Linux starts to act weird around 200,000 concurrent connections in my experience, even with aggressive sysctl tuning. You end up with weird edge cases like netstat literally taking 15 minutes of CPU time (in kernel) before it dumps the list of connections to stdout.

Not sure about FreeBSD or any other OSes.

Not saying netstat isn't slow, but:

# time sh -c 'netstat -tn | wc -l' 486206

real 0m13.538s user 0m1.698s sys 0m10.380s

It still works with a whole lost of connections. (in fairness, only about 130k were connected)

The problem is that it doesn't scale linearly. There's some O(n4) algorithm being used in netstat or some kernel syscalls or something. Once you go over 250k, things get _really_ weird.

try ss -nt instead.. it uses netlink sockets instead of /proc and generally scales much better

Thanks for the tip!