Experimental Physics and Industrial Control System
On Fri, 2007-01-05 at 15:24 -0500, Kay-Uwe Kasemir wrote:
> On Jan 5, 2007, at 15:06 , Kurt Biery wrote:
> > With EPICS Base R3.14.8.2, we have experimented with string and
> > waveform records and have noticed that messages are not always
> > delivered to monitor processes when the messages are quickly
> > written to the PV.
> You're right.
> I understand the reason is that only the most recent string or
> waveform is cached on the server
> side in order to preserve memory, because strings or waveforms can be
> long
> (well, EPICS strings are still fixed to char[40]).
> Double or other 'small' types are queued up, so all your values
> are eventually sent out,
> but for strings, your N quick string updates end up as
> a single monitor for the last string.
>
> In a similar case (string record for log messages...) I ended up writing
> then to my own queue, then a little sequence copied
> those values slowly (<=10Hz) to the actual PV.
>
> Problem 1: If you run out of memory because you queue up more than
> you ever sent out, you're of course dead.
>
> Problem 2: What is slow enough? When is it OK to send another value?
>
> Problem 3: Even with 'double' or other data types which are queued up
> on the server side, you can get into 'flow control' when
> the client doesn't read fast enough.
> There is no indication, no log message nor any other info
> I'd know of that will tell you that you are or ever
> were in flow control. You simply loose a few messages
> without ever knowing about it.
Hopefully, Jeff will add some info about this in Channel Access.
>
> -Kay
>
- References:
- soft IOC string and array records Kurt Biery
- Re: soft IOC string and array records Kay-Uwe Kasemir
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