On Sat, 2012-09-15 at 22:32 +0100, Alan Cox wrote: > > Yes, postgress performs loads better with it's spinlocks, but due to > > that, it necessarily _hates_ preemption. How the is the scheduler > > supposed to know that any specific userland task _really_ shouldn't be > > preempted at any specific time, else bad things follow? > > You provide a shared page for a process group so it can write hints to > which is kernel mapped so the scheduler can peek..
Or perhaps a flag ala SCHED_RESET_ON_FORK to provide a not necessarily followed hint. That hint could be to simply always try the LAST_BUDDY thing with flagged tasks, since we know that works (postgress inspired LAST_BUDDY). Even with postgress like things, fast mover kthreads etc punching through isn't necessarily a bad thing, you just need to avoid the punch leaving a gigantic hole. Oh, while I'm thinking about it, there's another scenario that could cause the select_idle_sibling() change to affect pgbench on largeish packages, but it boils down to preemption odds as well. IIRC pgbench _was_ at least 1:N, ie one process driving the whole load. Waker of many (singularly bad idea as a way to generate load) being preempted by it's wakees stalls the whole load, so expensive spreading of wakees to the four winds ala WAKE_BALANCE becomes attractive, that pain being markedly less intense than having multiple cores go idle while creator or work waits for one. -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/
