Experimental Physics and
Industrial Control System

Mark Rivers <[email protected]> · Tue, 18 Mar 2014 17:05:08 +0000

Hi Brian,

How big are your waveform records?

Mark

-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Brian Bevins
Sent: Tuesday, March 18, 2014 11:27 AM
To: [email protected]
Subject: Re: Prioritizing Channel Access per Record?

On 03/18/14 12:03, Ralph Lange wrote:
> On 18.03.2014 16:47, Brian Bevins wrote:
>> Is there any way to specify that CA delivery of updates from certain 
>> records should be prioritized over other records on the same ioc?
>>
>> I've got an application where a bunch of ai's are processed at 10Hz 
>> and some relatively big (for us) waveforms are processed at about 
>> 0.3-0.5 Hz. The ioc is not heavily loaded and all record processing 
>> seems to happen in a timely way, but from the client end (EDM) the 
>> "fast" records all pause for ~0.5 sec whenever the waveforms push out 
>> their CA updates.
>>
>> The ai's are already being processed at a higher priority, but this 
>> doesn't seem to help their updates get pushed out with higher priority.
>>
>> Is there some tinkering I can do with the priorities of the CA 
>> threads to get what I want? How can I tell which thread will get 
>> which record updates? Is there something more obvious I'm missing?
>>
>> Thanks,
>> --Brian
>>
>
> The CA thread pairs on the server (IOC) are created per client.
> The client can set the priority as an argument to the 
> ca_create_channel() call [1].
> If the client-set priorities are honored on the server depends on the 
> server side OS and/or configuration.
>
> On real-time Linux, the MCoreUtils module [2] (EPICS 3.15 only) 
> contains a rule-based engine to tinker with IOC thread priorities and 
> policies.
>
> Cheers,
> ~Ralph
>
> [1] 
> http://www.aps.anl.gov/epics/base/R3-14/12-docs/CAref.html#ca_create_channel
> [2] http://epics.sourceforge.net/mcoreutils/

Thanks for the quick reply, Ralph.

Using client-set priorities occurred to me, but then it seems I would 
need to write my own client to monitor and display the slow (or fast) 
records. Standard clients are all pretty much going to use a default 
priority in ca_create_channel(). If there is an existing general purpose 
client that allows setting the priorities, I'd love to hear about it. 
I'm currently using R3.14.12.3 on RTEMS, and I don't yet know if that 
supports client-set priority. If it does, then writing a little display 
program might work.

If I were to switch to rt linux and make use of mcoreutils, it sounds 
like I would essentially run two virtual iocs on the same machine, one 
at a higher priority. Is that right?

Thanks,
--Brian

-- 
Brian S. Bevins, PE
Computer Scientist / Mechanical Engineer
Thomas Jefferson National Accelerator Facility

      "The urge to save humanity is almost always only a false-face
       for the urge to rule it."
                                      -- H. L. Mencken

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System