From: Sébastien <seb.morand@gmail.com>
Subject: Task vs Protected Type
Date: Fri, 30 May 2008 14:12:49 +0000
Date: 2008-05-30T14:12:49+00:00 [thread overview]
Message-ID: <g1p214$296$1@registered.motzarella.org> (raw)
Hi guys,
I would like to understand a sentence of the wikibook.
In the presentation of Task and Protected we can see the following:
-------------------
-- CODE 1
-------------------
task type Semaphore_Task_Type is
entry Initialize (N : in Natural);
entry Wait;
entry Signal;
end Semaphore_Task_Type;
...
task body Semaphore_Task_Type is
Count : Natural;
begin
accept Initialize (N : in Natural) do
Count := N;
end Initialize;
loop
select
when Count > 0 =>
accept Wait do
Count := Count - 1;
end Wait;
or
accept Signal;
Count := Count + 1;
end select;
end loop;
end Semaphore_Task_Type;
-------------------
-- CODE 2
-------------------
protected type Semaphore_Protected_Type is
procedure Initialize (N : in Natural);
entry Wait;
procedure Signal;
private
Count : Natural := 0;
end Semaphore_Protected_Type;
...
protected body Semaphore_Protected_Type is
procedure Initialize (N : in Natural) is
begin
Count := N;
end Initialize;
entry Wait
when Count > 0 is
begin
Count := Count - 1;
end Wait;
procedure Signal is
begin
Count := Count + 1;
end Signal;
end Semaphore_Protected_Type;
Ok the two codes provides the same functionality but obviously not in
the same way. In the first one, a thread is dedicated to manage the
semaphore. In the second one, this is a protected type with exclusive
mutex on entry and procedure and absolutly no thread implementation (as
far as I understood internal mechanism of protected type)
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.
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.
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?
Thanks very much.
Sebastien
next reply other threads:[~2008-05-30 14:12 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2008-05-30 14:12 Sébastien [this message]
2008-05-30 14:33 ` Task vs Protected Type Dmitry A. Kazakov
2008-05-30 16:09 ` Sébastien
2008-05-30 15:15 ` Jean-Pierre Rosen
2008-05-30 17:30 ` Jeffrey R. Carter
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