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

From: Linus Torvalds
Date: Fri Sep 14 2012 - 17:43:01 EST

On Fri, Sep 14, 2012 at 2:27 PM, Borislav Petkov <bp@xxxxxxxxx> 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?

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