On Mon, Sep 26, 2011 at 10:10 AM, Parmenides
[off-list ref] wrote:

2011/9/26 Mulyadi Santosa [off-list ref]:

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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. :-)

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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.