FreeBSD Listen Queue Overflows - can't increase max queue size

The kern.ipc.somaxconn might not do what you think it does. It is the limit to outstanding and unhandled connections. (e.g.. it is not a connection limit, but the pending handle connection limit).

To use a non computer analogy: It is the maximum number of ringing phones (before they are picked up and answered), not the number of maximum simultaneous phone calls.

If you have backlog that is that big then your need to let your application pick up the phone more often (e.g. give it more resources, more CPU, better framework, whatever).

Note that the http://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/configtuning-kernel-limits.html">FreeBSD handbook section on kernel limits states this: (Emphasis mine).

The kern.ipc.somaxconn sysctl variable limits the size of the listen queue for accepting new TCP connections. The default value of 128 is typically too low for robust handling of new connections in a heavily loaded web server environment. For such environments, it is recommended to increase this value to 1024 or higher. The service daemon may itself limit the listen queue size

I have no experience with Nginx, but also check its configuration files for above mentioned limit from the application side.

May be listen queue limited to 128 in nginx config

Look nginx config for settings like:

listen 80 backlog=128;

And either remove backlog (default is -1 = use system limit) or change to bigger value (8192 should be enough even for loaded server). If even with increased listen queue your still see listen queue overflow it may indicate, that nginx is blocked for a long time (because of slow/overloaded HDD, or by bad written 3rd-party module).