Experimental Physics and
Industrial Control System

Kate Feng <[email protected]> · Tue, 02 Nov 2010 11:47:32 -0400

Hi Tim and Jeff,
 Linux and Solaris are not real-time operating systems to ensure the
order

of the events, unless there is a real-time extension (e.g. real-time Linux).

Another option is to rewrite the application code to ensure the order of
events.

The code written for vxWorks usually relies on O.S. to ensure the order.

For a non real-time O.S., one would have to write extra code at the user

application level to ensure the order. If one uses EPICS, then EPICS

is considered as part of the application levels.
Cheers,
Kate
Jeff Hill wrote:

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
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-----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