On Wed, 2012-09-19 at 16:54 +0200, Ingo Molnar wrote: > * Mike Galbraith <efa...@gmx.de> wrote: > > > On Sun, 2012-09-16 at 06:35 +0200, Mike Galbraith wrote: > > > > > 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. > > > > Enabling SMT on little E5620 box says that's the deal. > > pgbench as run is 1:N, and all you have to do is disable > > select_idle_sibling() entirely to see that for _this_ (~odd) > > load, max spread and lower wakeup latency for the mother of > > all work itself is a good thing. > > > > pgbench -i pgbench && pgbench -c $N -T 10 pgbench > > > > N= 1 2 4 8 16 32 64 > > 1336 2482 3752 3485 3327 2928 2290 virgin 3.6.0-rc6 > > 1408 2457 3363 3070 2938 2368 1757 +revert reverted > > 1310 2492 2487 2729 2186 975 874 +revert + select_idle_sibling() > > disabled > > 1407 2505 3422 3137 3093 2828 2250 +revert + schedctl -B > > /etc/init.d/postgresql restart > > 1321 2403 2515 2759 2420 2301 1894 +revert + schedctl -B > > /etc/init.d/postgresql restart + select_idle_sibling() disabled > > > > Hohum, damned if ya do, damned if ya don't. Damn. > > As a test, could you mark that 'big PostgreSQL central work > queue process' with some high priority (renice -20?), to make > sure it's never preempted by wakees? Does that recover > performance as well?
schedctl -B started postgress SCHED_BATCH, so pgbench won't be preempted since it's the only SCHED_NORMAL task left in the lot. All others are postmaster, and SCHED_BATCH. -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/
