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

[Vincent Guittot]

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

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

A: w=2, r=4q    B: w=2, r=6q

     A  |-------<
     B  |-----------<

     ---+---+---+---+---+---+---+-----------
     V  ^

A runs 1st because its Vd is earlier

     A    |-----<
     B  |-----------<

     ---+---+---+---+---+---+---+-----------
     V   ^

     A    |-----<
     B    |---------<

     ---+---+---+---+---+---+---+-----------
     V    ^

     A      |---<
     B    |---------<

     ---+---+---+---+---+---+---+-----------
     V     ^


     A      |---<
     B      |-------<

     ---+---+---+---+---+---+---+-----------
     V      ^

If B increases its nice because it wants more bandwidth but still
doesn't care of latency

A: w=2, r=4q    B: w=4, r=6q

     A  |-----------<
     B  |---------<

     ---+-----+-----+-----+-----+-----+---+-----------
     V  ^

B runs 1st whereas A's latency nice is lower

     A  |-----------<
     B    |------<

     ---+-----+-----+-----+-----+-----+---+-----------
     V   ^

     A     |--------<
     B    |------<

     ---+-----+-----+-----+-----+-----+---+-----------
     V    ^

     A     |--------<
     B      |----<

     ---+-----+-----+-----+-----+-----+---+-----------
     V     ^

     A        |-----<
     B      |----<

     ---+-----+-----+-----+-----+-----+---+-----------
     V      ^

     A        |-----<
     B        |--<

     ---+-----+-----+-----+-----+-----+---+-----------
     V       ^

     A        |-----<
     B        |--<

     ---+-----+-----+-----+-----+-----+---+-----------
     V        ^

>
> Let us consider a 3 task scenario, where one task (A) is double weight
> wrt to the other two (B,C), and let them run one quanta (q) at a time.
>
> Each step will see V advance q/4.
>
> A: w=2, r=4q    B: w=1, r=4q    C: w=1, r=4q
>
>   1) A runs -- earliest deadline
>
>     A  |-------<
>     B  |---------------<
>     C  |---------------<
>     ---+---+---+---+---+---+---+-----------
>     V  ^
>
>   2) B runs (tie break with C) -- A is ineligible due to v_a > V
>
>     A    |-----<
>     B  |---------------<
>     C  |---------------<
>     ---+---+---+---+---+---+---+-----------
>     V   ^
>
>   3) A runs -- earliest deadline
>
>     A    |-----<
>     B      |-----------<
>     C  |---------------<
>     ---+---+---+---+---+---+---+-----------
>     V    ^
>
>   4) C runs -- only eligible task
>
>     A      |---<
>     B      |-----------<
>     C  |---------------<
>     ---+---+---+---+---+---+---+-----------
>     V     ^
>
>   5) similar to 1)
>
>     A      |---<
>     B      |-----------<
>     C      |-----------<
>     ---+---+---+---+---+---+---+-----------
>     V      ^
>
> And we see that we get a very nice ABAC interleave, with the only other
> possible schedule being ACAB.
>
> By virtue of the heaver task getting a shorter virtual deadline it gets
> nicely interleaved with the other tasks and you get a very consistent
> schedule with very little choice.
>
> Like already said, step 2) is the only place we had a choice, and if we
> were to have given either B or C a shorter request (IOW latency-nice)
> that choice would have been fully determined.
>
> So increasing w gets you more time (and the shorter deadline causes the
> above interleaving), while for the same w, reducing r gets you picked
> earlier.
>
> Perhaps another way to look at it is that since heavier tasks run more
> (often) you've got to compete against it more often for latency.
>
>
> Does that help?