PyEpics and Python threads

["Vigder, Mark" <Mark.Vigder@nrc-cnrc.gc.ca>]

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.

 

Mark

 

 

Mark Vigder

Canadian Neutron Beam Centre