On Mon, May 09, 2016 at 09:44:13AM +0200, Mike Galbraith wrote: > > Then a valid question is whether it is this selection screwed up in case > > like this, as it should necessarily always be asked. > > That's a given, it's just a question of how to do a bit better cheaply. > > > > > 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. > > In a perfect world, running only Chris' benchmark on an otherwise idle > box, there would never _be_ any work to steal.
What is the perfect world like? I don't get what you mean. > In the real world, we > smooth utilization, optimistically peek at this/that, and intentionally > throttle idle balancing (etc etc), which adds up to an imperfect world > for this (based on real world load) benchmark. So, is this a shout-out: these parts should be coordinated better? > > En... should we try remove recording last_wakee? > > The more the merrier, go for it! :) Nuh, really, this heuristic is too heuristic, :) The totality of all possible cases is scary. Just for a general M:N two-way waker-wakee relationship, not recording last_wakee may work well generally. E.g., currently, on a 2-socket (24-thread per socket) 1:24 and 1:48 can't really be differentiated, whereas 1:24 and 2:48 are completely different. Am I understanding correctly?
