RE: Preemptive callback required

["Jeff Hill" <johill@lanl.gov>]

> dbServiceIO: preemptive callback required for 
> direct memory interfacing

A minor clarification:
The message says "direct in memory interfacing" and not "direct memory
interfacing". There should probably have been a hyphen in "in-memory". I fixed
that for R3.15.

> I looked at where the message was coming from but am not really 
> any the wiser as to what it is trying to tell me. 

In R3.13 the vxWorks version of the ca client library was implemented with
preemptive calls to user registered callback methods, but this was not allowed
or implemented on any other OS.

In R3.14 preemptive callback is implemented on all OS, but this is optional, and
the default is non-preemptive callback for backwards compatibility to the widest
range of applications that do not run on vxWorks.

Within the iocCore execution environment there is an efficient direct in-memory
path between client requests and the database calls that implement them, and all
socket related overhead is eliminated. In R3.14 this feature is currently
requiring the preemptive callback execution environment, and it is not possible
to create a channel unless you have preemptive callback mode enabled. I must
admit that I don't clearly remember why I coded in this restriction so if its
important, I could look into this further and see if it is still required. 

However, given that Williams's application was probably running before under
vxWorks in the R3.13 vxWorks ca client callback environment, then I assume that
the simple solution will be to just enable preemptive callbacks when starting up
a ca client context in the pvload utility. This is assuming of course that the
pvload code was prepared to receive preemptive callbacks when it ran in the
vxWorks preemptive callback environment provided under R3.13.

Jeff


> -----Original Message-----
> From: Chris Mayer [mailto:cjm@observatorysciences.co.uk]
> Sent: Monday, June 09, 2003 1:49 PM
> To: tech-talk@aps.anl.gov
> Subject: Preemptive callback required
> 
> 
>  I have just built William Lupton's pvload utility under 3.14.2 so that
> I
> can call it from iocsh on a Linux box. I ran it with a test file and the
> records got set correctly at startup however, I got the following
> message
> 
> dbServiceIO: preemptive callback required for direct memory interfacing
> of CA channels to the DB.
> 
>  I looked at where the message was coming from but am not really any the
> wiser as to what it is trying to tell me. Before I start looking at the
> pvload code can someone explain what circumstances would trigger this
> message.
> 
>   Regards
> 
>    Chris Mayer
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