Why do processes with higher priority to be allocated more timeslice?

[Jeff Donner]

On Mon, Sep 26, 2011 at 10:10 AM, Parmenides
<mobile.parmenides at gmail.com> wrote:
> 2011/9/26 Mulyadi Santosa <mulyadi.santosa at gmail.com>:
>>>Actually, the CFS scheduler which
>>> is a new scheduler in Linux kernel also does the same thing. But, I
>>> think this way does not fit with scheduler's principle.
>>
>> remember the keyword you ask? "fairness"? that is being  fair to all
>> processes.... but since, there are always more processes than
>> processors, unfairness always happen.....
>>
>
> In fact, I am interested in the length of timeslice rather than
> fairness at this point. :-)
>
>>>This way ensures
>>> lower latency. It is also necessary that CPU-bound processes are to be
>>> allocated longer timeslice to improve throughput owing to less process
>>> switch costs. That means lower priority processes (CPU-bound) should
>>> be allocated longer timeslice, whichs obviously conflicts with the
>>> actual practice taken by the Linux's scheduler. Any explanation?
>>
>> What you refer initially is the time when time slice assignment is
>> strictly derived from the static/nice level. So e.g process with nice
>> level 0 has lesser time slice that nice level -5.
>>
>> But as you can see, situation change dynamically during run time, thus
>> static prio must be taken into dynamic priority. And dynamic priority
>> itself, must take another factor for time slice calculation. Here,
>> sleep time comes into play.
>>
>
> Ok, suppose that there is a CPU-bound process and a I/O-bound process,
> both of them are allocated the same nice level 0. After some time, the
> I/O-bound process will receive higher dynamic priority owing to its
> frequent sleeping. Now that the I/O-bound process more like to sleep,
> why does the scheduler give it longer timeslice? After all, it really
> does not need more time.

Well, if it doesn't need more time then it doesn't matter what its priority is,
when it goes to sleep waiting for some IO it yields back the
remainder of its time. You could give it as long a timeslice
as you like; it won't use more than it needs, because it mostly waits on IO.

> On ther other hand, the CPU-bound process will receive lower dynamic
> priority as its punishment because it costs more CPU time. Lower
> dynamic priority indicates this process is more 'CPU-bound', that is
> this process need more CPU time. If the scheduler allocates longer
> timeslice for this process, the frequency of process switch will be
> reduced. I think that will help to improve throughput of the entire
> system.

A lot of the time the IO process won't be runnable, as it's waiting on IO.
When the kernel is looking to dole out CPU time at those times, well the
CPU-bound process is the only one that can take it. So the kernel
gives it to it, lower priority or not.

CFS doesn't distort anything.