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

[Mike Galbraith]

On Fri, 2012-09-14 at 23:59 +0200, Peter Zijlstra wrote: 
> On Fri, 2012-09-14 at 23:56 +0200, Peter Zijlstra wrote:
> > On Fri, 2012-09-14 at 14:44 -0700, Linus Torvalds wrote:
> > > On Fri, Sep 14, 2012 at 2:40 PM, Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> wrote:
> > > >
> > > > The problem the patch is trying to address is not having to scan an
> > > > entire package for idle cores on every wakeup now that packages are
> > > > getting stupid big.
> > > 
> > > No, it does something *else* too. That whole "left-right" logic to
> > > (according to the commit message) "prevent bouncing" is entirely new,
> > > afaik.
> > > 
> > > So it is *not* just about avoiding to have to scan the whole package.
> > > It changes actual semantics too. No?
> > 
> > Both things change semantics, not looking at the entire package is new
> > too. But yeah I guess you could look at the exact cross-stitching as an
> > enhancement to the 'idle_buggy' thing.
> 
> What I'm saying is that having an idle_buggy means you have to assign
> one in the first place, his left-right stuff might not be the simplest
> means to do that -- in fact I suggested he do a simple shift first time
> I saw that patch.

Shift just means that upon perturbation, tasks shift their way around
the package vs random bounce around, that's why I cross wired.
> So if not the left-right thing, you still need to do _something_ to make
> the idle_buggy work at all. So its not entirely separate.

Yeah.

-Mike


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