Re: Linux IPv6 router table size.

[Eric Dumazet <eric.dumazet@gmail.com>]

Le samedi 15 janvier 2011 à 22:11 +0100, Jan Hugo Prins a écrit :
> Hi netdev kernel developers,
> 
> In the beginning of December the number of IPv6 routes in the BPG router
> tables got somewhere close to 4096 and this number is somewhat magical
> in Linux. Why is it magical? Well, the default setting for
> net.ipv6.route.max_size is hardcoded in the linux kernel to be exactly
> this number, this in contradiction with the IPv4 net.ipv4.route.max_size
> which is set based upon the size of your memory. Actually, in the IPv4
> part of the kernel sources their is a route.c with a nice formula
> telling the kernel how big this number should be. For IPv6 their is also
> a route.c with some nice formula in it, the formula says exactly the
> following:
> net->ipv6.sysctl.ip6_rt_max_size = 4096;
> 
> The result of all this magic was that when the BGP RIB in my quagga
> routers went over the 4096 routes, quagga started complaining that it
> couldn't put the routes in the kernel FIB anymore and this went all into
> syslog.
> 
> It took some digging to find the reason for these error messages, but
> after changing this sysctl to something more useful, everything was fine
> again.
> 
> Maybe it is a good idea to change this in the IPv6 tree to something
> more useful in the same direction of the solution that is in the IPv4 tree.
> 

Hi

IPv4 does an auto memsize tuning because it uses a hash table, and this
table cant be resized after boot.

IPv6 is different, and you can change 
/proc/sys/net/ipv6/route/gc_thresh & /proc/sys/net/ipv6/route/max_size
whithout downsides.

echo 32768 >/proc/sys/net/ipv6/route/max_size
echo 8192 >/proc/sys/net/ipv6/route/gc_thresh

(or equivalent lines in /etc/sysctl.conf)

We probably could change default values, but they are "not hardcoded" ;)