Re: timeouts

["Steve" <nospam_steved94@comcast.net>]

What version of GNAT?
I vaguely recall a gnat bug having to do with time.  Unfortunately I can't
recall the particulars.  Maybe someone else on the list will remember.

Steve
(The Duck)


"Brian May" <bam@snoopy.apana.org.au> wrote in message
news:sa4smakuhzq.fsf@snoopy.apana.org.au...
> Hello,
>
> I have a written a communications protocols, for Windows, in Ada,
> using the GNAT compiler for Windows. This is my attempt at
> implementing the MAP27 protocol (used for talking to trunk radios).
>
> The main protocol is implemented as a separate task along the lines of
> the following code. This code is simplified from my actual code, and
> only shows the what I consider relevant to this bug, based on print
> messages. Any mistakes in syntax are probably copy errors.
>
> I should be able to provide full code on request, but need to confirm
> this first.
>
> task body State_Machine_Task is
>      timer_enabled : Boolean := True;
>      expire_time : Duration := clock + 0.1;
> ...
> begin
>    ...
>    Put(Log.Debug,"----------------------------------------");
>    Put(...);
>    Put(timer_enabled,expire_time);
>    Put(...);
>
>    while Continue loop
>        case SMT.DTE_State is
>        when ... =>
>             select
>                 accept ...
>                 ...
>             or
>                 ...
>             or
>                when timer_enabled =>
>                delay until expire_time;
>
>                print ("In.Link_Establishment_Timeout");
>
>                expire_time := clock + 0.1;
>             end select;
>
>        when ... =>
>             ...
>
>        ...
>
>        end case;
>    end loop;
> end;
>
>
> Generally speaking, the program does all this fine, and constantly
> prints the expired message, as required.
>
> However, occasionally something goes wrong, and while it prints the
> expiration time OK, it takes ages to print the "expired"
> message. e.g. one occasion the event was 7 hours late. This puzzles
> me, because every time the timer is incremented by 0.1 seconds, and my
> understanding is that the timer should trigger if the current time is
> greater then the expiration time.
>
> To be precise, at 1:50:57 am the timer was incremented, and the select
> reentered, and at 9:12:59 the timer expired. The actual log produced
> is below:
>
> 1:50:57 DLL DEBUG Activity_Timer: disabled
> 1:50:57 DLL DEBUG Link_Failure_Detection_Timer: disabled
> 1:50:56 DLL DEBUG In.Link_Establishment_Timeout
> 1:50:56 DLL DEBUG Out.Link_Request( 14, 1, 1)
> 1:50:56 T50 DEBUG TX Packet Packet( 22  16  2  1  14  1  1  16  3  138
 9 )
> 1:50:57 DLL DEBUG Out.Link_Request return
> 1:50:57 DLL DEBUG ----------------------------------------
> 1:50:57 DLL DEBUG looping state=RESET_WAIT
> 1:50:57 DLL DEBUG Link_Establishment_Timer time 1:50:57
> 1:50:57 DLL DEBUG Retry_Timer: disabled
> 1:50:57 DLL DEBUG Acknowledgement_Timer: disabled
> 1:50:57 DLL DEBUG Activity_Timer: disabled
> 1:50:57 DLL DEBUG Link_Failure_Detection_Timer: disabled
> 9:12:59 DLL DEBUG In.Link_Establishment_Timeout
> 9:12:59 DLL DEBUG Out.Link_Request( 14, 1, 1)
> 9:12:59 T50 DEBUG TX Packet Packet( 22  16  2  1  14  1  1  16  3  138
 9 )
> 9:12:59 DLL DEBUG Out.Link_Request return
> 9:12:59 DLL DEBUG ----------------------------------------
> 9:12:59 DLL DEBUG looping state=RESET_WAIT
> 9:12:59 DLL DEBUG Link_Establishment_Timer time 9:12:59
> 9:12:59 DLL DEBUG Retry_Timer: disabled
> 9:12:59 DLL DEBUG Acknowledgement_Timer: disabled
> 9:12:59 DLL DEBUG Activity_Timer: disabled
> 9:12:59 DLL DEBUG Link_Failure_Detection_Timer: disabled
>
> The fact that the time decrements from 57 seconds to 56 seconds seems
> weird in itself. It could be due to some obscure round of error in my
> code to print the time, but I can't imagine where this would occur. In
> all cases the current value of Clock is used, and it isn't stored
> anywhere. Or it could be some sort of time synchronisation tool on the
> computer, I am not aware of anything like this though.
>
> During this wait, all evidence points to the fact that the
> timer_enabled and expire_time variables are correct, they cannot be
> changed from other tasks, and that it was waiting in the select
> statement. During this time, the task appears to respond to other
> select events fine. I haven't yet tested if the timeout starts working
> again after entering the select again, as the structure of the code
> doesn't make this easy.
>
> What is going on? Strict real-time behaviour is not required, but 7
> hours instead of 0.1 seconds is slightly on the extreme side.
>
>
> Any ideas?
> Thanks in advance.
>
>
> PS. In hindsight, the "timer_enabled" variable may not be required, as
> I can use the state variable instead and probably simplify some of the
> code, but I am skeptical this is causing the problem.
> -- 
> Brian May <bam@snoopy.apana.org.au>