Re: [Xen-devel] [PATCH] Modified RTDS scheduler to use an event-driven model instead of polling.

[Meng Xu <xumengpanda@xxxxxxxxx>]

Hi Dagaen,

I just comment on the summary of scheduler design you proposed at the
end of the email. I'm looking forward to Dario's more insightful
reply.
>
>
> >
> >> I simply
> >> don't see how it can
> >> be done without heavy interaction and information sharing between them
> >> which really
> >> defeats the purpose.
> >>
> > No, it doesn't.
>
> Ok this last line is the zinger.
>
> Almost this entire email was built on the premise that you would NOT like the
> idea of the replenishment handler having basically the same decision logic
> as the scheduler and then tickling the pCPU to pick up the new vCPU. Actually,
> with it done this way, I would have a hard time calling the
> tickle-invoked method
> the "scheduler." It would be more like the mover, with the
> replenishment function
> being essentially the scheduler. In this case, I'm not too sure
> actually how much
> different this would be from what we have now. It feels like, from
> what you want,
> that we could get the same behavior by modifying rt_schedule to do
> replenishments
> first, then check if a reschedule is needed (these two parts would be in this
> proposed replenishment timer routine) and the perform any move if necessary. I
> know this isn't exactly what you want, but that sounds close right?
>
> But the scheduler will have to decide which to move in, so that logic
> is done twice.
> Also, if these are done back-to-back and require the locks, isn't it
> really the same
> as having one long hot path? If you want maintainability, couldn't we just do
> replenishment then schedule and move (if necessary) in one timer (the
> one we have
> now) and move them to functions. It seems this can be done with one
> timer neatly.
>
> So here's my proposal, lets see if it fits what you want:


I will leave this to Dario to answer if it fits what he wants. :-P


>
> 1.) The scheduler does not do any timing,


Not really. The rt_schedule does the budget enforcement. When a
current VCPU runs out of budget, the rt_schedule will be invoked by a
timer (you can refer to the schedule function in xen/sched/schedule.c
to have a look how the timer is armed/disarmed.). When the rt_schedule
is invoked, it needs to:
a) update the budget of the current running VCPU and move it from runq
to depleted queue;
b) pick the current highest VCPU from runq and return it as the snext VCPU.

So scheduling logic is still  involved in the rt_schedule function.

>
> 2.) replenishments are scheduled via timer at each [next]
> replenishment period. Each
> replenishment resorts the replenished vCPUs, and if any of the first
> #CPUs in the
> runq change, we tickle a pCPU for each change


This is right.

>
>
> In this case, we can use one timer.


We actually have two: one for budget enforcement and the other for
budget replenishment.


>
> We could use the current one as a
> replenishment
> timer, and make it so rt_schedule isn't the default invoked method.
>
> Does this sound similar to what you are suggesting?


I don't think so, but I will leave it for Dario's for his opinion.

In Dario's suggestion, you just simply remove the update_budget
function out of rt_schedule. This is because budget enforcement, which
is the work of rt_schedule, does not naturelly involves budget
replenishment.

In order to achieve budget replenishment, we need another timer to
invoke another function (budget_replenish function), that is dedicated
to that.

>
> I have to ask
> because I thought
> you wouldn't like the idea,


I guess Dario won't like this idea. :-P (I'm kidding, but it should be
the case.)


>
> and its not really *that* far off from
> what we have now, Its
> a little restructuring so that replenishments occur before any
> scheduling activity and
> the handler checks if switching is needed (basically acting as the
> scheduler) and then
> calls tickle. Sounds like what you had in mind?


Thanks and best regards,

Meng
-- 


-----------
Meng Xu
PhD Student in Computer and Information Science
University of Pennsylvania
http://www.cis.upenn.edu/~mengxu/

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