* Yury Norov <ynorov@xxxxxxxxxxxxxxxxxx> wrote: > On Tue, May 16, 2017 at 10:30:42AM +0200, Ingo Molnar wrote: > > > > * Yury Norov <ynorov@xxxxxxxxxxxxxxxxxx> wrote: > > > > > I collected about 700 results in dmesg, and took 600 fastest. > > > For the vanilla kernel, the average value is 368, and for patched > > > kernel it is 388. It's 5% slower. But the standard deviation is > > > really big for both series' - 131 and 106 cycles respectively, which > > > is ~ 30%. And so, my conclusion is: there's no benefit in using > > > sched_find_first_bit() comparing to find_first_bit(). > > > > Erm, so you in essence claim: > > > > "according to measurements the new code is 5% slower, with a high, 30% > > stddev, hence the new code is better!" > > > > Basic logic fail... > > > > Thanks, > > > > Ingo > > No, in essence I claim that scatter is so big (in both cases, and in > case of vanilla kernel even bigger) that 5% is not a meaningful > difference. To be specific - new measured value is inside the > confidence interval of previous one. Firstly, the high spread is due to the poor measurement method: by increasing the number of measurements the standard deviation can be reduced. Secondly, and most importantly, the claim you made based on the numbers is simply false: > > > And so, my conclusion is: there's no benefit in using > > > sched_find_first_bit() comparing to find_first_bit(). you _measured no benefit_, and in fact the result you got is leaning towards it being a benefit. When doing a proper measurement it might strengthen, vanish or turn around - we simply don't know. Thanks, Ingo
