Re: [PATCH 08/17] sched/fair: Implement an EEVDF like policy

[Peter Zijlstra]

On Thu, Mar 30, 2023 at 07:05:17PM +0200, Vincent Guittot wrote:
> On Thu, 30 Mar 2023 at 10:04, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >
> > On Wed, Mar 29, 2023 at 04:35:25PM +0200, Vincent Guittot wrote:
> >
> > > IIUC how it works, Vd = ve + r / wi
> > >
> > > So for a same weight, the vd will be earlier but it's no more alway
> > > true for different weight
> >
> > Correct; but for a heavier task the time also goes slower and since it
> > needs more time, you want it to go first. But yes, this is weird at
> > first glance.
> 
> Yeah, I understand that this is needed for bounding the lag to a
> quantum max but that makes the latency prioritization less obvious and
> not always aligned with what we want

That very much depends on what we want I suppose :-) So far there's not
been strong definitions of what we want, other than that we consider a
negative latency nice task to get it's slice a little earlier where
possible.

(also, I rather like that vagueness -- just like nice is rather vague,
it gives us room for interpretation when implementing things)

> let say that you have 2 tasks A and B waking up simultaneously with
> the same vruntime; A has a negative latency nice to reflect some
> latency constraint and B the default value.  A will run 1st if they
> both have the same prio which is aligned with their  latency nice
> values but B could run 1st if it increase its nice prio to reflect the
> need for a larger cpu bandwidth so you can defeat the purpose of the
> latency nice although there is no unfairness
> 
> A cares of its latency and set a negative latency nice to reduce its
> request slice

This is true; but is it really a problem? It's all relative anyway :-)

Specifically, if you latency-nice harder than nice it, you win again,
also, nice is privilidged, while latency-nice is not (should it?)

The thing I like about EEVDF is that it actually makes the whole thing
simpler, it takes away a whole bunch of magic, and yes the latency thing
is perhaps more relative than absolute, but isn't that good enough?


That said; IIRC there's a few papers (which I can no longer find because
aparently google can now only give me my own patches and the opinion of
the internet on them when I search EEVDF :/) that muck with the {w,r}
set to build 'realtime' schedulers on top of EEVDF. So there's certainly
room to play around a bit.