Experimental Physics and
Industrial Control System

"Jeff Hill" <[email protected]> · Tue, 26 Oct 2010 15:53:54 -0600

> if I fire two ca_get_callback on the
> same channel in a row, do I get two callbacks, too?

yes

> Are there any combinations which will fail 
> ("sorry, pv is active")?

No, and in fact if you produce {get, get callback, 
or put callback} requests at a rate higher than 
than they can be consumed by the channel's record then
the producer (the client app) will eventually block
until the record consumes enough buffer space to allow 
for additional requests to be enqueued. 

> so will interleaving two ca_put_callbacks also fail?

Negative, but the blocking scenario is however more likely 
with put callback because, if a put callback is already in 
progress for the same channel, the server's per-client 
thread will wait until record processing (both synchronous 
or asynchronous), and all side effect record processing
(both synchronous or asynchronous) finish before initiating 
another put callback request.

In contrast get and get callback only need to obtain the db 
scan lock before they can execute. In current versions of 
EPICS a get/get callback do _not_ initiate record processing.

> What if I interleave a synchronous ca_get
> (i.e. I wait for completion using ca_pend or somesuch) with a 
> ca_get or ca_get_callback issued by another thread?

Only a thread which has initialized the library in 
non-preemptive callback mode can call ca_pend_io to 
rendezvous with completion of a batch of no-callback 
gets which have been requested; furthermore, it's not 
allowed to attach additional threads to a non-preemptive 
callback mode ca context.

So while one can mix ordinary get and callback get in
a non-preemptive mode ca context, a preemptive mode ca
context is a no-callback get free zone.

> Cheers (head smoking)

:-)

This probably raises some concerns about documentation 
gaps so I created a bug entry at lp.

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

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> -----Original Message-----
> From: [email protected] [mailto:[email protected]]
> On Behalf Of Ben Franksen
> Sent: Tuesday, October 26, 2010 2:18 PM
> To: EPICS Techtalk
> Subject: Some Channel Access Questions
> 
> I am again hacking on the sequencer internals and thereby came to ask
> myself how CA handles these cases:
> 
> - multiple ca_put_callback calls on the same channel
> - multiple ca_get_callback calls on the same channel
> - multiple (blocking) ca_get calls on the same channel
> - a mixture of any of the above
> 
> I assume exactly one channel here, which is possibly accessed from
> multiple threads.
> 
> The only unproblematic case is the simple ca_put, as it is fire and
> forget; for ca_monitor I happen to know that multiple subscriptions are
> fine and each subscription get its own callback on each event. Do the
> other calls behave similarly, i.e. if I fire two ca_get_callback on the
> same channel in a row, do I get two callbacks, too? Are there any
> combinations which will fail ("sorry, pv is active")? I remember
> reading some time ago that only one *db*_put_callback can be active at
> a time on a record (on the IOC), so will interleaving two
> ca_put_callbacks also fail? What if I interleave a synchronous ca_get
> (i.e. I wait for completion using ca_pend or somesuch) with a ca_get or
> ca_get_callback issued by another thread?
> 
> Cheers (head smoking)
> Ben
> --
> "Never confuse what is natural with what is habitual."
>                                    -- attributed to Mahatma Gandhi

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System