Today, a PowerDNS (open source dns, www.powerdns.com) deployment ran into
trouble with large amounts of IPv6 users. It appears a large telco 'flicked
the switch'. We had around 8000 DNS queries/s over IPv6, and everything
slowed to a crawl. 100% CPU utilization, most of it in the kernel. The same
amount of queries over IPv4 causes no problems.
Note, this system is not functioning as a router or anything. It is just
serving IPv6 DNS to a reasonable number of clients.
Thanks to diligent debugging and rapid help from friends over at SUSE, who
suggested setting net.ipv6.route.max_size to a higher than default value,
all problems were quickly resolved (thanks!).
From a quick reading of ip6_dst_gc, it is obvious that exceeding the
max_size of the IPv6 routing table quickly becomes painful, causing non-stop
net.ipv6.route.max_size defaults to 4096. The equivalent setting for IPv4
defaults to 'millions' or is even dynamically sizing in modern kernels.
Now I know distributions can set this sysctl at will, but it appears that
many of them don't. It does appear odd that we still assume at a kernel
level that IPv6 is 'rare', a thousand times more rare than IPv4.