[ECOS] When or on what condition does "deschedule" happen?

ariga masahiro ariga@link-lab.co.jp
Mon May 19 05:32:00 GMT 2008


Hello everyone,

Thank you,Andrew, for your reply.
There is one unclear point that I would make it clear.

In explaning it ,let me use the scenario that
Anthony J. Massa elaborate in his book
"Embedded Software Development With Ecos".
(supposing since I bought the book I don't infringe its licence)

In case of Multilevel Queue Scheduler,
three threads are created like next.

ThreadC -- priority 30(lower)
ThreadA,B -- priority 0(highest)

    - ThreadA - Timeslice - ThreadB -Deschedule - ThreadA -
  | 
|
Preemption 
Deschedule
  | 
|
ThreadC 
ThreadC

Next is Anthony J. Massa's description.
"three threads-Thread A, Thread B, and Thread C-
are configured during creation of the threads at priority levels 0, 0, and 
30, respectively.
 --- starts with Thread C executing.
 Next, Thread A becomes able to run, causing Thread C to be preempted and a 
context
switch occurs. During the execution of Thread A, Thread B also becomes able 
to run. Thread A
continues until its timeslice period expires. Then, another context switch 
occurs allowing Thread
B to run. Thread B completes within its given timeslice period. The 
de-scheduling of a thread
can happen for various reasons; for example, by waiting on a mutex that is 
not free or delaying
for a specified amount of time. Since Thread A has the highest priority of 
tasks waiting to execute,
a context switch occurs and it runs next. After Thread A has completed, a 
context switch
takes place allowing Thread C to execute."

My question is last part of Deschedule when ThreadC resumes execution.

Am I correct in assuming that among your conditions, only two conditions 
should be fullfiled to make it possible
for ThreadC to resume execution ?
1.When the thread blocks.(How does scheduler recognise this ?)
2.When a thread exits.

If ThreadA does not finish and enter looping state,then isn't there any way 
for ThreadC to resume execution ?

Please kindly forgive my ignorance and enlighten me.
Thanks in advance.

Masahiro Ariga



> On Fri, May 16, 2008 at 05:12:30PM +0900, ariga masahiro wrote:
>> Hello everyone,
>>
>> Please teach me about eCos Thread Scheduler.
>> When or on what condition does "deschedule" happen ?
>
> When the thread blocks.
>
> When a higher priority thread becomes runnable.
>
> When a thread uses up its time slice and there is another thread of
> the same priority which is runnable.
>
> When a thread calls yield and there is another thread of the same
> priority which is runnable.
>
> When a thread exits.
>
> There is nothing unusual here. Any good book on operating systems will
> teach you the same.
>
>     Andrew
>
> -- 
> Before posting, please read the FAQ: http://ecos.sourceware.org/fom/ecos
> and search the list archive: http://ecos.sourceware.org/ml/ecos-discuss
>
>
> 



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