Requeue

Requeue

[Anatoly Chernyshev]

Hello, everybody,

Actually, this question is for Ada gurus. I've read in two places
(Coronado tutorial and J. English's "The craft of object oriented
programming") that there is a reserved word "requeue" used in task
programming, but this is so much advanced, tricky and deep technique
that it isn't even worthy of mention. So, eventually I've got concerned
about what is this word used for.
Could someone post a simple yet practical example of using that
statement in Ada programming?

Thanks in advance,
Anatoly

Re: Requeue

[Pat Rogers]

"Anatoly Chernyshev" <rhezusfactor@yahoo.com> wrote in message
news:3CB0CD37.52B43088@yahoo.com...
> Hello, everybody,
>
> Actually, this question is for Ada gurus. I've read in two places
> (Coronado tutorial and J. English's "The craft of object oriented
> programming") that there is a reserved word "requeue" used in task
> programming, but this is so much advanced, tricky and deep technique
> that it isn't even worthy of mention. So, eventually I've got concerned
> about what is this word used for.
> Could someone post a simple yet practical example of using that
> statement in Ada programming?

Think of various tasks interacting in a client-server organization.  These tasks
synchronize and communicate via the language's mechanisms to implement this
relationship, in which client tasks request a service of the server task.  For
example, imagine a server task that allocates discrete resources upon request.
These resources can be anything, but there is a finite number of them.  As such,
not all requests can be satisfied when received, for example because the
requested allocation is larger than the amount currently available.

One way to program this is to use a form of synchronization in which the client
is held pending when a request cannot be satisfied.  In this approach, the
server has determined that the request cannot be satisfied and simply suspends
the caller in the middle of the service request.  Eventually (presumably) some
other event will occur, such that the pending request can now be satisfied, and
the service resumes for the original caller.  The primary characteristic is that
the server suspends the service execution on behalf of the client until the
request can be satisfied.  ("Condition variables" are a way of implementing this
approach. This can be messy but it works.)

Ada uses "avoidance synchronization", which means that the service requested is
not supposed to start before the request can be fulfilled.  That's a problem for
Ada's mechanisms because they use the parameters of the synchronization
mechanism to communicate the requested allocation -- but there is no way to read
the parameters until the mechanism executes, so it is too late to stop if the
request cannot be fulfilled.  (The guards on select statements, for example,
cannot refer to the formal parameters of accept statements.)  In Ada 83 this led
to a two-rendezvous approach -- one to get the request via the parameters, and a
second to actually satisfy the request when the conditions allow.  That's both
inefficient and open to issues of abort: what if the second call never comes in?

So, enter the requeue statement for Ada 95.  You can think of a requeue as being
similar to what happens when you call a large corporation.  The first person to
answer is a receptionist -- they have no idea how to handle your request, but
they can send you to the person who does, so they forward your call to the
proper person and then hang up.  The receptionist is done with you, and can go
on to answer the next incoming call.  But from your point of view, you've made
one call and are now waiting for the other person to answer your (forwarded)
call.  You've been requeued.

There are other details, such as being able to say whether or not abort is
deferred, but that is the gist of it.

I recommend this book for concurrent programming with Ada:
A. Burns and A. J. Wellings, Concurrency In Ada, Second ed: Cambridge University
Press, 1998.

It talks about this issue -- and any other aspect of concurrency in Ada -- in
lucid detail.  You can find examples there too.

Hope that helps,

---
Patrick Rogers                       Consulting and Training in:
http://www.classwide.com          Real-Time/OO Languages
progers@classwide.com               Hard Deadline Schedulability Analysis
(281)648-3165                                 Software Fault Tolerance

Re: Requeue

[Robert Dewar]

Anatoly Chernyshev <rhezusfactor@yahoo.com> wrote in message news:<3CB0CD37.52B43088@yahoo.com>...
> Could someone post a simple yet practical example of 
> using that statement in Ada programming?

That's what the rationale is for, always look there first!

Re: Requeue

[Ehud Lamm]

"Robert Dewar" <dewar@gnat.com> wrote in message
news:5ee5b646.0204072100.1f3707a0@posting.google.com...
> Anatoly Chernyshev <rhezusfactor@yahoo.com> wrote in message
news:<3CB0CD37.52B43088@yahoo.com>...
> > Could someone post a simple yet practical example of
> > using that statement in Ada programming?
>
> That's what the rationale is for, always look there first!

Right!
This is always true, and requeue is explained beautifully in the Rationale.
It may be useful to keep in mind that Requeue can be used to solve radically
different problems. It may be best to try and understand each type of use,
as explained in the rationale, separately. Suddenly the Aha moment will come
when you see why all these different things can be done using requeue.

Ehud Lamm

Re: Requeue

[Darren New]

Anatoly Chernyshev wrote:
> Could someone post a simple yet practical example of using that
> statement in Ada programming?

Part of the need for requeue comes from the fact that a guard can't be
based on the parameters of the call. So if I want to do something like

  entry Save_In_Buffer(Octets : in Stream_Element_Array) 
    when Octets'Length < My_Buffer.Remaining_Space is
  begin .... end Save_In_Buffer;

I can't do that. Octets'Length isn't allowed in the guard.
So instead I do
  entry Save_In_Buffer(...)
    when True is
  begin
    if My_Buffer.Remaining_Space < Octets'Length then
      requeue Save_Wait
    end if;
    ...
  end Save_In_Buffer;

Then Save_Wait has a guard that is gets set to false whenever I put
something into the buffer and true whenever I take something out. Inside
Save_Wait, once you get in, you check again, and if there's still no
room, you requeue once again. Ugly but it works.

I'd be happy to put the code somewhere if you want to look.

-- 
Darren New 
San Diego, CA, USA (PST). Cryptokeys on demand.
      Remember, drive defensively if you drink.

Re: Requeue

[Anatoly Chernyshev]

Thanks a lot for your answers - they were extremely helpful.
What abou reading rationale - that is true, I had to read it.
But then, why  don't one to close this newsgroup and leave just universal
notice with large bold letters "RTFM" in it?

Regards,
Anatoly.

Re: Requeue

[Larry Kilgallen]

In article <3CB32A54.A3A9E975@yahoo.com>, Anatoly Chernyshev <rhezusfactor@yahoo.com> writes:
> Thanks a lot for your answers - they were extremely helpful.
> What abou reading rationale - that is true, I had to read it.
> But then, why  don't one to close this newsgroup and leave just universal
> notice with large bold letters "RTFM" in it?

The fact that requeue is dealt with in the Rationale does not
mean that _all_ Ada questions are handled there.

Re: Requeue

[Anatoly Chernyshev]

> The fact that requeue is dealt with in the Rationale does not
> mean that _all_ Ada questions are handled there.

Well, then they are explained elsewhere. Otherwise, how the people knows these things.
By "FM" I understood not only Rationale, but any publication one could peep into.
Merely publish the list of references in CLA and sleep in rest.

Re: Requeue

[Marin David Condic]

Hmmmmmmm..... Seems a little out of geek character to pass up on an
opportunity to demonstrate superior knowlege in all subjects. :-) Sort of
like passing up a chance to get a calculator with even more buttons on it.
Usually, one can toss a question out here and get some really great
condescention with an answer in it somewhere as in "Its intuitively obvious
to even the most casual observer that blah blah blah, you eediot!!!!" to
which, the proper response is "My cheeks burn with shame at the display of
your superior intellect..." or something equally obsequious. Its sort of the
price one pays for "free" consulting. :-)

MDC
--
Marin David Condic
Senior Software Engineer
Pace Micro Technology Americas    www.pacemicro.com
Enabling the digital revolution
e-Mail:    marin.condic@pacemicro.com


"Anatoly Chernyshev" <rhezusfactor@yahoo.com> wrote in message
news:3CB349CB.7CF3391A@yahoo.com...
>
> Well, then they are explained elsewhere. Otherwise, how the people knows
these things.
> By "FM" I understood not only Rationale, but any publication one could
peep into.
> Merely publish the list of references in CLA and sleep in rest.
>

Re: Requeue

[Pat Rogers]

"Anatoly Chernyshev" <rhezusfactor@yahoo.com> wrote in message
news:3CB349CB.7CF3391A@yahoo.com...
>
>
>
> > The fact that requeue is dealt with in the Rationale does not
> > mean that _all_ Ada questions are handled there.
>
> Well, then they are explained elsewhere. Otherwise, how the people knows these
things.
> By "FM" I understood not only Rationale, but any publication one could peep
into.
> Merely publish the list of references in CLA and sleep in rest.


Ya nye penemiyu.  I gave you a rather lengthy intro to the requeue semantics as
well as a pointer to a good textbook dedicated to concurrency with Ada.  Others
did as well.  The Rationale would have been a good thing for me to have
included, and it is not the "FM", so are you complaining about someone pointing
you to the Rationale, and, if so why?

Re: Requeue

[Anatoly Chernyshev]

> Ya nye penemiyu.  I gave you a rather lengthy intro to the requeue semantics as
> well as a pointer to a good textbook dedicated to concurrency with Ada.  Others
> did as well.  The Rationale would have been a good thing for me to have
> included, and it is not the "FM", so are you complaining about someone pointing
> you to the Rationale, and, if so why?

Poimite zhe nakonec. I appreciate your efforts (frankly, no kidding) and comments of
other people and I'm not complaining a bit.
I simply issued a distracted remark (mainly in response to R. Dewar's post) meaning
that everyone could become Ada guru just pointing others to Rationale and if people
will always follow
his advice, there will be no need for CLA. Especially for periodically arisen
questions like "How to convert integer to string".
But I never expected that great programmers are so offensive.

Re: Requeue

[Pat Rogers]

"Anatoly Chernyshev" <rhezusfactor@yahoo.com> wrote in message
news:3CB36065.1BC7FF6B@yahoo.com...
> > Ya nye penemiyu.  I gave you a rather lengthy intro to the requeue semantics
as
> > well as a pointer to a good textbook dedicated to concurrency with Ada.
Others
> > did as well.  The Rationale would have been a good thing for me to have
> > included, and it is not the "FM", so are you complaining about someone
pointing
> > you to the Rationale, and, if so why?
>
> Poimite zhe nakonec. I appreciate your efforts (frankly, no kidding) and
comments of
> other people and I'm not complaining a bit.
> I simply issued a distracted remark (mainly in response to R. Dewar's post)
meaning
> that everyone could become Ada guru just pointing others to Rationale and if
people
> will always follow
> his advice, there will be no need for CLA. Especially for periodically arisen
> questions like "How to convert integer to string".
> But I never expected that great programmers are so offensive.

I'm sure he meant no offense;  I was remiss in not mentioning the Rationale to
you myself.  Definitely worth reading because (by definition) it explains why
the language contains what it does, which helps people use the language well.  I
very much appreciated "Studies In Ada Style" (for Ada 83) for the same reason.

Best regards,

Pat

Re: Requeue

[Ehud Lamm]

> Well, then they are explained elsewhere. Otherwise, how the people knows
these things.
> By "FM" I understood not only Rationale, but any publication one could
peep into.
> Merely publish the list of references in CLA and sleep in rest.

When someone asks for a simple and practical code example, I don't see much
harm in pointing him to the Rationale (which is available online).

Ehud