Subject: Re: ip_flow.c (ip_fastforward)
To: Joe Smith <>
From: Jason R Thorpe <>
List: tech-net
Date: 05/18/2000 14:45:09
On Thu, May 18, 2000 at 05:37:26PM -0400, Joe Smith wrote:

 > We're using ip_fastfowarding,  on a system routing 20k-30k packets per
 > second between two intel pro/100+ cards. When we enable it,  it causes
 > strange system behaviour;  System gets sluggish,  pauses from time
 > to time.  It may be because IPFLOW_MAX is set to 256.  (IPFLOW_MAX
 > determines the MAX # of active routes stored in the table?) We attempted
 > to change it to 8192,  but the system halts immediately after
 > enabling fastforward.

Right, so your system is spending a LOT of time fowarding packets
(30k/sec is a pretty decent clip, after all), and since the whole
point of fastforward is to do it immediately when the packet arrives
(i.e. still at interrupt level), which means less time for user
processes to run.

Do you see any degredation in the rate at which packets are fowarded?

The reason you're losing when you crank IPFLOW_MAX up to 8192 is
probably because you're running out of KVA space.  Can you send me
the exact error message that occurs?  (I'm assuming it panics.)

Also, what rev of the OS are you running?

 > System has BGP4 running with 72,000 routes in kernel.

In any case, impressive!

        -- Jason R. Thorpe <>