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    SubjectRe: 20% performance drop on PostgreSQL 9.2 from kernel 3.5.3 to 3.6-rc5 on AMD chipsets - bisected
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    On Wed, 2012-09-19 at 16:54 +0200, Ingo Molnar wrote: 
    > * Mike Galbraith <efault@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



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