Re: 20% performance drop on PostgreSQL 9.2 from kernel 3.5.3 to3.6-rc5 on AMD chipsets - bisected

[Mike Galbraith]

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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