[StarCluster] limiting the number of ipengines started on each node?

[Jascha Sohl-Dickstein]

Hello,

Due to limitations on the number of simultaneous active network connections, IPython.parallel is unable to handle more than about 250 engines.  (see the discussion at http://python.6.x6.nabble.com/IPython-User-ipcluster-Too-many-open-files-tcp-listener-cpp-213-td4978725.html )

On an mx.large instance, with 8 cores, and with an engine started for each core, this means that an ipcluster is limited to around 30 nodes.  However, a single ipengine is capable of using all the cores on a node (for instance, if it's doing matrix algebra).  I would like to be able to start a cluster with fewer ipengines than cores.  That is, I would like the ipcontroller call on each node to have a "-n [constant number of engines]" rather than "-n [number of cores]".

This is probably easy to change in a config file somewhere, so apologies if this should be obvious, but I don't (yet) know where to look.

Thank you very much for any help!

-Jascha