Messages in this thread | | | Date | Mon, 9 Oct 2023 16:41:25 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 00/15] sched: EEVDF and latency-nice and/or slice-attr |
| |
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.
| |