Experimental Physics and
Industrial Control System

Matt Newville <[email protected]> · Fri, 16 Sep 2011 16:30:03 -0500

Hi Emmanuel,

On Fri, Sep 16, 2011 at 3:09 PM,  <[email protected]> wrote:
>> pvlist = []
>> for pvName in pvNames:
>>     pvlist.append( epics.PV(pvName) )
>>
>> for pvs in pvlist:
>>     val = pv.get()
>>     <store value>
>>
>> With this sort of approach, I typically see on the order of 10ms per
>> PV connection on startup.   That is, if I create and connect to 5K
>> PVs, it takes ~50 seconds (meaning 40 to 80 seconds) to get initial
>> values.  I believe that is mostly the CA library, not the python part,
>> and I believe it would scale, suggesting that any save/restore process
>> that runs once and then quit would take 10 minutes for 50K PVs.
>
> Are you saying that 10 min for 50K PVs is also what is observed with C utilities?
> 10 min for 50K is rather long ...

I'd be happy to be shown wrong, and I'm certainly not the right person
here to answer for sure,  but I think this might be the case.

> If that is the case, then one whould be careful to backup only PV which are of interest
> and not the 'entire' machine.
> How is this being managed at large installation?

I think they break up the set of variables across
processes/machines/NICs and/or have a long-running process that
repeatedly saves variables instead of
    start process; connect to 50K PVs; write values; end process.

> I understand your points, but somehow I am having a hard time believing this.
> Did anyone benchmark their backup platform?
> (I remember seeing a few powerpoints on this issue a while back...)

For what it's worth, and also gladly proven wrong, etc:  I recall that
with non-preemptive-callback context, creating and connecting PVs took
exactly 30ms, to very high precision and reproducibility.  I never
fully understood that....   From that view, the 10 ms average with the
preemptive-context is an improvement.

It's probably easy to tell whether it is closer to 1, 10, or 30 ms
even with a few hundred PVs.  But even at 1ms / PV connection, you
would probably want to not start a new connection per PV for saving
50K PVs.

> PS:
> Is there a difference between a connection to 50K Pvs versus, 50K connections to the same PVs?

Yes, these are definitely different.

--Matt

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System