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Date:   Sun, 8 Oct 2023 00:04:00 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Youssef Esmat <youssefesmat@chromium.org>
Cc:     Daniel Jordan <daniel.m.jordan@oracle.com>, mingo@kernel.org,
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Subject: Re: [PATCH 00/15] sched: EEVDF and latency-nice and/or slice-attr
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On Thu, Oct 05, 2023 at 02:05:57PM +0200, Peter Zijlstra wrote:

> t=10 V=4                                                t=10 V=4
>  A       |----<                                          A       |----<
>  B      |<                                              >B      |<
> >C   |----------------<                                  C   |----------------<
>    |---*-----|---------|---------|---------|----           |---*-----|---------|---------|---------|----
>                                                         

>                                                         
> t=52 V=18                                               t=36 V=13
>  A                   |----<                              A                   |----<
> >B                  |<                                   B                    |<
>  C                     |----------------<               >C   |----------------<
>    |---------|-------*-|---------|---------|----           |---------|--*------|---------|---------|----
>                                                         

>                                                         
> BAaaBCccccccccBBBAaaBBBAaaBB                            BBAaaBBBAaaBBBAaaBCccccccccB
> 
> 
> 
> As I wrote before; EVDF has worse lag bounds, but this is not
> insurmountable. The biggest problem that I can see is that of wakeup
> preemption. Currently we allow to preempt when 'current' has reached V
> (RUN_TO_PARITY in pick_eevdf()).
> 
> With these rules, when EEVDF schedules C (our large slice task) at t=10
> above, it is only a little behind C and can be reaily preempted after
> about 2 time units.
> 
> However, EVDF will delay scheduling C until much later, see how A and B
> walk far ahead of V until t=36. Only when will we pick C. But this means
> that we're firmly stuck with C for at least 11 time units. A newly
> placed task will be around V and will have no chance to preempt.

Playing around with it a little:

EEVDF					EVDF

slice 30000000				slice 30000000
# Min Latencies: 00014                  # Min Latencies: 00048
# Avg Latencies: 00692                  # Avg Latencies: 188239
# Max Latencies: 94633                  # Max Latencies: 961241
                                        
slice 3000000                           slice 3000000
# Min Latencies: 00054                  # Min Latencies: 00055
# Avg Latencies: 00522                  # Avg Latencies: 00673
# Max Latencies: 41475                  # Max Latencies: 13297
                                        
slice 300000                            slice 300000
# Min Latencies: 00018                  # Min Latencies: 00024
# Avg Latencies: 00344                  # Avg Latencies: 00056
# Max Latencies: 20061                  # Max Latencies: 00860


So while it improves the short slices, it completely blows up the large
slices -- utterly slaughters the large slices in fact.

And all the many variants of BIAS_ELIGIBLE that I've tried so far only
manage to murder the high end while simultaneously not actually helping
the low end -- so that's a complete write off.


By far the sanest option so far is PLACE_SLEEPER -- and that is very
much not a nice option either :-(