Experimental Physics and
Industrial Control System

Benjamin Franksen <[email protected]> · Wed, 12 Jun 2013 18:10:15 +0200

On Monday, June 10, 2013 17:22:12 Carl Lionberger wrote:
> We have several instances of a sequencer that occasionally start getting
> messages such as this, every 10 seconds or so:
>
> sevr=fatal pvGet(ss sscanner, var PVL[23], pv SR08U___GDS1HS_BC03): failed
> (timeout waiting for other get requests to finish)
>
> examining that channel with seqChanShow shows:
>
> #84 of 99:
>   Variable name: "PVL[23]"
>     type = long
>     count = 1
>   Value = 0
>   Assigned to "SR08U___GDS1HS_BC03"
>   Connected
>   Not monitored
>   Not sync'ed
>   Status = 10
>   Severity = 2
>   Message = get completion timeout
>   Time stamp = 2013-06-08 22:00:24.857905
> Next? (+/- skip count)
>
> Looking at the code in seq_ca.c and seq_if.c, it seems that if a
> synchronous pvGet ever fails to get a callback from channel access, the
> "get completion timeout" message is set in the channel metadata and all
> subsequent attempts to do pvGets on that channel will fail as in the first
> message.  The latter occurs because there is a get semaphore for each
> channel that the callback is supposed to give.
>
> My thought is that if the callback doesn't come the semaphore should be
> released in the same code that sets the "get completion timeout" message.

This is a difficult question.

Suppose I make this change in the sequencer code.

Now imagine this scenario: your program will continue to issue pvGet request
(even though the first one failed). Now, suppose the CA callback /does/
arrive, only late. Maybe right after you issued your second request. Then the
sequencer will think that the response (from CA) is to your second request,
when in reality it is the response to the first request.

I think what is really needed is a way to /cancel/ the request. I am not sure
but I think the CA client API does not offer this possibility, at least not
when using ca_get_callback.

I find it very hard to judge what the correct behaviour is, here.

Maybe Jeff can comment?

> Or is there something the operator should be able to do, short of
> restarting the sequencer?

There is not, currently, but I could add something. Maybe letting the
programmer explicitly "recover" from timeouts is better than doing it
automatically, at least as long as (and assuming that) canceling pvGet
requests is not supported by the CA client API.

Cheers
--
Ben Franksen
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Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System