Messages in this thread | | | Date | Wed, 22 Dec 2004 06:39:42 -0600 | From | "K.R. Foley" <> | Subject | Re: Gurus, a silly question for preemptive behavior |
| |
jesse wrote: > --- Con Kolivas <kernel@kolivas.org> 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 > > > > - > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ >
- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |