question regarding strange kernel stats under high (network) load

question regarding strange kernel stats under high (network) load

[Willem de Bruijn]

Hi,

  for a network monitoring project that I'm working on (see my sig if you're 
interested) I have to compare system load under high throughput. Directly 
comparing my implementation to tcpdump (LSF) seems fine, our user% and 
kernel% are lower. However, when I added an idle system I noticed some 
strange behaviour that remains on re-executing the tests: 

the kernel% is actually higher when nothing is running than when my code is 
running! Overall processing is lower (idle% is higher on the idle system), 
but that is thanks to the high %si (soft ints) that my code generates. 

In order to better explain my problem here is the output of top (the new one, 
with %si, %hi and %wa) for a 600Mbit throughput, the first line is for the 
idle system, the second for FFPF (my system) and the third for Linux Socket 
Filters:

Mbit		iperf in	iperf out		capt	recv	kern drop	% drop		tcpdump %	iperf %	user %	
kernel %	idle %																																																																																																																																																																																																																																																	
600		608	608								33	6	30	40																																																																																																																																																																																																																																																	
600		608	608		514084	517314	33	0.63		17	20	13	15	25																																																																																																																																																																																																																																																	
600		609	608		421215	517597	96382	31.4		76	33	17	49.5	0																																																																																																																																																																																																																																																	

the usage statistics are user%, kernel% and idle% (I haven't saved the si% 
results). What's odd is that although idle is higher for the idle system, 
kernel% is actually lower. I can understand that my code generates extra soft 
interrupts, but not that it has an effect on the `other' kernel load. Perhaps 
I don't understand the values good enough, in which case I would greatly 
appreciate your help. However, it could also signal a bug in the statistics 
reporting (although I don't hope that's the case, these results are great for 
my paper :). FYI, I used the command

top -b -n 5 -d 3 | grep "Cpu\|tcpdump\|iperf"

to generate these results.

thanks for your help,

  Willem
-- 
Willem de Bruijn
wdebruij_at_dds.nl
http://www.wdebruij.dds.nl/

current project : 
Fairly Fast Packet Filter (FFPF)
http://ffpf.sourceforge.net/

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