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Message ID: 269     Entry time: 13 Jun 2006
Author: Stefan Ritt 
Topic: Info 
Subject: Scheduler changed for slow control equipment 
The schedule in mfe.c is used both for "normal" front-ends and for "slow-control" front-ends. Unfortunately it was only optimized for the first class. This lead to the fact that the slow control equipment was read out at different speed depending if the run is started or not. Furthermore, the maximum readout speed was somehow limited. This has been changed in the current version of mfe.c (SVN revision 3146). There are now two ways to control the readout speed of slow control equipment:

1) The "event limit" in the equipment list can be used as minimum time between readouts. I'm not happy about the "mis-use" of this variable, but it has been there since the beginning. If I would change it now, all front-ends on this world would have to be changed, which I maybe not a good idea. If this event limit is set to let's say 10, then the slow control equipment is read out with a maximum speed of 1/10ms = 100Hz. That means up to 100 variables (not complete equipments) are read out per second. If an equipment has 200 variables, each variable is then read out every two seconds of course. This number can be used to limit the readout speed differently for different equipments. Like one might want to read a sensitive pressure as often as possible, but some beamline magnet values only once every minute.

2) By default, the scheduler runs now at "full speed" when slow control equipment is present, resulting in a 100% CPU usage. To avoid this, following code can be added into the frontend_loop function:

BOOL frontend_call_loop = TRUE;

INT frontend_loop()
/* don't eat up all CPU time */
return cm_yield(10);

This limits the readout speed of all slow control equipment again to 100Hz, but avoids the 100% CPU usage. On most operating systems, the minimum time is 10ms as shown above, since this is the basic time slice of a process.

The readout scheme of slow control equipment will be re-visited this summer, when multi-threaded slow control front-ends will be implemented.
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