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

Subject: Re: PyEpics and Python threads
From: Andrew Johnson <[email protected]>
To: Matt Newville <[email protected]>, "Vigder, Mark" <[email protected]>
Cc: [email protected]
Date: Wed, 20 Apr 2011 15:45:43 -0500
Hi Matt & Mark,

On Wednesday 20 April 2011 15:06:33 Matt Newville wrote:
> On Wed, Apr 20, 2011 at 1:04 PM, Vigder, Mark
> <[email protected]> wrote:
> > The only difference is I created some arbitrary unrelated PV before
> > starting the thread. I'm still
> > trying to understand CA contexts, so I'm not sure those calls are
> > necessary, but
> > for some things I tried they seemed to be required. Other little changes,
> > such as creating the
> > PV object inside the thread rather than passing it in to the thread, also
> > seem to eliminate the 30 second delay.
> > Trying on my PC rather than linux co-hosted with IOC gives about a 10
> > second delay.
> >
> > Any help or explanations are well appreciated.
> 
> The default timeout for pv.put() in "put-with-wait" mode is 30
> seconds, which is probably where that time is coming from.
> 
> It appears that the put-with-wait is not announcing completeness
> across "contexts" (that is you create a PV in the main thread, then
> create a new context in the created thread and do the put-with-wait on
> the PV from the original thread).  It's possible thatt the
> put-with-wait is not complete enough for dealing with all thread cases
> right now.

Assuming that epics.ca.context_create() results in a call to 
ca_context_create() then I worry that Mark's code ends up trying to use a chid 
created in one CA context (associated with the object m) in a thread that is 
running with a different CA context.  Internally a chid has a reference to the 
context it was created in, and I have no idea what happens if you use one this 
way, but I'm fairly sure you're not supposed to.

I would recommend that you share a single CA context across all the threads in 
your program unless you have a very good reason to not do that, it saves 
resources.  To do that you'll need to find out what the PyEpics equivalents 
are of the routines ca_current_context() and ca_attach_context(), and pass the 
context pointer returned by calling ca_current_context() from the main thread 
into the new thread and there passing it to ca_attach_context().

HTH,

- Andrew
-- 
An error is only a mistake if you don't learn from it.
When you learn something from it, it becomes a lesson.

Replies:
RE: PyEpics and Python threads Vigder, Mark
References:
PyEpics and Python threads Vigder, Mark
Re: PyEpics and Python threads Matt Newville

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