How can I find the reason network writes fail with ENOMEM on 7.x?

[Eugene Perevyazko]

I'm getting network write failures on a host running 7.2-PRERELEASE
(I know, I should update it to STABLE, but I've heard of similar reports
on 7-S too)

  Failures are expressed for example in BIND
named[72084]: /usr/src/lib/bind/isc/../../../contrib/bind9/lib/isc/unix/socket.c:1567: unexpected error:
named[72084]: internal_send: 192.168.71.91#1049: Cannot allocate memory
named[72084]: client 192.168.71.91#1049: error sending response: out of memory

and on ssh session spontaneously breaking with "Write failed: Cannot allocate memory"

Frequency of those failures clearly correlates with network load for the host,
which is mainly doing dummynet and ng_nat.

How can I find what to tune in this case?

-- 
Eugene Perevyazko