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Path: utzoo!utgpu!water!watmath!clyde!cbosgd!ucbvax!SEI.CMU.EDU!John.Goodenough
From: John.Goodenough@SEI.CMU.EDU
Newsgroups: comp.lang.ada
Subject: Preemption and priorities
Message-ID: <8801221730.AA23176@cs.sei.cmu.edu>
Date: 22 Jan 88 17:30:36 GMT
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MFeldman asks:

>    Suppose a lower-priority task has had control of the CPU and no
>    timeslicing is in effect; suppose that task doesn't do anything to block
>    itself; suppose the higher priority task was waiting on a delay which
>    expires. Now - does [9.8(4)] require that we immediately discover the
>    "eligibility to run"? I have always understood the answer was "no" - that
>    the decision is allowed to be deferred until the next "synchronization
>    point". At _that_ point surely the higher-priority task will be scheduled.

AI-00032, which has been fully approved, directly addresses this point:

   !standard 09.08    (04)                                 87-03-16  AI-00032/09
   !class ramification 85-05-18
   !status approved by WG9/AJPO 87-03-10
   !references AI-00045, 83-00059, 83-00517, 83-00518
   !topic Preemptive scheduling is required

   !summary 87-02-23

   If an implementation supports more than one priority  level,  or  interrupts,
   then it must also support a preemptive scheduling policy.

   !question 85-01-31

   If a task is executing when a task with a higher priority becomes  executable
   because  a delay has expired, must execution of the lower priority task cease
   immediately so execution of the higher priority task can continue?

   !response 87-02-23

   9.8(4) says:

           If two  tasks with  different priorities  are both  eligible  for
           execution and could sensibly be executed using the same  physical
           processors and  the  same  other processing  resources,  then  it
           cannot be  the case  that the  task with  the lower  priority  is
           executing while the task with the higher priority is not.

   Expiration of a delay  means  a  task  is  eligible  for  execution.   If  an
   implementation  determines that a delay has expired for a high priority task,
   then that task must be scheduled for execution.  If  the  physical  processor
   and other processing resources needed to execute the higher priority task are
   currently being used by a task of lower  priority,  execution  of  the  lower
   priority task must be suspended.

   If preemptive scheduling is not feasible in a particular implementation, then
   the implementation should not provide multiple priority levels (see AI-00045)
   or support interrupts.

   (Note:  the phrase "could sensibly be executed" refers to situations in which
   the  high  priority  task  can  actually  make use of the processor and other
   resources being used  by  the  lower  priority  task.   In  some  distributed
   processing  situations,  a  high  priority task may not be able to execute on
   some processors.  Preemption is only required for  processing  resources  the
   high priority task can use.) 

(AI-00045 says an implementation need not provide any priority levels at all.)

At one point, my understanding of Ada was the same as Feldman's, but I have
been made to see the error of my ways.