Here's some practical advice for running dpb with, say, 4 hosts and
16 cores each.

> For instance, ssh multiplexing to remote hosts breaks down.

dpb opens a ControlMaster ssh connection to the remote hosts and
a multiplexed session for each build job.  By default, sshd limits
this to 10 sessions.  For more jobs per host, sshd_config needs to
be tweaked, e.g. by adding this stanza at the bottom:

# dpb jobs
Match User root
       MaxSessions 16

> I take it that dpb runs into resource limits,

dpb itself requires something like 18 + 2*jobs file descriptors.
(I don't know the exact formula, the number of hosts probably figures
in there.  You can try two different numbers, check fstat -p <dpb>,
and use your calculator's linear regression feature.)

The default file descriptor limit for root is 128, so to run a total
of 64 jobs, something like "ulimit -Sn 192 && dpb" is needed.

-- 
Christian "naddy" Weisgerber                          naddy@mips.inka.de