Thanks for the reply, budget enforcement in the scheduler timer makes sense. I think I have an idea of what he wants done now.
~Dagaen On Jun 17, 2015 1:45 AM, "Meng Xu" <xumengpa...@gmail.com> wrote: > 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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