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From: Niklas Holsti <niklas.holsti@tidorum.invalid>
Newsgroups: comp.lang.ada
Subject: Re: Ada.Execution_Time
Date: Fri, 17 Dec 2010 10:49:26 +0200
Organization: Tidorum Ltd
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>>> On Wed, 15 Dec 2010 16:05:16 -0600, Randy Brukardt wrote:
>>>
>>>> I think you are missing the point of CPU_Time. It is an abstract 
>>>> representation of some underlying counter. There is no requirement that this 
>>>> counter have any particular value -- in particular it is not necessarily 
>>>> zero when a task is created.

Are you sure, Randy? RM D.14 13/2 says "For each task, the execution 
time value is set to zero at the creation of the task."

>>>> So the only operations that are meaningful on a 
>>>> value of type CPU_Time are comparisons and differences. Arguably, CPU_Time 
>>>> is misnamed, because it is *not* some sort of time type.

Additions of CPU_Time values should also be meaningful. As I understand 
it, Ada.Execution_Time is meant for use in task scheduling, where it is 
essential that when several tasks start at the same time on one 
processor, the sum of their CPU_Time values at any later instant is 
close to the real elapsed time, Ada.Real_Time.Time_Span. Assuming that 
CPU_Time starts at zero for each task, see above.

Dmitry A. Kazakov wrote:
> CPU_Time has no physical meaning. 2s might be 2.5s
> real time or 1 year real time.

CPU_Time values have physical meaning after being summed over all tasks. 
The sum should be the real time, as closely as possible.

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
       .      @       .