In one of our slave DNS servers BIND, version bind910-9.10.0P2_5, constantly get killed with the following message in /var/log/messages:
Nov  9 05:11:14 zsusun2 kernel: pid 2509 (named), uid 53, was killed: out of swap space

This service runs on a FreeBSD 10.0 VM in XCP 1.6, it has 922MB of system memory.

At this moment swapinfo return this:
Device          1K-blocks     Used    Avail Capacity
/dev/ada0p3        944128     9060   935068     1%

I don't think it's a swap problem, it appears to be memory leakage, but I'm unsure.

This is one of two slave DNS servers, they only store the zones from the authoritative server and act as a recursive server for the internal users to the outside world. The number of clients is something between 700-1500 simultaneous users. Since we have a /21 internal space and a /23 public IPv4 space and there's no queries from the outside world, the port 53 is even blocked on the firewall to those machines.