NetNews Usenet Archive 1992 #16/NN_1992_16.iso/spool/comp/lang/ada/2201

Internet Message Format | 1992-07-27 | 2.4 KB

Path: sparky!uunet!mcsun!corton!cenaath.cena.dgac.fr!geant!bruniau
From: [e-mail] (Christophe Bruniau)
Newsgroups: comp.lang.ada
Subject: Re: Ada tasking rules and AST's...
Message-ID: <[e-mail]>
Date: 27 Jul 92 09:12:19 GMT
References: <[e-mail]>
Sender: [e-mail]
Organization: Centre d'Etudes de la Navigation Aerienne
Lines: 37
Originator: bruniau@piano


In article <[e-mail]>, [e-mail] (Larry King) writes:
|>     We are issueing VMS system calls from Ada.  These system calls
|> have associated AST's which I have defined and have gotten to work
|> correctly.  My question is how can I get Ada to do a rescheduling of the
|> tasks to get this new entry to run.  There is one process that takes
|> some amount of time but if the system call has finished we would like
|> the AST entry to run before this task normally finishes.  Currently we
|> issue a delay statement for the time
|> DURATION'SMALL but this causes us to lose 10 milliseconds of time just
|> to do the delay.  It does cause VAX Ada to run the AST entry but we would like
|> to avoid losing the 10 milliseconds if possible.  Does VAX Ada (or any
|> Ada for that matter) allow you to force an execution of the next entry on
|> the queue before the current task has completely finished?  I know the
|> question is vague but I hope I am giving enough for someone to give me a
|> reply.  Thanks in advance...
|>
|> --
|> Larry King
|> GTE Government Systems, Needham, MA            [phone]
|> E-Mail : [e-mail]

I think the best way to do this is to assign a higher priority to the
task which handles the AST (this is coherent in that case because the task
handles system interrupts). When the AST is issued, the executive should
do a rescheduling, so the AST handling task would execute before the previous
"current" one.
This mechanism is valid only in the case of a preemptive executive (If my memory
doesn't fail, this is the case of DEC-Ada, and it is enforced by Ada rules).
This will prevent you from time wasting using delays, or CPU consuming using
busy waiting (select ... else ...).
Another way to help this mechanism work is to enable time-slicing, but it
involves overhead and may cause you some tuning problems.


Christophe BRUNIAU   CENA Toulouse
([phone].   [e-mail]