Re: [ECOS] Thread activation disturbed by lower priority threads]

[Andrew Lunn <andrew at lunn dot ch>]

On Tue, Aug 07, 2007 at 04:02:34PM +0200, Alois Z. wrote:
> Hi, 
> 
> as I got no response to me questions (see below) I may have to add a
> few things for clarification.
>
> First of all I'm running an an AT91M5580A processor (thy phytec
> board). I changed the ecos settings so that the timer tick is now
> 1ms. The reason for this is that I need such a small tick for my
> application. Does this anyhow influence the scheduling
> algorithm. Are there settings that need to be adjusted appart from
> denominator, nominator and timesclice value?
> 
> I did more measurements and found out that the timer DSR is really
> stable. even more stable than on some other systems (non ecos) I'm
> using. The problem is that the time between posting on the semaphore
> (the thread is waiting on) until the thread starts executing is
> varying largly. It seems that it is prolonged by other execution
> elements. And this even when the thread under question is the thread
> with the highest priority.  would be great if this clearifies my
> problem a little bit more.

If it is the highest priority runnable thread, as soon as the DSR
finished it should get to run. The only exception i can think of is if
some other thread has the scheduler locked. This would prevent a
context switch until the scheduler was unlocked.

How to you do your timing between the DSR timer and thread running? 

Does this high priority thread need to acquire a mutex etc? It could
be that something else has the mutex. So it has to wait for it to be
released. Priority inversion then happens. The lower priority thread
which holds the mutex gets boosted in priority to the priority of the
waiting thread. This should allow the low priority thread to finish
what it is doing and release the mutex. However there is one
wrinkle. eCos only undoes priority inversion when the thread releases
all its mutex, not just the mutex of interest. 

    Andrew


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