Normal CPU usage with a PPP connection

Nikos Vassiliadis nvass at
Tue Feb 26 11:51:04 UTC 2008

On Sunday 24 February 2008 14:34:37 Dylan Smith wrote:
> Hey guys, i'm running 6.3-R with (i think) userland ppp to connect to my
> ADSL provider over PPPoE.
> I build a number of graphs(mrtg) for system stats and i am noticing that
> under heavy load on my ppp connection, that is sustained 150KB/s for an
> hour or so my cpu usage, based on my graph, hovers around 20% (haven't
> looked at it in top yet but if there isn't a simple solution/reason for
> this i will look into what sort of usage it is). 

You should check top and tcpdump, it might be a big number of small
packets causing the load.

> The box is a home 
> router/server so at any 1 time isn't doing anything else that i can see
> would account for this. I also noticed that this usage increase does not
> occur with traffic coming/going on the inward facing interface, which
> regularly has transfer speeds around 10 MB/s.
> I'm running a AMD X2 3800+ with 2GB ram. Is this sort of usage normal?

Can't really tell if it's normal, but it is known that userland ppp uses
much resources. I have seen it too using 20% CPU time running on
a much slower machine(I faintly remember that it was a Pentium
Celeron at 300MHz) and a much slower(384/128?) ADSL line.

> Is there something i can do to bring it down?

Fortunately yes, you could use net/mpd. The CPU load then,
will probably be near zero for normal traffic, the 150KB/s you
mention. If a million of small packets arrive at your router the
load will be higher, nevertheless orders of magnitude lower
than the load caused by userland ppp.

HTH, Nikos

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