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Re: When or on what condition does "deschedule" happen?


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

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