Thread question

[Joel Sherrill <joel@OARcorp.com>]

Kenneth Peters wrote:
> At 10:23 AM 12/13/2004 -0600, Joel Sherrill <joel at OARcorp.com> wrote:
> 
>> Steve Holle wrote:
>>
>>> At 05:05 PM 12/10/2004, Angelo Fraietta wrote:
>>>
>>>> Steve Holle wrote:
>>>>
>>>>> If I disable task 1A, 1B keeps running even with the streaming 
>>>>> audio running.
>>>>>
>>>>> I assumes that since 1A and 1B where set up to timeslice that 
>>>>> whatever time was left 1A and 1B would get serviced.  That is 
>>>>> obviously a false assumption.
>>>>
>>>>
>>>>
>>>> I am pretty sure that if a higher priority thread takes control at 
>>>> any stage, the thread that was switched out will get switched back 
>>>> in; however, it's timeslices will start again at zero. It is 
>>>> possible in that case for one thread to get starved because the 
>>>> thread of equal priority keeps getting switched back with it's 
>>>> timeslice count set back to the start
>>>
>>>
>>> I'm not sure I understand this.  Why should it's timeslice restart?
>>> That seems counter-intuitive.  That would explain the symptoms we are 
>>> seeing but I'm not sure I understand the purpose and how to use 
>>> timeslicing if this is the case.
>>
>>
>> THe way timeslicing is defined in the Classic API, it is just a limit on
>> how long a thread can run without giving up the processor.  If it ever
>> gives up the CPU, it will have its timeslice reset when it runs again.
>>
>> POSIX has a slightly different definition of timeslice quantum
>> handling.  It is not reset until it is actually exhausted.
>>
>> A quick hack to try the POSIX version is to change the taskcreate.c
>> call to _Thread_initialize to use EXHAUST_TIMESLICE not RESET_TIMESLICE.
>> There has been a wishlist for a while to be able to specify which
>> algorithm to use.  Maybe you have finally got a case where it matters. :)
>>
>> --joel
> 
> 
> FYI, I've been using a hacked version of rtems_task_mode() to set these 
> parameters after creating classic tasks (but before starting them 
> executing). It has worked fine for me. See diffs against 4.5.0 
> taskmode.c below.

Does this address the situation he is describing?

> I did it this way because I did not want to hack the OS itself (for 
> administrative reasons we need to use an "oficial release" as much as 
> possible), so I made my own copy of taskmode.c to build into my own code 
> and changed the name to reveal the guilty.

I don't really have a problem with this feature and think it already has
a PR.  If a patch were to show up against the CVS source with:

   + a modified task create
   + a modified task mode
   + updated documentation

Something tells me it would get merged. :)

> Ken Peters
> 
> 12c12
> <  *  $Id: taskmode.c,v 1.2 1999/11/17 17:50:27 joel Exp $
> ---
>  >  *  $Id: electra_rtems_taskmode.c,v 1.3 2004/09/16 18:06:06 kenp Exp $
> 30a31,32
>  > #include "../include/electra_task_control.h"
>  >
> 32a35,43
>  >  *  electra_rtems_task_mode
>  >  *    Copied from rtems-4.5.0 "taskmode.c" and modified by Ken Peters 
> 03 Oct 2003
>  >  *    to allow changing a task's budget_algorithm for timeslicing to
>  >  *    not reset the task's timeslice every time the task is dispatched.
>  >  *    That functionality is supported by the RTEMS POSIX interface, 
> but not
>  >  *    the classic interface.
>  >  *    Also set the task's "Start.budget_algorithm", so if the task is 
> restarted
>  >  *    it keeps the selected timeslicing method.
>  >  *
> 48c59
> < rtems_status_code rtems_task_mode(
> ---
>  > rtems_status_code electra_rtems_task_mode(
> 51c62,64
> <   rtems_mode *previous_mode_set
> ---
>  >   rtems_mode *previous_mode_set,
>  >   boolean     use_timeslice_exhaustion,
>  >   boolean     *previous_use_timeslice_exhaustion
> 76a90,94
>  >   if ( executing->budget_algorithm == 
> THREAD_CPU_BUDGET_ALGORITHM_EXHAUST_TIMESLICE )
>  >     *previous_use_timeslice_exhaustion = TRUE;
>  >   else
>  >     *previous_use_timeslice_exhaustion = FALSE;
>  >
> 85,87c103,115
> <     if ( _Modes_Is_timeslice(mode_set) )
> <       executing->budget_algorithm = 
> THREAD_CPU_BUDGET_ALGORITHM_RESET_TIMESLICE;
> <     else
> ---
>  >     if ( _Modes_Is_timeslice(mode_set) ) {
>  >         if ( use_timeslice_exhaustion ) {
>  >         executing->budget_algorithm
>  >                 = THREAD_CPU_BUDGET_ALGORITHM_EXHAUST_TIMESLICE;
>  >               executing->Start.budget_algorithm
>  >                 = THREAD_CPU_BUDGET_ALGORITHM_EXHAUST_TIMESLICE;
>  >         } else {
>  >         executing->budget_algorithm
>  >                 = THREAD_CPU_BUDGET_ALGORITHM_RESET_TIMESLICE;
>  >               executing->Start.budget_algorithm
>  >                 = THREAD_CPU_BUDGET_ALGORITHM_RESET_TIMESLICE;
>  >         }
>  >     } else {
> 88a117
>  >       }
> 
> 


-- 
Joel Sherrill, Ph.D.             Director of Research & Development
joel at OARcorp.com                 On-Line Applications Research
Ask me about RTEMS: a free RTOS  Huntsville AL 35805
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