Experimental Physics and
Industrial Control System

Ralph Lange <[email protected]> · Mon, 01 Nov 2010 17:27:04 -0400

 On 01.11.2010 17:09, Tim Mooney wrote:

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?

Short answer: yes.
The event queue is allocated in chunks. Each chunk has room for a number
of subscriptions, and holds the values that are on the queue for these
subscriptions.

The event task (per client) simply loops over the event queue chunks for
this client (lines ~940 in dbEvent.c), and calls event_read() for each
chunk, which loops over the queue entries in a chunk (line ~815) and
pushes everything it finds there to the CA server (via user_sub, line ~865).

Depending on which subscriptions from which records are held in which
event queue chunk, the order on the network may vary.
You are almost correct: The only order that you can rely on is the order
of updates for a single subscription. (Two subscriptions for different
fields of the same record may end up in different event queue chunks and
get sent out-of-order.)
Ralph

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System