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Subject: Re: 20% performance drop on PostgreSQL 9.2 from kernel 3.5.3 to
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From: Mike Galbraith <efault@gmx.de>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Borislav Petkov <bp@alien8.de>, Peter Zijlstra <a.p.zijlstra@chello.nl>,
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        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@kernel.org>,
        Suresh Siddha <suresh.b.siddha@intel.com>
Date: Tue, 25 Sep 2012 05:20:35 +0200
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On Mon, 2012-09-24 at 20:10 -0700, Linus Torvalds wrote: 
> On Mon, Sep 24, 2012 at 7:49 PM, Mike Galbraith <efault@gmx.de> wrote:
> >
> > Ah.  That's what I did to select_idle_sibling() in a nutshell, converted
> > the problematic large L3 packages into multiple ~core2duo pairs, modulo
> > shared L2 'course.  Bounce proof, and on Westmere, the jabbering back
> > and forth in L3 somehow doesn't hurt as much as expected, so the things
> > act (more or less, L2 traffic _does_ matter;) like the real deal.
> 
> Right. But your patch *only* looked at the pair.
> 
> Which may be bounce-proof, but we also saw that it was unacceptable.

Yes.  Cross wiring traverse _start_ points should eliminate (well, damp)
bounce as well without killing the 1:N latency/preempt benefits of large
L3 packages.  You'll still take a lot of L2 misses while doing futile
traverse when fully committed, but that's a separate issue.

-Mike