LISTSERV mailing list manager LISTSERV 16.0

Help for TEAM-ADA Archives


TEAM-ADA Archives

TEAM-ADA Archives


TEAM-ADA@LISTSERV.ACM.ORG


View:

Message:

[

First

|

Previous

|

Next

|

Last

]

By Topic:

[

First

|

Previous

|

Next

|

Last

]

By Author:

[

First

|

Previous

|

Next

|

Last

]

Font:

Proportional Font

LISTSERV Archives

LISTSERV Archives

TEAM-ADA Home

TEAM-ADA Home

TEAM-ADA  April 1999

TEAM-ADA April 1999

Subject:

Re: Protected types and Ada 95

From:

Samuel Mize <[log in to unmask]>

Reply-To:

Samuel Mize <[log in to unmask]>

Date:

Thu, 22 Apr 1999 11:16:19 -0500

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (122 lines)

> Ada 95 introduced protected types, and
> information on the new concurrent standard, which I
> heard was the thing most changed from 83,

It sounds as if you may not have the best information available.

You'll find a lot of info about Ada at http://www.adahome.com and
http://www.acm.org/sigada, and those have pointers to other web sites.

You may also want to look at the Ada 95 Rationale, which explains a
lot of the design decisions that went into Ada 95.  You can get an
electronic copy at either of those sites.

- - - - - - - - - -

> One
> issue I had, was that I read somewhere that protected
> procedures and so forth, when executed, will run until
> completed, and not be interrupted by time slicing and
> so forth.

Well, this is confused.

I'll answer you publicly so others can correct any mis-statements,
if any.  I don't want to leave you MORE confused!

- - - - - - - - - -

Perhaps your source meant that operations that can CHANGE the
protected object are serialized.  Several tasks may have concurrent
read access to the object, or one task may have exclusive write
access (no reads during a write access).

Or, your source may have confounded together the following:

1) It is an error for a protected action to wait for another task to
   do something, as this could cause deadlock.  (That's a VERY loose
   statement of the formal language rule:  "During a protected action,
   it is a bounded error to invoke an operation that is potentially
   blocking."  (Ada Reference Manual, 9.5.1 paragraph 8.)

   In other words, once a task gets hold of the protected object, it
   shouldn't just hang on indefinitely -- it should do its job and get
   out, so other tasks can get at the resource.  If the resource needs
   processing that is extended or involves several tasks at once, it's
   too big for a simple critical region -- it should be protected by
   its own task.

2) Annex D of the Reference Manual says that, once a task gets the
   CPU, it runs until it hits a "task dispatching point."  These are
   points where the task may be expected to give up the CPU, for
   example when it is waiting to synchronize with another task.

   But conformance to Annex D is NOT required.  It's an optional set
   of constraints for compilers aimed at a real-time environment, and
   it describes one well-defined, predictable task scheduling method.

   A given compiler may provide Annex-D-style task scheduling, or it
   may not.  It may give you a choice between that and other
   scheduling methods like time-slicing.

   The core Ada 95 standard sets no task scheduling policy at all.

- - - - - - - - - -

> Is there a specific way [time slicing] happens in the
> 95 standard, or is it implementation dependent?

If the compiler supports another scheduling discipline, it's
specified with a "pragma" statement.

> I'm also curious if there is a compiler out there
> which does enforce consistant time slicing, since Gnat
> supposedly doesn't.

I don't know which (if any) compilers support time-slicing.

However, GNAT should, with a manual trick.

A task can give up the CPU by executing a "delay" statement.  If I
recall correctly, using a compiler that offers compliance with Annex
D -- like GNAT -- a higher-priority task that is in a delay will take
the CPU back from a lower-priority task as soon as the delay expires.

So, make a highest-priority task that repeatedly delays for a
time-slice, then executes a task dispatching point.  It looks like:

    task body Slicer is
    begin
        loop
            delay 0.01; -- seconds
        end loop;
    end Slicer;

(which I expect you can read even if you're unfamiliar with Ada.)

A delay statement IS a dispatching point.  The other tasks, if they
are of equal priority, will round-robin share the processor.

(Strictly speaking, if one of the tasks gives up the CPU early by
executing a dispatching point, the next one won't get a full slice --
just the rest of the current slice.  But this guarantees that all
tasks will periodically get SOME CPU cycles, which is usually the
point of time slicing.)

- - - - - - - - - -

I hope you found this to be of interest.  I urge you to look at the
web sites, and follow their pointers to the available books and
on-line tutorials.  I think you'll find Ada to be very effective for
multitasking and multiprocessing systems.

> -Matt Goulet
> aka The Mildly Infamous Blue Herring

Best,
Sam Mize

--
Samuel Mize -- [log in to unmask] (home email) -- Team Ada
Fight Spam: see http://www.cauce.org/ \\\ Smert Spamonam

Top of Message | Previous Page | Permalink

Advanced Options


Options

Log In

Log In

Get Password

Get Password


Search Archives

Search Archives


Subscribe or Unsubscribe

Subscribe or Unsubscribe


Archives

December 2017
November 2017
October 2017
September 2017
June 2017
May 2017
April 2017
January 2017
December 2016
November 2016
October 2016
August 2016
July 2016
June 2016
May 2016
April 2016
March 2016
February 2016
January 2016
December 2015
November 2015
October 2015
September 2015
August 2015
June 2015
May 2015
April 2015
March 2015
February 2015
January 2015
December 2014
November 2014
September 2014
August 2014
July 2014
June 2014
May 2014
April 2014
March 2014
February 2014
January 2014
December 2013
November 2013
October 2013
September 2013
June 2013
May 2013
April 2013
March 2013
February 2013
January 2013
December 2012
November 2012
October 2012
September 2012
August 2012
July 2012
June 2012
May 2012
April 2012
February 2012
January 2012
December 2011
November 2011
October 2011
September 2011
August 2011
July 2011
June 2011
May 2011
April 2011
December 2010
November 2010
October 2010
September 2010
June 2010
May 2010
April 2010
March 2010
February 2010
January 2010
December 2009
November 2009
October 2009
September 2009
August 2009
July 2009
June 2009
May 2009
April 2009
February 2009
January 2009
December 2008
November 2008
October 2008
September 2008
August 2008
June 2008
May 2008
April 2008
March 2008
February 2008
January 2008
December 2007
November 2007
October 2007
September 2007
June 2007
May 2007
March 2007
February 2007
December 2006
November 2006
October 2006
September 2006
August 2006
July 2006
June 2006
May 2006
April 2006
March 2006
February 2006
January 2006
December 2005
November 2005
October 2005
August 2005
July 2005
June 2005
May 2005
April 2005
March 2005
February 2005
January 2005
December 2004
November 2004
October 2004
September 2004
August 2004
July 2004
June 2004
May 2004
March 2004
February 2004
January 2004
December 2003
November 2003
October 2003
September 2003
August 2003
July 2003
June 2003
May 2003
April 2003
March 2003
February 2003
January 2003
December 2002
November 2002
October 2002
September 2002
August 2002
July 2002
June 2002
May 2002
April 2002
March 2002
February 2002
December 2001
November 2001
October 2001
September 2001
August 2001
July 2001
June 2001
May 2001
April 2001
March 2001
February 2001
January 2001
December 2000
November 2000
October 2000
September 2000
August 2000
July 2000
June 2000
May 2000
April 2000
March 2000
February 2000
January 2000
December 1999
November 1999
October 1999
September 1999
August 1999
July 1999
June 1999
May 1999
April 1999
March 1999
February 1999
January 1999
December 1998
November 1998
October 1998
September 1998
August 1998
July 1998
June 1998
May 1998
April 1998
March 1998
February 1998
January 1998
December 1997
November 1997
October 1997
September 1997
August 1997
July 1997
June 1997
May 1997
April 1997
March 1997
February 1997
January 1997
December 1996
November 1996
October 1996

ATOM RSS1 RSS2



LISTSERV.ACM.ORG

Secured by F-Secure Anti-Virus CataList Email List Search Powered by the LISTSERV Email List Manager