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

[Parmenides]

2011/9/26 Mulyadi Santosa <mulyadi.santosa at gmail.com>:
> Hi :)
>
>>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.

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.