Re: Gurus, a silly question for preemptive behavior

[K.R. Foley]

jesse wrote:
> --- Con Kolivas <kernel@xxxxxxxxxxx> wrote:
>
>
> > jesse wrote:
> >
> > > Paulo:
> > >
> > >   I already said in the messsage that my user
> >
> > space
> >
> > > application has a low nice priority, i set it to
> >
> > 10.
> >
> > > since my application has low priority compared to
> > > other user space applications, it is supposed to
> >
> > be
> >
> > > interrupted. but it is not.
> >
> > If your task is better priority the scheduler will
> > make it preempt the 
> > worse priority task. It sounds to me like you are
> > complaining that the 
> > worse priority task is still getting cpu? If so, you
> > misunderstand 
> > priority - it orders tasks according to priority
> > giving lower latency 
> > and preemptive behaviour to the better task, and
> > gives _more_ cpu but 
> > not all the cpu. The cpu must still be shared, but
> > with more cpu 
> > distributed to the better priority task. If you want
> > your better 
> > priority task to get _all_ the cpu you have to use
> > real time scheduling.
> >
> > Cheers,
> > Con
>
>
> ok, Con, your explaining makes some sense to me , but
> still not very well.
>
>    suppose I have five high process: A1, A2, A3, A4,
> A5 all have nice = 0. and I also have a low priority
> process B with nice = 10.
>
>     a) when process B is scheduled to run, it is given
> a short time slot based on its priority, for example 5
> secs. because at that point, A1/2/3/4/5 are not
> started yet. B will get CPU and run at full speed. 
>     b) at the end of time slot(5 secs), scheduler
> finds higher priority A1/A2/A3/A4/A5 are ready,
> scheduler will interrupt process B and starts to pick 
> a process from group A, even though B still needs CPU
> cycle.
>     c)unfortunately, process A1/2/3/4/5 are so active,
> thus process B should never get opportunity to run
> again, in consequence, CPU Usage% of Process B should
> be very Low.
>     
>    However, The above theretic assumption is in
> contrary to what i observed. in my LAB, the low
> priority process B seems to hold the CPU forever and
> Top command always shows Process B with a 90% CPU
> usage.
>   
>   If _more_ cpu but not _all_ the cpu are given to
> Process A1/2/3/4/5, Process B shouldn't have a 90% CPU
> usage. 

This statement is not exactly true. If process B is always ready to run 
(CPU bound, not I/O bound) it could easily consume more CPU than all the 
other processes combined. It all depends on what A1/2/3/4/5 are doing. 
Just because A1 has a higher priority doesn't mean it is ready to run. 
If processes A1/2/3/4/5 spend most of their time waiting for I/O or 
sleeping (not ready to run) and B does a lot of computation or just busy 
spins, B could easily consume more CPU than the others.

>   Thus, i can't help asking why low priority process B
> gets most CPU cycle.
>
>   thanks in advance. 
>
> jesse
>
>  
>
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