Experimental Physics and
Industrial Control System

[email protected] · Fri, 16 Sep 2011 15:38:31 -0700

With same request files ( ~11K Pvs), I have
sddscasr 2 sec
burtrb   2 sec
python  10 sec
(with gigabit network)

I am wondering what is the reason for the difference.
As you can see below, just the connection to PV is comparatively 'slow'

debug output:
[Fri Sep 16 16:11:15 2011] import epics start
[Fri Sep 16 16:11:15 2011] import epics end
[Fri Sep 16 16:11:15 2011] request read
[Fri Sep 16 16:11:21 2011] connected to pv
[Fri Sep 16 16:11:23 2011] fetched PV values
[Fri Sep 16 16:11:23 2011] write on disk
[Fri Sep 16 16:11:23 2011] write completed

with

#!/usr/bin/env python 

WRITE_ON_DISK = True
USE_SQLITE = False

import time

print "[%s] import epics start" % time.asctime( time.localtime(time.time()) )
import epics
print "[%s] import epics end" % time.asctime( time.localtime(time.time()) )

pvNames = []
pvs = []
vals = []

with open("burt.req") as fp:
    for line in iter(fp.readline, ''):
        pvNames.append(line[:-1])

print "[%s] request read" % time.asctime( time.localtime(time.time()) )

for pvName in pvNames:
    pvs.append(epics.PV(pvName))

print "[%s] connected to pv" % time.asctime( time.localtime(time.time()) )

for pv in pvs:
    vals.append(pv.get())

print "[%s] fetched PV values" % time.asctime( time.localtime(time.time()) )

if not WRITE_ON_DISK : exit()

print "[%s] write on disk" % time.asctime( time.localtime(time.time()) )

if USE_SQLITE :
    from pysqlite2 import dbapi2 as sqlite

    connection = sqlite.connect('test.db')
    cursor = connection.cursor()
    cursor.execute('CREATE TABLE backup (id INTEGER PRIMARY KEY, pvname VARCHAR(50), value VARCHAR(50))')

    for index, pvName in enumerate(pvNames):
        sqlquery = "INSERT INTO backup VALUES (null, '%s','%s')" % (pvName, vals[index] )
        #cursor.execute('INSERT INTO names VALUES (null, "%s" % pvName, "%s" % vals[index])')
        cursor.execute(sqlquery)

    connection.commit()
    cursor.close()
    connection.close()

else:
    with open("test.txt",'w') as fp:
        for index, pvName in enumerate(pvNames) :
            print >>fp , "%s %s" % ( pvName, vals[index] )

print "[%s] write completed" % time.asctime( time.localtime(time.time()) )

On 16:30 Fri 16 Sep     , Matt Newville wrote:
> 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
> 

-- 
Emmanuel

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System