Re: [Xen-devel] [PATCH RFC v1] xen:rtds: towards work conserving RTDS

[Meng Xu <xumengpanda@xxxxxxxxx>]

On Mon, Aug 7, 2017 at 1:35 PM, Dario Faggioli
<dario.faggioli@xxxxxxxxxx> wrote:
> On Sat, 2017-08-05 at 17:35 -0400, Meng Xu wrote:
>> >
>> > > @@ -966,8 +1001,16 @@ burn_budget(const struct scheduler *ops,
>> > > struct
>> > > rt_vcpu *svc, s_time_t now)
>> > >
>> > >      if ( svc->cur_budget <= 0 )
>> > >      {
>> > > -        svc->cur_budget = 0;
>> > > -        __set_bit(__RTDS_depleted, &svc->flags);
>> > > +        if ( is_work_conserving(svc) )
>> > > +        {
>> > > +            svc->priority_level++;
>> > >
>> >
>> >                ASSERT(svc->priority_level <= 1);
>>
>> I'm sorry I didn't see this suggestion in previous email. I don't
>> think this assert makes sense.
>>
>> A vcpu that has extratime can have priority_level > 1.
>> For example, a VCPU (period = 100ms, budget = 10ms) runs alone on a
>> core. The VCPU may get its budget replenished  for 9 times in a
>> period. the vcpu's priority_level may be 9.
>>
> Ah, ok. Yes, I missed this, while I see this now.
>
> But doesn't this mean that, at a certain time t, between both CPUs that
> are both in 'etratime mode' (i.e., they've run out of budget, but
> they're running because they have extratime set), the one that has
> received less replenishments gets priority?

Yes.

>
> Is this wanted or expected?

It is wanted.

A VCPU i that has already got budget_i * priority_level_i time has
higher priority than another VCPU j that got budget_j *
priority_level_j time, where priority_level_j > priority_level_i.

For the unreserved resource, a VCPU will gets roughly budget/period
proportional unreserved CPU time.


> Basically, if I'm not wrong, this means that the actual priority,
> during the extratime phase, is some combination of deadline and budget
> (which would make me think to utilization)... is this the case?

Yes.
The higher utilization a VCPU has, the more extra time it will get in
the extratime phase.

>
> I don't care much about the actual schedule during the extratime phase,
> in the sense that it doesn't have to be anything too complicated or
> super advanced... but I at least would like:
> - to know how it works, and hence what to expect,
> - for it to be roughly fair.

The unreserved resource is proportionally allocated to VCPUs roughly
based on VCPU's budget/period.

Best,

Meng


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

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