Experimental Physics and
Industrial Control System

"Gibbs, Matt" <[email protected]> · Tue, 19 Nov 2013 13:19:58 -0800

Thanks for the info.

I am using node-ffi to create an interface between the libca C library, and the javascript code in my node.js server.  I am using preemptive callbacks right now, and when a libca monitor event callback fires, I run a javascript function.  After reading the channel access docs, I think the problem I am having is that node.js doesn’t like it when you call a javascript callback from anything but the main thread, and libca runs callbacks from its worker threads.  Sounds like I need to either disable preemptive callbacks, or come up with a more sophisticated way to deal with the multithreaded-ness of CA.

Back to the drawing board!
—Matt
On Nov 11, 2013, at 8:52 AM, Andrew Johnson <[email protected]> wrote:

> 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
> -- 
> Advertising may be described as the science of arresting the human
> intelligence long enough to get money from it. -- Stephen Leacock

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System