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Date:	Tue, 18 Dec 2007 08:37:26 -0500
From:	Mark Seger <Mark.Seger@...com>
To:	netdev@...r.kernel.org
Subject: 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


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