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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Task vs Protected Type
Newsgroups: comp.lang.ada
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Date: Fri, 30 May 2008 16:33:09 +0200
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Date: 2008-05-30T16:33:09+02:00
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On Fri, 30 May 2008 14:12:49 +0000, in comp.lang.ada you wrote:

> I would like to understand a sentence of the wikibook.
[...] 
> The first one has the advantage to force the user to Initialize the task 
> because of the first entry, but there is no such mechanism in the 
> protected type.

Of course there is many. For example:

   protected type Semaphore_Protected_Type (N : Natural) is
      ...
   private
      Count : Natural := N;
   end Semaphore_Protected_Type;

> So in the wikibook, there is this sentence:
> "Alternatively, semaphore functionality can be provided by a protected 
> object, with major efficiency gains."
> 
> This is between the two codes, letting the reader (me at least) 
> understand protected type is a better approach, but I can't understand why.

It is sometimes better, because seizing a semaphore implemented by a
protected object does not require context switching, as it can accomplished
on the caller's context. A rendezvous with the semaphore's monitor task
would usually require switching the context twice.

> For example, you have a web application and want to develop an object 
> pool. You could use one of both example, since it's the same purpose 
> with some GetObject and ReleaseObject enties/procedure in a task or in a 
> protected. But which one is the better approach?

Neither. It is a bad idea to use tasks for implementation of low-level
synchronization primitives (like semaphores). Your web application would
probably do much more things than just locking. Instead of Wait, you would
likely have something like Service (Object), which is more like a
transaction. Because the time required for handling higher level requests
is sufficiently greater than mere switching contexts, the difference might
become negligible. At the same time protected objects tend to be too
low-level. Further, protected actions shall be very short. This requirement
forces a very heavy design, when you have to do some prologue as a
protected action, continue work outside it, and complete it again as a
protected action again. Such things, and semaphore is a typical example of,
are exposed to various nasty problems.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de