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  August 1999

TEAM-ADA August 1999

Subject:

Re: Annex D Compliance

From:

Roger Racine <[log in to unmask]>

Reply-To:

Roger Racine <[log in to unmask]>

Date:

Thu, 12 Aug 1999 15:44:47 -0400

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (161 lines)

At 01:37 PM 8/12/1999 , Tucker Taft wrote:
>Roger Racine wrote:
>> ...
>
>Note that in Ada 83, there were no guarantees about the number of
>priorities, and that is why we moved the entire priority model
>to the RT Annex, while at the same time imposing additional requirements,
>such as a minimum of 30 priority levels.
>
>> ... For a very clear
>> example, we used to be able to run multitasking programs in application
>> mode on every platform (Ada83).  Now, for GNAT using Linux threads (not
>> validated for Annex D), one must be in supervisor mode.
>
>I don't think these two things are directly related.
>

Here is how they are related.  For most implementations of Ada83, tasking
was implemented by the runtime system, using one OS process.  It did not
matter what the OS did with priorities (such as changing them to guarantee
service).  With Ada95, it appears that implementers have said: "Well, if
the user wants Annex D, they must want real-time performance.  That is the
whole point of the annex.  If they want real-time performance, they must be
in supervisor mode.  So if they are not in supervisor mode, anything can
happen, since we will not validate Annex D in user mode."  For example, on
Linux if you are in user mode, you are not allowed to change your priority,
except downward.  So an Ada runtime implemented using Linux Threads must be
in supervisor mode to even get priorities set up correctly.

>You may need to be in supervisor mode to disable time-slicing and get
>true "real-time" behavior.  However, you should certainly be able
>to "run multitasking programs" on all Ada 95 platforms without being
>in supervisor mode.  It is possible that you won't get the same guranatees
>about priority-based preemption, but that is presumably because those
>guarantees can't be provided without being in supervisor mode.
>I presume that if you run in non-supervisor mode, the Ada run-time
>does the best it can.  If not, that sounds like an implementation bug,
>not a language bug.
>

When not in supervisor mode, the implementation supplies the core language
behavior (i.e. no priorities).  Not a bug.  In Ada83, the core language
included priorities.  This behavior would have been a bug.  OK, an
implementor could say they were only providing one priority, but I never
saw that implementation.

You say that "guarantees can't be provided", which needs expansion.  They
can be provided, just not if one implements tasking using services of the
OS that require supervisor mode.  For example, many Ada83 vendors had their
own implementation of tasking.  This was because A) OSs did not support
multiple tasks in the early 80s; or B) those OSs that did support multiple
tasks did not support Ada semantics well enough.

>> ...
>> 2) In Ada83, tasks were very much defined to be concurrent processing
>> paths.  Problems occurred when calling system services (the whole program
>> blocked), but one got used to that and worked around it.  If a program
>> depended on different than concurrent behavior, it was erroneous.  It made
>> for quite portable applications and generally safe designs (i.e. if the
>> priority of a task changed, it did not affect data consistency).  We
>> typically made sure we tested our programs with time-slicing on to make
>> sure it was designed well.  Now it appears that Ada95 requires
>> run-till-blocked semantics, which will lead to designs depending on that
>> semantics.
>
>Ada 95 certainly doesn't require run-till-blocked semantics.  That
>is only required in the presence of the Task_Dispatching_Policy pragma
>specifying FIFO_Within_Priorities.  In the absence of that pragma,
>any scheduling of same-priority tasks is permitted.

How many vendors provide more than one policy?  I admit I have limited
experience with Ada95 compilers (GNAT), but it would be a pain to support
more than one, and the vendors haven't changed that much from Ada83, and
they have never liked pain. :-)

>Also, the only
>"dependence" on run-till-blocked semantics that I imagined was a
>schedulability dependence.  It is not surprising that schedulability
>would depend on the details of context switching.  I haven't
>heard of using run-till-blocked as a way to get "cheap" mutual
>exclusion, for example.  On the other hand, the *implementation*
>of protected types might be able to take advantage of run-till-blocked
>to reduce the overhead of getting a lock, by simply raising and
>lowering priorities while a lock is held.
>
>> ...  And second, Ada vendors insisted on taking
>> priorities out of the core language so that it would be easier to implement
>> the core on top of existing threads-based operating systems.  ...
>
>That was not the motivation.  We wanted to create additional portability
>relating to priorities, by specifying a requirement for more priorities
>and for clearer definition of the priority model.  However, we did not
>feel it was appropriate to impose that on all environments, given that
>Ada 83 allowed implementations to have a single priority.
>

You fixed something that was not broken.  I would love to see a list of
vendors, host and target pairs, and the number of priorities implemented
for each, for Ada83.  Yes, the number varied, but the vast majority had
more than one.  My guess is that the vast majority had at least 8, and a
majority had at least 30.  To make the minority move up to 30 would have
been a minor change (compared with all the other changes needed for Ada95).

>In any case, based on what you say, it appears that there were some
>*unintended* consequences which effectively made multitasking programs with
>priorities *less* portable.  If true, I certainly agree that is
>undesirable, and we should do something to fix it.  This is the
>first time I have heard this complaint about actual loss of portability.
>Earlier complaints had been about *potential* loss of portability,
>but those seemed unfounded since essentially all vendors made it
>clear they were going to support the real-time annex anyway, and Ada 83
>made no guarantees about supporting multiple priorities.
>

The language did not, but virtually all the vendors did implement some
number of priorities.  For our purposes, there were enough.

>There is an "ARG" meeting coming up in September, and perhaps we can
>try to address this apparent loss of portability.  If you could give
>a very specific example that would help.  For example, what happens
>when you run your program without being in supervisor mode?  Does it
>behave strangely?  What actually are the "fall-back" semantics when
>run in non-supervisor mode?  etc...
>

As I mentioned above, in user mode the core language is implemented.
Pragmas defined in Annex D are essentially ignored.  No warning message.
Just "strange" behavior (all tasks have equal priorities).  As far as I can
tell, this is permitted by the language.

I do not have a lot of trust in the "ARG" (has it changed its name?),
especially on this.  If what I have been told about implementations is
true, vendors will be very resistant to change, and it would likely require
a major change.  Besides, some application developers will argue that they
have gotten used to using system services in tasks.  Others will argue that
Ada tasking with priorities is no different than using threads in C.
Unfortunately, these are all valid arguments now.

My specific application is a non-real-time simulation of a real-time
application.  I model the real-time clock by using a low-priority task that
has a loop.  Each time through the loop this task rendezvous with a
high-priority task that increments the clock.  All other tasks use a
service to run cyclically, essentially like the "delay until" mechanism.
On the real target, this performs in real time.  On the host, the task is
put on a queue to be woken when the simulated clock reaches the correct
time.  Each time the clock ticks, the queue is checked, and all tasks that
should be released are released.

The effect of this, if priorities work correctly (on a uniprocessor), is
that the clock will never tick unless all the application tasks are
blocked, either waiting for something (like an event to occur, implemented
as a protected record or rendezvous), or waiting on the timer queue.

This has been extremely useful, for us, for initial development of
real-time embedded systems.
Roger Racine
Draper Laboratory, MS 31
555 Technology Sq.
Cambridge, MA 02139, USA
617-258-2489

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