Dom0 xvif mbuf issues

[Harry Waddell <waddell%caravaninfotech.com@localhost>]

I have a server where Dom0 started becoming unusable as of a few months ago
where previously it ran for years with few issues. 

netbsd-7 branch, never more than a month behind. 
BRIDGE_IPF is enabled and these options set in sysctl.conf: 

kern.sbmax=1048576
net.inet.tcp.recvbuf_max=1048576
net.inet.tcp.sendbuf_max=1048576
kern.mbuf.nmbclusters=300000
kern.maxfiles=3000

Xen 4.8.3 similarly updated.

One of the xvif devices "could not allocate a new mbuf". I enabled MBUF debugging
and netstat didn't seem to point to a leak on any of the devices. It hung again, but with a new 
error scrolling on the console. "xennetback: got only 63 new mcl pages"

My suspicion is that either one of the guests started doing a lot more nfs activity OR
that a VM I created which uses a fuse filesystem to move large dumpfile to azure blob
storage may be what pushed this previously working system off the edge. 

I'm moving the azure fuse system to another server and plan to disable ipf on the bridge. 

Beyond that, and any suggestions? Should I just upgrade to netbsd 8 and/or xen 4.11?
( even if it's just to make debugging easier since this is where current work is taking place? )

This is a production system with about 30 guests. I just want it to work like it used to. 

HW