linux-kernel - Re: 20% performance drop on PostgreSQL 9.2 from kernel 3.5.3 to 3.6-rc5 on AMD chipsets

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Message-ID: <CA+55aFwn3UhTDDch4+-=9=T3rHpDvqphY+NLtf2Ljq-7eG+gYw@mail.gmail.com>
Date:	Fri, 14 Sep 2012 14:42:44 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Borislav Petkov <bp@...en8.de>,
	Nikolay Ulyanitsky <lystor@...il.com>,
	Mike Galbraith <efault@....de>, linux-kernel@...r.kernel.org,
	Andreas Herrmann <andreas.herrmann3@....com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: 20% performance drop on PostgreSQL 9.2 from kernel 3.5.3 to
 3.6-rc5 on AMD chipsets - bisected

On Fri, Sep 14, 2012 at 2:27 PM, Borislav Petkov <bp@...en8.de> wrote:
>
> as Nikolay says below, we have a regression in 3.6 with pgbench's
> benchmark in postgresql.
>
> I was able to reproduce it on another box here and did a bisection run.
> It pointed to the commit below.

Ok. I guess we should just revert it. However, before we do that,
maybe Mike can make it just use the exact old semantics of
select_idle_sibling() in the update_top_cache_domain() logic.

Because the patch in question seems to do two things:
 (a) cache the "idle_buggy" logic, so that we don't have those costly loops
 (b) change it to do that "left-right" thing.

and that (b) thing may be what causes a regression for you.

So my gut feel is that the patch was wrong to begin with, exactly
because it did two independent changes. It *should* have treated those
two issues as independent changes and separate commits.

Maybe I'm mis-reading it. Mike? Peter?

            Linus
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