Re: PyEpics and Python threads

[Matt Newville <newville@cars.uchicago.edu>]

Hi Mark,

On Wed, Apr 20, 2011 at 1:04 PM, Vigder, Mark
<Mark.Vigder@nrc-cnrc.gc.ca> wrote:
> I'm working on getting PyEpics and Python threads to work together. I get a
> lot of
> inconsistent behaviour, but the following seems to be somewhat consistent.
> At least
> today it is. I have a motor record, 'L3:m1' and run the following:
>
> from threading import Thread
> import epics
> import random
> def th(pv, sp):
>     epics.ca.context_create()
>     pv.put(sp, wait=True)
>     epics.ca.context_destroy()
> if __name__ == '__main__':
>         d1=random.randrange(360)
>         m = epics.PV('L3:m1')
>         t1 = Thread(target=th, args=(m, d1))
>         t1.start()
>
> When executing the 'pv.put' statement, this (almost) always sits for about
> 30 seconds
> doing nothing, and then moves the motor correctly. Why the 30 second delay?
>
> However, the following program usually (though not always) executes
> correctly
> without the 30 second delay.
>
> from threading import Thread
> import epics
> import random
> def th(pv, sp):
>     epics.ca.context_create()
>     pv.put(sp, wait=True)
>     epics.ca.context_destroy()
> if __name__ == '__main__':
>         d1=random.randrange(360)
>         m = epics.PV('L3:m1')
>         t1 = Thread(target=th, args=(m, d1))
>         anyPV = epics.PV('L3:scan1')
>         t1.start()
>
>
> 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.

--Matt Newville