Weirdness - FBSD 7, Routing, Packet generator, em taskq

[gnn at freebsd.org]

At Thu, 26 Jun 2008 23:25:18 -0400,
Paul wrote:
> 
> I have a FreeBSD router set up with Full BGP routes and I'm doing some 
> tests on  using it for routing.
> 
> 7.0-RELEASE-p1 FreeBSD 7.0-RELEASE-p1 #6: Thu Apr 17 18:11:49 EDT 2008  
> amd64
> 
> oddness..:
> 
> Use a packet generator to generate random source ips and ports and send 
> traffic through the router to a destination on the other side, single ip.
> What happens is the 'em0 taskq'  starts to eat cpu... but the funny 
> thing is immediately when I start the traffic (say, 100,000 pps) em0 
> taskq is about 15% cpu.. and then over the course of 2 minutes or so it 
> climbs to 60% cpu..  This makes no sense.. The packets per second are 
> continuous and it just routed 100kpps for 60 seconds with less cpu so 
> why in the world would it slowly climb like that? 
> 
> It's an observation I suppose and I was hoping if someone could 
> enlighten me on WHY.. :)   I did test it on 3 different machines by the way.
> It even does this with just a handful of routes in the routing table , I 
> tried that too just to rule that out.  
> I don't remember Freebsd 4/5 doing this??
> 

What are you using to measure the CPU time?  Some tools take time to
gather up enough samples.  Also, have you tried to do any profiling on
the kernel to see why this might be the case?

http://www.watson.org/~robert/freebsd/netperf/profile/

Best,
George