rapid fire? From a general purpose monitoring perspective, since I read hundreds of counters every second doing it 10 times/sec is way too much overhead and special processing for netowork counters would also be pretty painful. The general problem of the counters only changing once a second means you'll never do that well when you monitor close the the interval and you can't ever get accurate counters at lower rates. In fact, if you try to treat the network counters like any other and if you monitor say every .2 seconds, you see a rate of 0 for 4 of the 5 intervals and 500MB/sec for the 5th. -mark