Experimental Physics and
Industrial Control System

"Jeff Hill" <[email protected]> · Mon, 1 Nov 2010 15:41:52 -0600

Hi Tim,

The Channel Access publish and subscribe system is a classic
consumer/producer situation. The producer is of course record
processing and the consumer is the Channel Access client side 
application. In basic queuing theory, it is known that no
matter how big the finite buffer capacity between the producer
and the consumer, all of it will be consumed if the production
rate is even a marginal amount faster than the consumption rate.
Furthermore, to prevent one client from disrupting subscription 
updates sent to another client, each client attaching to an IOC 
is serviced out of a per-client dedicated event queue connecting 
the higher priority database processing threads with the lower
priority per-client server threads. So to avoid consuming all
available memory this event queue is finite length, and if full
capacity is reached the last event on the queue for a particular
subscription is replaced; this has the net impact of causing an 
intermediate subscription update to be discarded.

So one can easily see, given the above design with intermediate 
events being discarded when the producer out-paces the consumer,
that events will sometimes be delivered out of order when comparing
different channel subscriptions, but that events will never be 
delivered out of order within a single channel subscription.

HTH

Jeff
______________________________________________________
Jeffrey O. Hill           Email        [email protected]
LANL MS H820              Voice        505 665 1831
Los Alamos NM 87545 USA   FAX          505 665 5107

Message content: TSPA

With sufficient thrust, pigs fly just fine. However, this is
not necessarily a good idea. It is hard to be sure where they
are going to land, and it could be dangerous sitting under them
as they fly overhead. -- RFC 1925

> -----Original Message-----
> From: [email protected] [mailto:[email protected]]
> On Behalf Of Tim Mooney
> Sent: Monday, November 01, 2010 3:10 PM
> To: EPICS tech-talk
> Subject: monitors received out of order
> 
> Dear folks,
> 
> I have two records, and a separate task monitoring a field that both
> records
> post, all in the same IOC.  Most of the time, my task receives monitors in
> the
> order in which they were posted.  But sometimes, the task receives a
> monitor
> from record B before it receives a previously posted monitor from record
> A.
> (I know for sure which record is posting first, because record A posts
> before
> causing record B to process.  Also, I've modified the record to set its
> time
> stamp immediately before posting, and I get another time stamp on entry to
> the monitor routine.  The time ordering of those stamps does not agree.)
> I've seen this on solaris and Linux, but not on vxWorks.
> 
> I have code that misbehaves when this happens, so I started digging around
> and have convinced myself that I should not be relying on the time
> ordering of
> monitors received from different records (even when those records are
> running
> in the same task).  I now think I can rely on posts from a single record
> arriving in
> time order, but not posts from different records.  I think this because
> events from
> different record go into different queues, and there doesn't seem to be
> any code
> in the vicinity that seems worried about time ordering across event
> queues.
> 
> Am I right about this?
> 
> --
> Tim Mooney ([email protected]) (630)252-5417
> Software Services Group, Advanced Photon Source, Argonne National Lab.

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System