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

From: Peter Zijlstra
Date: Fri Sep 14 2012 - 17:41:35 EST

On Fri, 2012-09-14 at 23:27 +0200, Borislav Petkov wrote:
> I was able to reproduce it on another box here and did a bisection run.
> It pointed to the commit below.
> And yes, reverting that commit fixes the issue here.

Hmm, cute. What kind of machine did you test it on? Nikolay's machines
look to be smallish AMD X6 or ancient Intel c2d (the patch will indeed
have absolutely no effect on a dual core).

I'll see about running pgbench on a bigger Intel tomorrow if Mike
doesn't beat me to it.

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.

Regressing Postgres otoh isn't nice either..

Anyway, I guess I'm fine with nixing this patch until we figure out
something smarter..

I'm also curious to know wth postgres does that this patch makes such a
big difference...
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