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
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