INFO-ADA Digest V88 
Author Message
 INFO-ADA Digest V88

Now I'm really baffled. John quotes a recent interpretation to the effect
that an implementation supporting multiple priorities must support
preemption. Maybe my naivete about operating systems is showing, but
that seems to be to require that an implementation with multiple
priorities _must_ be done on a platform providing timer interrupts.
Otherwise, how would expiration of a delay be detected (not by continuous
polling, I hope!)? I've always thought that it was quite reasonable to
allow one of a number of cooperating tasks to run until it blocked, _then_
have the scheduler see who's eligible and schedule the one with the
highest priority. It seems to me that - given the "weak" nature of
priorities in Ada anyway - priorities and preemption can be treated
independently. Seems a bit too stringent to tie them together: priorities
are only "advisory" in nature, and static, in any case.

Oh, well. So let me get this straight: does a system supporting multiple
priorities have to detect _immediately_ (and what does _that_ mean?) the
expiration of a delay so that it can pre-empt the running task if the
one that just "woke up" has a higher priority?

Is this policy now being implemented in an ACVC? I have seen one recently
pre-validated implementation which indeed seems to follow the policy
(Janus/Ada), in the sense that it has a "run till blocked" scheduling
policy and only a single priority level. It looked to me like they
were just trying to simplify their scheduler. But now I get the
impression that the single priority scheme is a consequence of this
recent ruling. Meridian, on the other hand, has a "run till blocked"
scheduler but multiple priorities - so do many others, I think. Clearly
ACVC 1.8 permitted it (Meridian validated under it).
Does 1.9 forbid it? I can see a _lot_ of work
coming for implementers validating under post-1.8 ACVC's. Or am I
missing something important?

Fri, 19 Jun 1992 17:37:00 GMT  
 [ 1 post ] 

 Relevant Pages 

1. INFO-ADA Digest V88 - Nyberg's and Goodenough's notes

2. INFO-ADA Digest V88 #31

3. INFO-ADA Digest V88 #15

4. INFO-ADA Digest V88 #198

5. INFO-ADA Digest V88 #145

6. [umd5!!jmoody: Re: INFO-ADA Digest V88 #135]

7. INFO-ADA Digest V88 #47

8. INFO-ADA Digest V88 - tasking again

9. INFO-ADA Digest V88 #43 Robert Dewar's response

10. INFO-ADA Digest V92 #56 (Re: Why Ada)

11. INFO-ADA Digest V90 #89 - Ada Coding Conventions

12. INFO-ADA Digest - 4 Sep 1998 - Special issue (#1998-224)


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