Re: [PATCH 00/15] sched: EEVDF and latency-nice and/or slice-attr

[Peter Zijlstra]

On Sun, Oct 08, 2023 at 12:04:00AM +0200, Peter Zijlstra wrote:
> 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 :-(

And this can be easily explained by the fact that we insert tasks around
0-lag, so if we delay execution past this point we create an effective
DoS window.