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SubjectRe: [PATCH v2 2/4] sched/fair: add util_est on top of PELT
On 15-Dec 13:53, Peter Zijlstra wrote:
> On Fri, Dec 15, 2017 at 12:14:17PM +0000, Patrick Bellasi wrote:
> > On 13-Dec 17:16, Peter Zijlstra wrote:
>
> > > > + /*
> > > > + * Skip update of task's estimated utilization when its EWMA is already
> > > > + * ~1% close to its last activation value.
> > > > + */
> > > > + util_est = p->util_est.ewma;
> > > > + if (abs(util_est - util_last) <= (SCHED_CAPACITY_SCALE / 100))
> > > > + return;
> > >
> > > Isn't that computation almost as expensive as the stuff you're trying to
> > > avoid?
> >
> > Mmm... maybe slightly simpler. I'll profile it again but I remember
> > I've added it because it was slightly better on backbench.
> >
> > This code at the end it's just a "sub" and a "compare to constant" and
> > it's likely to bail early for all "almost regular" tasks.
> >
> > Are you worried about the branch overhead?
>
> Its a subtract, a test for sign, a conditional branch on test, a negate,
> a subtract constant and another conditinoal branch.

Close enough, the actual code is:

util_est = p->util_est.ewma;
5218: f9403ba3 ldr x3, [x29,#112]
521c: f9418462 ldr x2, [x3,#776]
if (abs(util_est - util_last) <= (SCHED_CAPACITY_SCALE / 100))
5220: eb010040 subs x0, x2, x1
5224: da805400 cneg x0, x0, mi
5228: f100281f cmp x0, #0xa
522c: 54fff9cd b.le 5164 <dequeue_task_fair+0xa04>

>
> Branch overhead certainly matters too.
>
> > > > + p->util_est.last = util_last;
> > > > + ewma = p->util_est.ewma;
> > > > + if (likely(ewma != 0)) {
> > >
> > > Why special case 0? Yes it helps with the initial ramp-on, but would not
> > > an asymmetric IIR (with a consistent upward bias) be better?
> >
> > Yes, maybe the fast ramp-up is not really necessary... I'll test it
> > without on some real use-cases and see if we really get any noticiable
> > benefit, otheriwse I'll remove it.
> >
> > Thanks for pointing this out.
> >
> > > > + ewma = util_last + (ewma << UTIL_EST_WEIGHT_SHIFT) - ewma;
> > > > + ewma >>= UTIL_EST_WEIGHT_SHIFT;
> > > > + } else {
> > > > + ewma = util_last;
> > > > + }
> > > > + p->util_est.ewma = ewma;
>
> And this, without the 0 case, is shift, an add, a subtract and another
> shift followed by a store.

Actual code:

p->util_est.last = util_last;
5230: f9018061 str x1, [x3,#768]
if (likely(ewma != 0)) {
5234: b40000a2 cbz x2, 5248 <dequeue_task_fair+0xae8>
ewma = util_last + (ewma << UTIL_EST_WEIGHT_SHIFT) - ewma;
5238: d37ef440 lsl x0, x2, #2
523c: cb020002 sub x2, x0, x2
5240: 8b010041 add x1, x2, x1
ewma >>= UTIL_EST_WEIGHT_SHIFT;
5244: d342fc21 lsr x1, x1, #2
p->util_est.ewma = ewma;
5248: f9403ba0 ldr x0, [x29,#112]
524c: f9018401 str x1, [x0,#776]
5250: 17ffffc5 b 5164 <dequeue_task_fair+0xa04>

>
> Which is less branches and roughly similar arith ops, some of which can
> be done in parallel.
>
> I suspect what you saw on the profile is the cacheline hit of the store,
> but I'm not sure.

Yes likely, looking at the two chunks above and considering the
removal of the 0 case, it's probably worth to remove the first check.

I'll give it a try again to measure hackbench overheads with the cache
alignment fixed.

Cheers Patrick

--
#include <best/regards.h>

Patrick Bellasi

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