Experimental Physics and
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<== Date ==>		<== Thread ==>

Subject:	Re: Many PV connections from CA client
From:	Matt Newville via Tech-talk <[email protected]>
To:	Tong Zhang <[email protected]>
Cc:	"[email protected]" <[email protected]>
Date:	Thu, 21 Mar 2019 12:06:55 -0500

Hi Tong,

On Thu, Mar 21, 2019 at 10:05 AM Tong Zhang via Tech-talk <[email protected]> wrote:

Hello,

As you can see from the attached screenshot, the warning messages from
ca module of pyepics happened when I was doing tasks described as below:

1. I created many epics.PV objects (few hundreds to a thousand);
2. Capture all the readings from these created PV objects in a for loop.

It looks like I can use caget_many(<PV list>) to achieve the goals, but
the whole logic needs to be well refactored, so what if I want do this
in the for-loop way, how can I avoid this UserWarnings, anyone can help
me understand this issue?

Those warning messages are coming because those PVs are not connected, or have not connected yet. There are several possible reasons for that, including an IOC that is currently down or slow to respond. It might work to add a short wait before fetching values, and it might be useful to test whether PVs are actually connected.

It's a little hard for me to tell what you're doing, but it should definitely be possible to create several hundred PVs and capture all those values in a loop. Perhaps something like this:

import epics

import time

# create some PVs

pvs = []

for i in range(1000, 1200):

pvs.append(epics.get_pv('XXX:RFC_%d:E_RD_CAVS' % i)

# wait for a brief time, to give a chance for connections to happen

time.sleep(0.05)

# read out the PV values

for step in range(10):

# read a dict of {PVname:Value}

# version 1: if you are pretty sure all PVs are connected

result = {pv.pvname: pv.value for pv in pvs}

# version 2: if you are not confident that pvs are connected:

result = {}

for pv in pvs:

if pv.connected:

result[pv.pvname] = pv.value

else:

result[pv.pvname] = '<not connected>'

# do something useful with the results

print(results)

time.sleep(1.0)

The recommended use is to create all the PVs once, as this will establish an Epics Channel that is internally, asynchronously monitored for changes, and then subsequent calls to PV.get() will report the latest value.

FWIW, `caget_many()` is designed especially for the use-case of wanting to get values for a very large number of PVs but *not* retain the Channel connections. That's really good for a script that runs to collect data on many PVs and then exists (as one might do in a cron job). But if the script or application is long-running and wants to have access to those PVs during the entire lifetime of the application, it is better to create and use a single PV per channel name.

Hope that helps. If not, maybe you can give a little more detail on what you're doing,

--Matt

References:: Many PV connections from CA client Tong Zhang via Tech-talk

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ANJ, 21 Mar 2019

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Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System