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?