Re: [PATCH v3 1/4] xen: add real time scheduler rtds

[Dario Faggioli <dario.faggioli@citrix.com>]

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On ven, 2014-09-19 at 09:11 -0400, Meng Xu wrote:
> 2014-09-18 14:08 GMT-04:00 Dario Faggioli <dario.faggioli@citrix.com>:

> > Actually, what you're saying about the budget, points at another lack of
> > the current implementation, i.e., payback for budget overrun (also
> > mentioned in the same email, I think). That basically means, in the case
> > you're describing, recognizing that the vcpu in question overran, by
> > allowing its budget to go negative, and making it pay a price for the
> > budget to become positive again, in terms on how far its deadline will
> > be set (this can be implemented quite easily, by slightly changing the
> > while loop we were discussing above).
> 
> Hmm, I think we have other ways to pay a price for the overrun budget.
> For example, we can reduce the budget from its next period to pay for
> the overrun budget in the current period if it overruns. When we add
> timers and avoid such MILLISECS(1) scheduling quantum, the overrun
> budget should not be too much, IMO. But anyway, this is something in
> the future to solve because it needs to incorporate the extra timers
> we will add in the future.
> 
Just FTR, we're talking about the same thing. In fact, I of course meant
that overrun must be payed in terms of less than full budget in the
following period. The point is, what it the overuun excedes the budget?
That's when the "pushing the deadline ahead" comes into play.

Say we have a 40ms budget vcpu that it (somehow) manages tor run for
90ms, going down to -50ms. If you push the deadline one period ahead,
and gives it a recharge of 40ms (i.e., it's full budget), that will
leave it with -10ms. What I was saying was, in this case, we give it
another 40ms recharge, but at the price of having its deadline one more
period ahead.

Anyway, that's for future development, as we agreed.

Regards,
Dario

-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)


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