Messages in this thread | | | Date | Fri, 31 Mar 2017 03:13:55 +0800 | From | Yuyang Du <> | Subject | Re: [RESEND PATCH 2/2] sched/fair: Optimize __update_sched_avg() |
| |
On Thu, Mar 30, 2017 at 04:14:28PM +0200, Peter Zijlstra wrote: > On Thu, Mar 30, 2017 at 02:16:58PM +0200, Peter Zijlstra wrote: > > On Thu, Mar 30, 2017 at 04:21:08AM -0700, Paul Turner wrote: > > > > > + > > > > + if (unlikely(periods >= LOAD_AVG_MAX_N)) > > > > return LOAD_AVG_MAX; > > > > > > > Is this correct in the iterated periods > LOAD_AVG_MAX_N case? > > > I don't think the decay above is guaranteed to return these to zero. > > > > Ah! > > > > Indeed, so decay_load() needs LOAD_AVG_PERIOD * 63 before it truncates > > to 0, because every LOAD_AVG_PERIOD we half the value; loose 1 bit; so > > 63 of those and we're 0. > > > > But __accumulate_sum() OTOH returns LOAD_AVG_MAX after only > > LOAD_AVG_MAX_N, which < LOAD_AVG_PERIOD * 63. > > > > So yes, combined we exceed LOAD_AVG_MAX, which is bad. Let me think what > > to do about that. > > > So at the very least it should be decay_load(LOAD_AVG_MAX, 1) (aka > LOAD_AVG_MAX - 1024), but that still doesn't account for the !0 > decay_load() of the first segment. > > I'm thinking that we can compute the middle segment, by taking the max > value and chopping off the ends, like: > > > p > c2 = 1024 \Sum y^n > n=1 > > inf inf > = 1024 ( \Sum y^n - \Sum y^n - y^0 ) > n=0 n=p
It looks surprisingly kinda works :)
> + c2 = LOAD_AVG_MAX - decay_load(LOAD_AVG_MAX, periods) - 1024; ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ But, I'm not sure this is what you want (just assume p==0).
Thanks, Yuyang
| |