RE: channel access

["Jeff Hill" <johill@lanl.gov>]

> Is this behavior reasonable? Does pend event do something fundamentally
> different from pend io?

Yes. If you have not made any of the following IO requests since the last
call to ca_pend_io() (or the start of the program whichever is first), then
ca_pend_io() will have no effect other than to flush the output queue,
and ca_pend_io() will return immediately without blocking for the duration
of the specified time out.

1) created a CA channel (without specifying a connection call back handler)
2) made a CA get request (without specifying an IO completion call back handler)

In contrast ca_pend_event() will always process CA background activity
for the full duration of the specified timeout and until all queued
labor has been processed by CA. 

You should call ca_poll() or ca_pend_event(1e-12) in the background at least 
a 10 Hz rate. Alternatively, in EPICS R3.14 if your application is thread safe, 
you can enable preemptive call backs and not bother with polling ca_pend_event().

Jeff

> In the course of testing edm, I have noticed the following behavior for
> some configurations (unfortunately, I don't mean anything precise when I
> say "some configurations"):
> 
>   I run a fresh copy of edm, bring up a display containing ~9000
>   references to the same pv  and execute it. This adds ~9000 events to the
>   ioc (or portable CAS). Performance is good.
> 
>   I now deactivate the display which clears all the events.
> 
>   From this point on, when this or any other display is executed, the
>   performance is such that it takes twice as long to complete the
>   process of adding all the events.
> 
> 
> 
> At this point the code looked something like the following:
> 
> ====================================
> 
>   for all pvs {
> 
>     add events
> 
>   }
> 
>   ca_pend_io( time )
> 
> ====================================
> 
> 
> 
> So, I changed this to
> 
> ====================================
> 
>   count=0
>   for all pvs {
> 
>     add events
>     if ( ++count > someNumber ) {
>       ca_pend_io( time )
>       count=0
>     }
> 
>   }
> 
>   ca_pend_io( time ) 
> 
> ====================================
> 
> 
> 
> This did not help at all. Then I change to code to
> 
> ====================================
> 
>   count=0
>   for all pvs {
> 
>     add events           
>     if ( ++count > someNumber ) {
>       ca_pend_io( time )
>       ca_pend_event( small-time )
>       count=0
>     }
>  
>   }
> 
>   ca_pend_io( time )
>   ca_pend_event( small-time )
> 
> ====================================
> 
> 
> 
> This made an amazing difference and now the performance is good all the
> time.
> 
> Is this behavior reasonable? Does pend event do something fundamentally
> different from pend io?
> 
> 
> John Sinclair
> sinclair@mail.phy.ornl.gov
> Oak Ridge National Lab
> 865-576-6362   865-574-1268 (fax)
>