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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Concurrency always is non-deterministic?
Date: Mon, 13 Feb 2012 19:04:23 +0100
Organization: cbb software GmbH
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Date: 2012-02-13T19:04:23+01:00
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On Mon, 13 Feb 2012 09:41:31 -0800 (PST), Long Ho�ng ��nh wrote:

> The author of this post said so. If Ada were non-deterministic at concurrency, I guess it couldn't be real-time, right?
> 
> http://ghcmutterings.wordpress.com/2009/10/06/parallelism-concurrency/
> 
> Please let me know your opinions about that post.

Concurrent and parallel were used as synonyms in the literature I read. The
author seems to think that concurrent = time sharing. It is not.

In any case the issue is unrelated to determinism and real-time.

Determinism is when the state of a program is determinable from its
previous state. A program that reads from the keyboard and then uses the
value in some if statement, is non-deterministic. A program that uses a
hardware random generator, a real-time clock is non-deterministic.

Note also that unpredictable /= non-deterministic. A program can be very
complex to predict (especially when you are looking for a bug to reproduce
(:-)) but still deterministic.

Real-time is a property of being related to the external real time source.
No more, no less.

In a narrower sense, RT is when the quality of a value (e.g. of a response)
depends on the real time. For example, when that value degrades as the time
passes. Hard real-time is when the value reaches 0 in bounded time. A
program is said to be real time, when it processes certain values before
their quality degrades below specified (usually by the application domain)
level.

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