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

Subject: Re: Preemptive callback required
From: Chris Mayer <cjm@observatorysciences.co.uk>
To: Jeff Hill <johill@lanl.gov>
Cc: tech-talk@aps.anl.gov
Date: Fri, 13 Jun 2003 07:58:01 +0100
Jeff,

thanks for the explanation. Late yesterday I had followed the old advice "If all else fails read the instructions" and discovered your updated 3.14 CA manual. I just changed the task_initialize call for context_create with preemptive callbacks enabled and sure enough it worked exactly as before. I haven't been through and replaced the other deprecated calls yet.

Chris

Jeff Hill wrote:

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
_______________________________________________________________________
Observatory Sciences Ltd.          Email: cjm@observatorysciences.co.uk
William James House                Tel: 44 - (0)1223 - 508257
Cowley Road,                       Fax: 44 - (0)1223 - 508258
Cambridge, CB4 0WX, UK             http://www.observatorysciences.co.uk



--
_______________________________________________________________________
Observatory Sciences Ltd.          Email: cjm@observatorysciences.co.uk
William James House                Tel: 44 - (0)1223 - 508257
Cowley Road,                       Fax: 44 - (0)1223 - 508258
Cambridge, CB4 0WX, UK             http://www.observatorysciences.co.uk


References:
RE: Preemptive callback required Jeff Hill

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