Re: [Xen-devel] A question on the credit scheduler

[gavin <gbtux@xxxxxxx>]

At 2011-12-16 19:04:19,"George Dunlap" <George.Dunlap@xxxxxxxxxxxxx> wrote:

>2011/12/16 zhikai <gbtux@xxxxxxx>:
>> Hi All,
>>
>> In the credit scheduler, the scheduling decision function csched_schedule()
>> is called in the schedule function in scheduler.c, such as the following.
>> next_slice = sched->do_schedule(sched, now, tasklet_work_scheduled);
>>
>> But, how often the csched_schedule() is called and to run? Does this
>> frequency have something to do with the slice of credit scheduler that is
>> 30ms?
>
>The scheduler runs whenever the SCHEDULE_SOFTIRQ is raised.  If you
>grep through the source code fro that string, you can find all the
>places where it's raised.
>
>Some examples include:
>* When the 30ms timeslice is finished
>* When a sleeping vcpu of higher priority than what's currently running wakes up
>* When a vcpu blocks
>* When a vcpu is migrated from one cpu to another
>
>30ms is actually a pretty long time; in typical workloads, vcpus block
>or are preempted by other waking vcpus without using up their full
>timeslice.

Thank you very much for your reply.
So, the vcpu is very likely to be preempted whenever the SCHEDULE_SOFTIRQ is raised. And we cannot find a small timeslice, such as a(ms), which makes the time any vcpu spending on running phase is k*a(ms), k is integer here. There is no such a small timeslice. Is it right?

Best Regards,
Gavin

>
> -George
>
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