Re: An Example for Ada.Execution_Time

[Simon Wright <simon@pushface.org>]

"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:

> On Thu, 30 Dec 2010 18:51:30 -0500, BrianG wrote:
>
>> Since D.16 defines CPU_Time as if it were a numeric value, is it too 
>> much to ask why a conversion to some form of numeric value wasn't 
>> provided?
>
> But time is not a number and was not defined as if it were. It is the time
> differences which are numeric. RM D.14 defines differences of CPU_Time as
> Time_Span. Time_Span is numeric.

This is certainly true. However, I remain surprised that .. even delving
back to v1.2 of the AI .. the proposer found it necessary to have this
type CPU_Time. The concept being looked for is the number of seconds
during which *this* task was executing, still sounds like a Duration
(OK, Time_Span if you must) to me.

> I think this is the core of misunderstanding. The thing you have in
> mind is "time interval since the task start according to the execution
> time clock."  It is not Ada.Execution_Time.Clock, it is:
>
>    Ada.Execution_Time.Clock - CPU_Time_First

I don't _think_ that CPU_Time_First is actually defined as the value of
Clock when the task starts (is initialized, put on the ready queue,
whatever).

D14(13/2) says "For each task, the execution time value is set to zero
at the creation of the task."; however, zero is clearly not a value of
CPU_Time, so I don't know what is meant.

> There is a direct correspondence between two, yet they are
> conceptually different things.

I have no idea what's meant by CPU_Time, then! Seems to me it's a giant
bug waiting to happen. It's clear that the CPU_Time of one task is
incommensurate with the CPU_Time of another. Given that, what's it
_for_?


Of course, I competely understand that the Standard is what it is, and
no one's going to change it at this point.