At 2011-12-16 23:58:26,"George Dunlap" <George.Dunlap@xxxxxxxxxxxxx> wrote:
>2011/12/16 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.
>
>It depends; if you have a cpu-burning vcpu running on a cpu all by
>itself, then after its 30ms timeslice, Xen will have no one else to
>run, and so will let it run again.
>
>But yes, if there are other vcpus on the runqueue, or the host is
>moderately busy, it's likely that SCHEDULE_SOFTIRQ will cause a
>context-switch.
>
>> 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?
>
>I'm sorry, I don't really understand your question. Perhaps if you
>told me what you're trying to accomplish?
I try to describe my idea as the following clearly. But I really don't know if it will work. Please give me some advice if possible.
According to the credit scheduler in Xen, a vCPU can run a 30ms timeslice when it is scheduled on the physical CPU. And, a vCPU with the BOOST priority will preempt the running one and run additional 10ms. So, what I think is if we monitor the physical CPU every 10ms and we can get the mapping information of a physical CPU and a vCPU. And also, we can get the un-mapping information that a physical CPU isn’t mapped to any vCPU. Thus, we can get the CPU usage by calculating the proportion of the mapping information to the total time when we monitored.
For example, if we monitor the physical CPUs every 10ms and we can get 100 pairs of pCPU and vCPU in a second, such as (pCPU_id, vCPU_id). If there is 60 mapping pairs that the pCPU is mapped to a valid vPCU, and 40 un-mapping pairs that we cannot find the pCPU to be mapped a valid vCPU. So, we can get the usage of the physical CPUs that is 60%.
Here, we monitor the physical CPUs every 10ms. We also can monitor them once less than the 10ms interval, such as 1ms interval. Whatever interval we choose, we must make sure no CPU content switch in the interval or the context switch always occur at the edge of interval. Only in this condition, can this idea work.
So, I am not sure whether we can find such a time interval that can meet this condition. In other words, whether we can find such a time interval that ensures all the CPU content switch occur at the edge of interval.
Thank you very much.
Gavin
> -George
>
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