On 2014/5/27 18:55, Mike Galbraith wrote: > On Tue, 2014-05-27 at 12:43 +0200, Peter Zijlstra wrote: >> On Tue, May 27, 2014 at 12:05:33PM +0200, Mike Galbraith wrote: >>> On Tue, 2014-05-27 at 11:48 +0200, Peter Zijlstra wrote: >>> >>>> So I suppose this is due to the select_idle_sibling() nonsense again, >>>> where we assumes L3 is a fair compromise between cheap enough and >>>> effective enough. >>> >>> Nodz. >>> >>>> Of course, Intel keeps growing the cpu count covered by L3 to ridiculous >>>> sizes, 8 cores isn't nowhere near their top silly, which shifts the >>>> balance, and there's always going to be pathological cases (like the >>>> proposed workload) where its just always going to suck eggs. >>> >>> Test is as pathological as it gets. 15 core + SMT wouldn't be pretty. >> >> So one thing we could maybe do is measure the cost of >> select_idle_sibling(), just like we do for idle_balance() and compare >> this against the tasks avg runtime. >> >> We can go all crazy and do reduced searches; like test every n-th cpu in >> the mask, or make it statistical and do a full search ever n wakeups. >> >> Not sure what's a good approach. But L3 spanning more and more CPUs is >> not something that's going to get cured anytime soon I'm afraid. >> >> Not to mention bloody SMT which makes the whole mess worse. > > I think we should keep it dirt simple and above all dirt cheap. The per > task migration cap per unit time should meet that bill, limit the damage > potential, while also limiting the good, but that's tough. I don't see
agree > any way to make it perfect, so I'll settle for good enough. > > -Mike > > > > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
