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 - 18:10:41 EST

On Fri, 2012-09-14 at 15:01 -0700, Linus Torvalds wrote:
> Sure, it doesn't take tsk_cpus_allowed() into account while setting up
> the cache (since it's not dynamic enough), but *assuming* the common
> case is that people let threads be on any of the cores of a package,
> it should be possible to make the cache 100% equivalent with no
> semantic change. No?

I'm not seeing how it could be. Only ever looking at 1 other cpu
(regardless which one) cannot be the same as checking 'all' of them.

Suppose we have the 6 core AMD chip, a task being woken on cpu0 would
look at cpus 1-6 (the entire package shares cache) to see if any of them
was idle. Only looking at a single cpu will avoid looking at the other

The chance of finding an idle cpu to run on is much bigger the more cpus
you look at (also more expensive).

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