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Subject: Re: EPICS base V4
From: Ralph Lange <Ralph.Lange@bessy.de>
To: Bob Dalesio <bdalesio1@comcast.net>
Cc: Andrew Johnson <anj@aps.anl.gov>, "Kasemir, Kay-Uwe" <kasemirk@ornl.gov>, Marty Kraimer <mrk@aps.anl.gov>, Benjamin Franksen <benjamin.franksen@bessy.de>, Eric Norum <norume@aps.anl.gov>, Jeff Hill <johill@lanl.gov>, Ned Arnold <nda@aps.anl.gov>, Matej Sekoranja <matej.sekoranja@ijs.si>
Date: Fri, 25 Feb 2005 15:25:25 +0100
Bob Dalesio wrote:

Marty pointed out then that the use of forward links and passive records
made the load leveling a very difficult problem. Since then, I would add
that there are times that you want to have records of a slower period still
all process at the same time.
But most probably not *all* records of that slower period, only those who belong to a certain subsystem, right? Hmm ... let's assume that every of these user named and created scanner threads can be configured to process its records either "synchronised en bloc" or "balanced". In that case the desired behaviour could be achieved by having your set of records all processed by a certain thread, which is configured to do it "en bloc", right?

Perhaps an alternative would be to specify the base rate for the periodic
scanner.
Use PHAS to set which of the sub-periods to use to process the record
So - if the base scan period is 60 Hz, and you define a 1 second scan
record, PHAS of 30 would be at the halfway point of the 60 Hz scanning.
Another interesting idea. I would rather have the load-balancing case being the default, though. At least I'd like the load-balancing setting to be easy, and the grouping to be the more elaborate configuration.

To bring this back to the functional specifications - the question seems to
be:
Do we need to be able to support "load balancing"

I think that it does not buy us much - but it is nice to give the database
engineer more control.
I have seen some cases where in case of a serial-type connection underneath a large number of IO-connected records in a slow task (e.g. the 10 second scan taking more than 1.5 seconds to finish) was filling up low level queues so that fast processing (going through the same queue) was heavily affected. With almost no way to change this behaviour.
Load-balancing would make the IOC a lot more "elastic" in this regard.

Ralph



References:
Re: EPICS base V4: iocCore database Marty Kraimer
Re: EPICS base V4: iocCore database Andrew Johnson
Re: EPICS base V4: iocCore database: Booleans Benjamin Franksen
EPICS base V4 Marty Kraimer
Re: EPICS base V4 Andrew Johnson
Re: EPICS base V4 Ralph Lange
Re: EPICS base V4 Bob Dalesio

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