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From: Greg Bond <bond@ee.ubc.ca>
Subject: Re: GNAT R/T Annex and Win95
Date: 1996/04/22
Message-ID: <317C0165.4487@ee.ubc.ca>#1/1
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references: <3174712D.71C7@ee.ubc.ca> <dewar.829885694@schonberg>
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organization: Dept. of Electrical Eng., UBC
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Date: 1996-04-22T00:00:00+00:00
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Robert Dewar wrote:
> 
> Greg says
> 
> "My understanding is that Win95 uses a timesliced scheduling policy for
> threads, and that GNAT tasks are mapped to Win95 threads. How is
> GNAT's fixed priority preemptive scheduling policy supported if this
> is the case?"
> 
> Time-slicing has nothing to do with fixed-priorities per se, or with
> preemption. It *does* have to do with run-till-blocked.

Or even more precisely, prioritized run-until-blocked scheduling and 
prioritized time-sliced scheduling can both be considered fixed priority 
preemptive scheduling policies, so my terminology is ambiguous.

However, I've found that the majority of the literature in rate 
monotonic analysis refers to fixed priority preemptive scheduling when I 
thought what was really being referred to was fixed priority, 
run-until-blocked scheduling. I've just adopted this terminology. To be 
precise I should have referred to the Ada RT Annex Fifo_Within_Priority 
dispatching policy (although other respondents to my posting knew that 
that is what I was referring too).

Wondering out loud: On the other hand, it only just occurred to me that 
rate monotonic analysis techniques might also apply equally well to 
prioritized time-sliced schedulers if each task is assigned a unique 
priority (disclaimer: I'm new to this area!).

> Ada 95 does not require run-till-blocked semantics unless the RT annex
> is supported fully. Not all OS's can support the real time annex, and
> clearly GNAT on such a system does not support the annex (this is
> assuming tasks are mapped to threads). On NT, you can map to the
> real time threads, and all is well, but I don't know if this applies
> to Win95. Anyway, Greg, you are making undocumented assjmptions here!

To follow up on my thoughts above: Although a system that only supports 
prioritized time-sliced scheduling (like Win95) does not support the  
Fifo_Within_Priority dispatching policy required by the RT annex, it 
seems that most (all?) of the other features of the RT annex are 
supportable. Is this the case for Win95? Counter-example anyone?


--
* Greg Bond                         * Dept. of Electrical Eng.  
* email: bond@ee.ubc.ca             * Univ. of British Columbia      
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