Re: Subscription updates halting?

[Andrew Johnson <anj@aps.anl.gov>]

Hi Matt,

On 11/10/2013 04:46 AM, Gibbs, Matt wrote:
> I have a simple test script where I create a channel, then create a
> new subscription, and pass in an event callback function which just
> prints the current value of the PV to the terminal.  Most of the
> time, this works great, but maybe 20% of the time, I get two updates,
> then nothing else for the duration of the program.  This is always
> using the same code, with the same PV.  If I run ‘camonitor’ on the
> PV repeatedly, I never see the same problem.
>
> I don’t get any exception messages, and the rest of the script
> doesn’t crash or halt, so it seems like the event callback just stops
> getting called.
>
> Any ideas about what might be happening, or things I can do to
> further debug this?

As Dirk said its easier for us to help if you can show us the code, but 
I can suggest a couple of possibilities.

Are you configuring the CA library to use pre-emptive callbacks or not? 
See http://www.aps.anl.gov/epics/base/R3-14/12-docs/CAref.html#Thread 
for an explanation. If you want libca run your callbacks from its 
internal threads at any time (pre-emptive enabled) you must handle any 
necessary locking to protect your data structures against race 
conditions. If on the other hand you disable pre-emptive callbacks you 
must call ca_pend_event() regularly because libca can only call your 
event callbacks while your main thread is inside that routine.

Note that the callbacks will still happen in the context of those 
internal libca threads, but you don't have to worry about using mutexes 
to protect your data in most cases.

There is a difference between ca_pend_io() and ca_pend_event(); the 
former will only result in monitor callbacks being run while the library 
is waiting for channel connections and ca_get() and ca_put() operations 
to complete. If the only I/O outstanding is waiting for monitor events 
to fire, ca_pend_io() will return immediately without processing them, 
thus you usually need to use ca_pend_event().

If that doesn't help we'll probably need to see your code to work out 
what's going on.

- Andrew
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