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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 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



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