[plug] CPU usage with PPP

[Michael Collard]

I've got my own little gateway/server that handles my pppoe connection
and provides a number of services. Its a celly 633 with 256Mb RAM. 

I go to use VoIP today and notice that the quality is crap, to the point
its dropping out. Hmm, thats odd, that never happens. I start some basic
troubleshooting. I notice in my log that my server's NIC is having
timeouts with the following showing up:

Jan 18 17:44:23 amy kernel: NETDEV WATCHDOG: eth0: transmit timed out

So I change the port on the switch its going through and that seems to
have resolved that problem. I go to use the VoIP again and its the same.
I'm not maxing the 8/1 link... Im doing barely 200K down, maybe 50K up,
and I use traffic shaping anyway so none of this would impact VoIP. I
start pinging things and notice high response times to my server and
anything it NATs. And when I say high on LAN I mean up to and over 20ms.

That just ain't right especially since other devices respond < .5ms
where I expect them too. I do a 'top' and notice that pppoe is sitting
these just as such: 

32241 root      25   0  1536  496  428 R 95.1  0.2   3:07.32 pppoe

So, I am at the stage now where I've killed pppoe and restarted it,
using the very same config file I've been using for years. Still the
same. I've killed the firewall (iptables) and shaping (tc) to no avail,
CPU usage maxes out at about 260Kb/sec on the PPP link. 

I am almost certain that a reboot will resolve this problem for now, but
I would rather spend some time, find the cause and fix it properly.
Anyone have any suggestions on where to start looking?

There is no APM in use so CPU isn't throttled. Other CPU intensive apps
(such as MP3 encoding) are fine and normal. It's just PPP as far as I
can tell right now.

Cheers