Re: Route cache performance under stress

["David S. Miller" <davem@redhat.com>]

   From: Ralph Doncaster <ralph@istop.com>
   Date: Tue, 10 Jun 2003 17:39:44 -0400 (EDT)
   
   Looking at Simon's profile numbers, these seem to contribute a lot more
   than fib_validate_source:

Ignore all the fib_rule*() and associated overhead, Simon
is going to turn of policy routing support so that stuff
drops out of the profiles.

The fib_lookup() will decrease significantly from the profiles
if fib_validate_source() is deleted, and this is what I want
confirmed from such an experiment.

     1237 ipt_route_hook                            19.3281
     3120 do_gettimeofday                           21.6667
     8299 ip_packet_match                           24.6994
     8031 fib_lookup                                25.0969
     1877 fib_rule_put                              29.3281
   
   What's the do_gettimeofday for?

Every packet records a timestamp.

   I traced back the fib_lookup to ip_forward_finish, which seems to only
   call ip_forward_options when there's IP options (go figure!), which would
   make sense for a SYN packet (juno).

You're thinking TCP options, not IP options.

   What I can't think of is why we'd want to have special routing
   considerations for TCP SYN packets (and other IP packets with
   options set).

ip_forward_options() has to update things like record-route
IP options, which record hop-by-hop information for diagnostic
tools like traceroute.