Re: Delay guarantees

[Bryce Bardin <bbardin@home.com>]

"Robert I. Eachus" wrote:
> 
> fmanning@my-dejanews.com wrote:
> 
> > If you have a delay relative statement, does the delay argument
> > guarantee a minimum delay? For example:
> >
> >    Delay 0.5;
> >
> > Does this guarantee a minimum 500 ms delay?
> 
>      Short answer: Yes.
> 
>      There are three reasons why the RM says that this is a minimum
> time:
> 
>    1) There is usually a non-zero time associated with a context switch,
> and
> so there is a minimum delay that is not associated with the clock delay.
> It is possible to ask for a shorter delay, but you will not get it.  of
> course, it is possible that a given switch will take longer...
> 
>    2) All processors on which this task can run are currently running
> tasks of equal or higher priority.  The implementation is not required
> to do an immediate task switch in this case.  (This, of course, does not
> apply if the delayed task is the only (eligable) task at the highest
> priority.
> 
>    3) Yes, the compiler is expect to delay "at least as long" as the
> requested delay.  If the clock used for delays has relatively long
> ticks, you should expect the delay (if case 2 doesn't apply) to be
> between the minimum number of ticks longer than the requested interval,
> and two ticks longer than that.  (Even if you request an exact multiple
> of the clock interval, the allowance for the delay in 1 above can add
> almost a whole tick.)  Of course, in a good real-time implementation you
> should expect that the clock used for computing delays allows timing to
> the nearest microsecond or better.  This does not mean that interrupts
> can occur at that interval, just that the run-time can use a clock with
> finer granularity to compute the minimum delay to request.
> 
> --
> 
>                                         Robert I. Eachus
> 
> with Standard_Disclaimer;
> use  Standard_Disclaimer;
> function Message (Text: in Clever_Ideas) return Better_Ideas is...

Case 3) is, of course, the effect of section 9.6(20) of the Ada
Reference
Manual, which says, in part:

For a delay_relative_statement, the expiration time is defined as
the current time, in the time base associated with relative delays, 
plus the value of the delay_expression converted to the type 
Duration, and then _rounded up to the next clock tick_.  
(Emphasis added.)