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<== Date ==> <== Thread ==>

Subject: Re: Changes to records during asynch processing
From: Marty Kraimer <[email protected]>
To: Benjamin Franksen <[email protected]>
Cc: [email protected]
Date: Fri, 29 Aug 2003 06:45:48 -0500
Benjamin Franksen wrote:


Hmmm. It depends of course on how much is deemed "significant runtime overhead". I imagine a solution along the following lines: All puts that occur during asynch processing (i.e. target record's PACT==TRUE) are redirected (by dbPut) into a queue (linked list or cicular buffer or some such thing), where pairs consisting of {field address, new value} are stored. Only when the record completes processing, the puts stored in this list actally happen. After that eventual re-processing is done.

The advantage is that for the 'normal' situation (either PACT==FALSE or
dbScanLock is active) the only additional overhead is the check for PACT
and the check for an empty queue after processing. These are effectively
two machine instructions. Only if no scan lock is active and PACT==TRUE,
real overhead will occur. Even this overhead is not too great a cost,
IMHO, if it solves the problem.

I haven't spent any time thinking about the most clever representation
of the 'deferred puts' in the queue but certainly this can be worked
out.

What do you think?

At the present time after initialization nothing in base except channel access server code allocates or frees memory. Channel access server code must because clients are connecting and disconnecting. The CA code manages memory VERY carefully.


In order to implement what you suggest the queue must be managed very carefully. But what goes into the queue? What if the value is an array? If it is an array record support routines are involved. What fields of a record are involved? Note that for the bo record only the VAL field is a problem because this is the only field that the bo record support compares with MLST.


Ben


P.S. I have the impression that nowadays asynchronous processing is
rather the rule than the exception, with the wide adoption of field
busses. Here at BESSY more than half the records with hardware device
support use CAN bus and so are asynchronous.

GTACS, the precurser of EPICS, did not have the concept of asynchronous records. Device support had to be written so that it was synchronous. Asyn support was invented for gpib devices. Once it was available people found many uses for it. Sigh!!


Marty



Replies:
Re: Changes to records during asynch processing Benjamin Franksen
Re: Changes to records during asynch processing Benjamin Franksen
References:
Does DISP work for DB OUT links? Benjamin Franksen
Re: Does DISP work for DB OUT links? Marty Kraimer
Re: Does DISP work for DB OUT links? Benjamin Franksen
Re: Does DISP work for DB OUT links? Marty Kraimer
Re: Does DISP work for DB OUT links? Related question Benjamin Franksen
Re: Does DISP work for DB OUT links? Related question Marty Kraimer
Changes to records during asynch processing Benjamin Franksen

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