Latency spikes on FreeBSD 8.2 with kernel scan function.

Ondrej Zajicek santiago at
Wed Nov 23 16:27:40 CET 2011

On Tue, Nov 22, 2011 at 09:43:52AM +0100, Pawel Tyll wrote:
> >> I do have dummynet configured, but like I said - every 20 seconds
> >> network freezes for about 0.5s. Guess what happens every 20 seconds...
> >> You got it! Protocol "kernel" scans the kernel routes. Also, after
> >> /usr/local/etc/rc.d/bird stop, problem is gone. What could be causing
> > bird is stopped, traffic forwarding is stopped (since dynamic routes
> > disappers) ?
> > Perhaps your configuration shapes ALL (e.g. network AND local) traffic
> > withing single queue? If so, when queue is full with network traffic
> > even local traffic is delayed ?
> No  no.  And  there's  almost  no traffic. There's one IBGP peer, this
> machine  isn't  in  production yet. It was for a moment, but had to be
> pulled because of this. I'm 100% certain this has something to do with
> 'scan' facility of 'kernel' protocol.

You can verify that by disabling 'kernel' protocol.

On BSD, kernel scan is implemented by requesting a big buffer and
copying several tens of MB of route data from kernel to BIRD.
I am not an expert on BSD, but perhaps there may be some kernel problem
on some BSD version?

Another possible explanation is that kernel scan for some reason generates
a lot of log messages that strains the disk/systen, but i would doubt that.

Elen sila lumenn' omentielvo

Ondrej 'SanTiago' Zajicek (email: santiago at
OpenPGP encrypted e-mails preferred (KeyID 0x11DEADC3,
"To err is human -- to blame it on a computer is even more so."
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