Experimental Physics and
Industrial Control System

Folks,

 

Andrew Johnson thought he remembered someone at the recent EPICS collaboration meeting talking about improvements to the EZCA extension to use 3.14 preemptive callbacks.  Has someone done some work on this?

 

Here’s why I want to know:

 

Thanks to help from Andrew I just discovered that the IDL interface (and probably other scripting languages like Matlab, etc.) can cause serious problems with an EPICS IOC.  The problem arises when the IDL client sets monitors on a PV (with caSetMonitor()), but then does not call ca_pend_event() frequently.  ca_pend_event() is called by the “caGet”, “caPut”, “cacheckMonitor”, etc. functions in IDL.  But if one calls caSetMonitor() and then just sits at the IDL prompt, ca_pend_event() is not being called.  When that happens the CA server is trying to send monitor events to the client.  These initially queue up on the client side, but eventually they begin to queue up on the server side.  As they queue on a vxWorks server they consume mbufs.  I have found that a single client in this state will consume all of the 256 byte clusters, and some of the 512 byte clusters.  If a few more clients are in this state then there are no mbufs available above a certain size.  When that happens no new CA connections can be made, other network services fail, etc.

 

I think this problem could be avoided if we changed the ezca C library that IDL calls to use preemptive callbacks, so that periodic calls to ca_pend_event are no longer needed.

 

Thanks,

 

Mark