Re: Channel Access Timeouts - ca(get|put) CA Priority

["Johnson, Andrew N. via Tech-talk" <tech-talk at aps.anl.gov>]

Hi Gary,

On Jul 23, 2021, at 9:51 AM, Yendell, Gary (DLSLtd, RAL, LSCI) via Tech-talk <tech-talk at aps.anl.gov> wrote:

We are having very occasional issues with CA timeouts (with a timeout of 3 seconds) from our control software (GDA). I think it is because the IOC sometimes doesn't respond when it is overwhelmed with requests and when we have multiple instances of EDM screens open it can produce a lot of requests. These timeouts can cause scans to fail, so responsiveness for the EDM screens is much less important. If we add an appropriate value for the CA Priority option to ca(get|put) in control software requests, would that make the IOC respond to it in preference to other clients and stop the timeout errors? Or am I misunderstanding what this option does?

The priority of a CA connection controls the priority of the EPICS threads in the IOC that are responsible for sending and receiving the CA messages. Whether and how the EPICS thread priority maps to that of the underlying OS depends on what OS the IOC is running on.

For IOCs running on Linux the EPICS thread priority only has an effect if the IOC has enough privilege to use the real-time scheduler (regular users generally don’t have this enabled). See this tech-talk message for a description and this how-to for detailed instructions. You might not even need to set the priorities of your CA connections to fix this though, just enabling the scheduler may be enough since the IOC thread that responds to name searches runs at a lower priority than the CA threads (which are lower again than the IOC’s threads that run the process database).

The other thing you might want to do is look at the CA search traffic on your subnet and see if it’s worth cleaning up any clients that are searching for PV names that don’t exist. If you don’t keep an eye on old GUI screens and other clients these dead searches can severely load down your IOCs. There are a couple of older tools I know of which can help to identify those PV names (caSnooper is probably best known), but the community might have others that I don’t know about.

HTH,

- Andrew

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