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From: "Alex R. Mosteo" <devnull@mailinator.com>
Newsgroups: comp.lang.ada
Subject: Re: Periodic tasks - organization
Date: Mon, 14 Jul 2008 11:08:33 +0200
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Date: 2008-07-14T11:08:33+02:00
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Maciej Sobczak wrote:

> Consider a program that has a couple of periodic tasks. Let's say
> there are two tasks, one with a period of 3s and another with a period
> of 4s. Let's say that exact triggering is not required.
> 
> The simplest way to do it is to just have appropriate delay statements
> in main loops of these tasks. The advantage of such a setup is that
> the tasks are completely self-contained and independent on any other
> program entity. The disadvantage is that they are bound to the way
> their periodicity is implemented.
> 
> Another way is to have additional task that will serve as a clock,
> "ticking" two protected objects that are countdown counters with
> single entry that waits for the counter to reach zero. In this case
> the tick would be 1s, as the greatest common divisor of two periods.
> The working tasks can then wait on respective entries in these
> protected objects, which are released when the counters reach zero.
> The advantage of this approach is that the notion of time is extracted
> from the working tasks themselves and can be easily changed to
> something else, like waiting on a different kind of clock or an
> interrupts or whatever without disturbing main working tasks. The
> disadvantage is that there are more program entities (+ two protected
> countdowns + 1 ticking task) to handle.
> 
> What can you say about these two approaches? Which would you recommend
> and when?

I would use "delay until", which prevents any drift and still avoids using
extra machinery.