Re: periods and deadlines in SCHED_DEADLINE

[Raistlin <raistlin@linux.it>]

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On Sat, 2010-07-10 at 12:28 +0200, Peter Zijlstra wrote:
> > Mmm... I see... Are you thinking of another scheduling class? Or maybe
> > just another queue with "higher priority" inside the same scheduling
> > class (sched_dl.c)?
> 
> Inside the same class, since as you say that would allow sharing lots of
> things, also conceptually it makes sense as the admission tests would
> really have to share a lot of data between them anyway.
> 
Ok, fine. So my plan is to let what I have out really as fast as I can
so that you can see the progresses we made, and then start working on
this new thing...

> Right, so that's a good point, I'm wondering if we should use two
> separate policies or use the one policy, SCHED_DEADLINE, and use flags
> to distinguish between these two uses.
>
> Anyway, that part is the easy part to implement and shouldn't take more
> than a few minutes to flip between one and the other.
> 
Yes, that will be need very low effort to change.

> But if you have a per-cpu bandwidth, and the number of cpus, you also
> have the total amount of bandwidth available to G-EDF, no?
> 
That's for sure true. If you like that I can add something like it quite
easily... I'll try to do that on a per-domain basis, instead of truly
fully global.

> So it really doesn't matter how we specify the group budget, one global
> clock or one clock per cpu, if we have the number of cpus involved we
> can convert between those.
> 
>    (c) use a 'global' bw pool and be blissfully ignorant of the
>        per-cpu things?
> 
Ok, if it's not a problem having a global pool I think I'll keep the per
CPU bw specification as well as per CPU bw availability. Not only
because I like multiplication more than divisions, but since I think
they could be useful if someone want to achieve a partitioned behaviour
through setting affinities accordingly.

> > Ok, that seems possible to me, but since I have to write the code you
> > must tell me what you want the semantic of (syswide and per-group)
> > sched_dl_{runtime,period} to become and how should I treat them! :-)
> 
> Right, so for the system-wide and group bandwidth limits I think we
> should present them as if there's one clock, and let the scheduler sort
> out how many cpus are available to make it happen.
> 
As said above, I agree and I'll do exactly that...

> So we specify bandwidth as if we were looking at our watch, and say,
> this here group can consume 30 seconds every 2 minutes. If the
> load-balance domains happen to be larger than 1 cpu, hooray we can run
> more tasks and the total available bandwidth simply gets multiplied by
> the number of available cpus.
> 
> Makes sense?
> 
Yep. Thanks! :-)

Dario

-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
----------------------------------------------------------------------
Dario Faggioli, ReTiS Lab, Scuola Superiore Sant'Anna, Pisa  (Italy)

http://blog.linux.it/raistlin / raistlin@ekiga.net /
dario.faggioli@jabber.org

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