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. -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/
