ksoftirq consumes >80% CPU on embedded platform

Hello,

we're designing SOHO router based on MIPS processor, wired up with 24-ports
switch. The CPU runs NAT (configured with iptables), iptables rules, dhcp
etc. it doesn't have any H/W acceleration for these functions. When testing
NAT in full-mesh mode (i.e. one WAN port and others are LAN port), we
observe the significant system's slowdown, especially console responds very
slowly, and there is also packets loss.
The 'top' shows that ksoftirqd consumes over 80% of CPU.

What can be the reason of such behaviour? As I know Linux NAT runs in kernel
space, so it should not be a problem of context switching?

--
Mark

["Followup-To:" header set to comp.os.linux.networking.]
On Wed, 2010-07-28, Mark wrote:
> Hello,
>
> we're designing SOHO router based on MIPS processor, wired up with 24-ports
> switch. The CPU runs NAT (configured with iptables), iptables rules, dhcp
> etc. it doesn't have any H/W acceleration for these functions. When testing
> NAT in full-mesh mode (i.e. one WAN port and others are LAN port), we
> observe the significant system's slowdown, especially console responds very
> slowly, and there is also packets loss.
> The 'top' shows that ksoftirqd consumes over 80% of CPU.

Get the mpstat(1) command and check the output from mpstat -P ALL <n>
to get a different view of it (or check the files in /proc which give
you the same info, except in raw form).

mpstat(1) is part of the sar package, I think.

> What can be the reason of such behaviour? As I know Linux NAT runs in kernel
> space, so it should not be a problem of context switching?

/Jorgen

--
// Jorgen Grahn <grahn@ Oo o. . .
\X/ snipabacken.se> O o .