On Sat, 2012-09-15 at 09:16 -0700, Andi Kleen wrote: > Mike Galbraith <efa...@gmx.de> writes: > > > > The only reason I can think of why pgbench might suffer is postgres's > > userspace spinlocks. If you always look for an idle core, you improve > > the odds that the wakeup won't preempt a lock holder, sending others > > into a long spin. > > User space spinlocks like this unfortunately have a tendency to break > with all kinds of scheduler changes. We've seen this frequently too > with other users. The best bet currently is to use the real time > scheduler, but with various tweaks to get its overhead down.
Yeah, that's one way, but decidedly sub-optimal. > Ultimatively the problem is that user space spinlocks with CPU > oversubcription is a very unstable setup and small changes can > easily disturb it. > > Just using futex is unfortunately not the answer either. 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? -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/
