> On Apr 6, 2021, at 07:49, Paul Irofti <p...@irofti.net> wrote: > >>>> The diff is obviously fine. But it is still a heuristic with no real >>>> motivation except for this particular ESXi VM case. So my question >>>> about why we choose the minimum instead of the median or the mean has >>>> not been answered. >>> >>> Because median or mean is affected by outliers. We actually see >>> some outliers in samples from the VMware. >>> >>> I suppose there is a better mesure, but I am currently no idia and had >>> not used that kind of measure in kernel. On the other hand, finding >>> the minimum is very simple. >> Using the median should take care of the outliers though. >> I'm not at all convinced that taking the absolute value of the >> difference makes sense. It probably works in this case since the >> actual skew on your VM is zero. So measurements close to zero are >> "good". But what if the skew isn't zero? Take for example an AP that >> is running ahead of the BP by 5000 ticks. In that case, the right >> value for the skew is -5000. But now imagine that the BP gets >> "interrupted" while doing a measurement, resulting in a delay of 10000 >> ticks between the two rdtsc_lfence() calls. That would result in a >> measured skew of around zero. And by taking the minimum of the >> absolute value, you end up using that value. > > Exactly!
I agree that the median is a better choice of skew than the absolute minimum or average. I think this means adding qsort to the kernel, though. Unless we want to do median of medians...
