Experimental Physics and
Industrial Control System

"Pearson, MR (Matthew)" <[email protected]> · Wed, 20 May 2009 11:23:01 +0100

Hi,

I'm currently investigating a possible race condition in our motor
control software. In our motor controller driver we are setting a
parameter by using a BO record writing to an Asyn port.

Then we are setting a series of other parameters using the Asyn motor
device support layer (via the motor record), which is talking through
the same Asyn port as the BO record.

Then at the end we are setting again the BO record flag.

So my question is really about guaranteeing the process ordering of
these records, as well as making sure that the same ordering is
preserved in Asyn.

I know channel access has a sequence ID that the channel access server
uses to re-order CA events on the IOC, and process them in sequence
order number (is that right?).

On my client side, I do a series of ca_put()s, and call ca_pend_event()
after each one. But I understand that the CA client is not guaranteed to
service the event when calling ca_pend_event(). So is the CA sequence
number assigned to an event at the ca_put() stage, or the at the
ca_pend_event() stage?

Now on to Asyn... The device support layers for the BO and motor records
are connecting to the same Asyn port. Does the asynManager set up
different queues for each device support layer type? Or is it one queue
per port?

I am worried that if there are different queues for the motor records
and the BO record, then it is possible that the processing could be done
out of order in the scan task at this low level, even though processing
was initiated *in* order at the record level.

In the asyn documentation is does say that a thread handling a queue is
dedicated to a specific port, so I think that means that there is indeed
only one queue per port.

Cheers,
Matthew

DLS Controls

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Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System