Re: [Xen-devel] Question about the credit2 sources in xen-unstable tree.

[peb1611 <peb1611@xxxxxxxxx>]

Thanks for your very kind explanation :-)

Now, I'm totally make sense of what the code would say and I'm sorry 
about asking you withoug thoughtful consideration.
and then, I hope that the credit2 works well for not only latency 
sensitive apps but also any other apps.
Thank you once again.

2011-04-05 ìí 6:12, George Dunlap ì ê:
> Hmm, I seem not to have documented that bit of code very well. :-)
>
> The idea is this:
> 1) Start with runtime = amount of time until this vcpu's credit reaches 0:
>     time = c2t(rqd, snext->credit, snext);
> 2) Then, if there's another vcpu on the runqueue, calculate how much
> time until the current vcpu's credit matches that vcpu's credit:
>     ntime = c2t(rqd, snext->credit - svc->credit, snext);
>     if ( time>  ntime )
>        time = ntime;
> 3) Finally, never let the time go outside of the minimum and maximum
> schedule times:
>     if ( time<  CSCHED_MIN_TIMER )
>        time = CSCHED_MIN_TIMER;
>     else if ( time>  CSCHED_MAX_TIMER )
>        time = CSCHED_MAX_TIMER;
>
> Does that make sense?
>
> So let's say snext has 10 ms of credit, and there's a vcpu on the
> runqueue with 5ms of credit.  Then it will be scheduled to run for 2
> ms (since right now MAX_TIMER is set to 2ms).  But if the vcpu on the
> runqueue has 9ms of credit, it snext will only be allowed to run for
> 1ms (so that when it finishes, both will have about 9ms of credit).
> If, however, the next vcpu on the runqueue has 9.9ms of credit, snext
> will be schedule to run for 0.5ms (since MIN_TIMER is set to 500us).
>
> Does that make sense?
>
> I chose MIN_TIMER and MAX_TIMER rather arbitrarily -- they probably
> could use some tweaking.
>
>   -George
>
> 2011/4/5 peb1611<peb1611@xxxxxxxxx>:
> > Hi, all
> >
> > I am analyzing the credit2 scheduler in order to adopt to my own private
> > cloud, which should satisfy the conditions of some latency sensitive
> > applications.
> >
> > In the latest xen-unstable repository, which is tip tags, I found some
> > codes that is not understandable to me.
> >
> > In the xen/common/sched_credit2.c,
> >
> > /* How long should we let this vcpu run for? */
> > static s_time_t
> > csched_runtime(const struct scheduler *ops, int cpu, struct csched_vcpu
> > *snext)
> > {
> > s_time_t time = CSCHED_MAX_TIMER;
> > struct csched_runqueue_data *rqd = RQD(ops, cpu);
> > struct list_head *runq =&rqd->runq;
> >
> > if ( is_idle_vcpu(snext->vcpu) )
> > return CSCHED_MAX_TIMER;
> >
> > /* Basic time */
> > time = c2t(rqd, snext->credit, snext);
> >
> > /* Next guy on runqueue */
> > if ( ! list_empty(runq) )
> > {
> > struct csched_vcpu *svc = __runq_elem(runq->next);
> > s_time_t ntime;
> >
> > if ( ! is_idle_vcpu(svc->vcpu) )
> > {
> > ntime = c2t(rqd, snext->credit - svc->credit, snext);
> >
> > if ( time>  ntime )
> > time = ntime;
> > }
> > }
> >
> > /* Check limits */
> > if ( time<  CSCHED_MIN_TIMER )
> > time = CSCHED_MIN_TIMER;
> > else if ( time>  CSCHED_MAX_TIMER )
> > time = CSCHED_MAX_TIMER;
> >
> > return time;
> > }
> >
> > As I understand, this function is used to determining the next vcpu's
> > running time-slice from now to the next scheduling event.
> >
> > So, what does this code mean?
> >
> > if ( time>  ntime )
> > time = ntime;
> >
> > actually, I think it is not different as
> >
> > if ( svc->credit>  0 )
> > time = ntime;
> >
> > If so, It means that if credits of the next guy on runqueue are not
> > negative caculated time-slice is snext's time, or time of the next guy
> > is selected and then eventually saturated by the CSCHED_MIN_TIMER.
> >
> > what is the implication of it?
> >
> > Am I correct ? If not, would you please kindly let me know about the
> > mecahnism of caculating the next-time slice?
> >
> > Thanks
> >
> > --
> > Eunbyung Park
> >
> >
> > _______________________________________________
> > Xen-devel mailing list
> > Xen-devel@xxxxxxxxxxxxxxxxxxx
> > http://lists.xensource.com/xen-devel
--
Eunbyung Park


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