On Mon, May 09, 2016 at 05:45:40AM +0200, Mike Galbraith wrote: > On Mon, 2016-05-09 at 02:57 +0800, Yuyang Du wrote: > > On Sun, May 08, 2016 at 10:08:55AM +0200, Mike Galbraith wrote: > > > > Maybe give the criteria a bit margin, not just wakees tend to equal > > > > llc_size, > > > > but the numbers are so wild to easily break the fragile condition, like: > > > > > > Seems lockless traversal and averages just lets multiple CPUs select > > > the same spot. An atomic reservation (feature) when looking for an > > > idle spot (also for fork) might fix it up. Run the thing as RT, > > > push/pull ensures that it reaches box saturation regardless of the > > > number of messaging threads, whereas with fair class, any number > 1 > > > will certainly stack tasks before the box is saturated. > > > > Yes, good idea, bringing order to the race to grab idle CPU is absolutely > > helpful. > > Well, good ideas work, as yet this one helps jack diddly spit.
Then a valid question is whether it is this selection screwed up in case like this, as it should necessarily always be asked. > > In addition, I would argue maybe beefing up idle balancing is a more > > productive way to spread load, as work-stealing just does what needs > > to be done. And seems it has been (sub-unconsciously) neglected in this > > case, :) > > > > Regarding wake_wide(), it seems the M:N is 1:24, not 6:6*24, if so, > > the slave will be 0 forever (as last_wakee is never flipped). > > Yeah, it's irrelevant here, this load is all about instantaneous state. > I could use a bit more of that, reserving on the wakeup side won't > help this benchmark until everything else cares. One stack, and it's > game over. It could help generic utilization and latency some.. but it > seems kinda unlikely it'll be worth the cycle expenditure. Yes and no, it depends on how efficient work-stealing is, compared to selection, but remember, at the end of the day, the wakee CPU measures the latency, that CPU does not care it is selected or it steals. > > Basically whenever a waker has more than 1 wakee, the wakee_flips > > will comfortably grow very large (with last_wakee alternating), > > whereas when a waker has 0 or 1 wakee, the wakee_flips will just be 0. > > Yup, it is a heuristic, and like all of those, imperfect. I've watched > it improving utilization in the wild though, so won't mind that until I > catch it doing really bad things. > > So recording only the last_wakee seems not right unless you have other > > good reason. If not the latter, counting waking wakee times should be > > better, and then allow the statistics to happily play. En... should we try remove recording last_wakee?
