linux-kernel - Re: [PATCH] x86: Consider multiple nodes in a single socket to be "sane"

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Date:	Tue, 16 Sep 2014 02:03:00 -0500
From:	Chuck Ebbert <cebbert.lkml@...il.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Peter Zijlstra <peterz@...radead.org>, Dave Hansen <dave@...1.net>,
	linux-kernel@...r.kernel.org, borislav.petkov@....com,
	andreas.herrmann3@....com, hpa@...ux.intel.com, ak@...ux.intel.com
Subject: Re: [PATCH] x86: Consider multiple nodes in a single socket to be
 "sane"

On Tue, 16 Sep 2014 08:44:03 +0200
Ingo Molnar <mingo@...nel.org> wrote:

> 
> * Chuck Ebbert <cebbert.lkml@...il.com> wrote:
> 
> > On Tue, 16 Sep 2014 05:29:20 +0200
> > Peter Zijlstra <peterz@...radead.org> wrote:
> > 
> > > On Mon, Sep 15, 2014 at 03:26:41PM -0700, Dave Hansen wrote:
> > > > 
> > > > I'm getting the spew below when booting with Haswell (Xeon 
> > > > E5-2699) CPUs and the "Cluster-on-Die" (CoD) feature 
> > > > enabled in the BIOS.
> > > 
> > > What is that cluster-on-die thing? I've heard it before but 
> > > never could find anything on it.
> > 
> > Each CPU has 2.5MB of L3 connected together in a ring that 
> > makes it all act like a single shared cache. The HW tries to 
> > place the data so it's closest to the CPU that uses it. On the 
> > larger processors there are two rings with an interconnect 
> > between them that adds latency if a cache fetch has to cross 
> > that. CoD breaks that connection and effectively gives you two 
> > nodes on one die.
> 
> Note that that's not really a 'NUMA node' in the way lots of 
> places in the kernel assume it: permanent placement assymetry 
> (and access cost assymetry) of RAM.
> 
> It's a new topology construct that needs new handling (and 
> probably a new mask): Non Uniform Cache Architecture (NUCA)
> or so.

Hmm, looking closer at the diagram, each ring has its own memory controller, so
it really is NUMA if you break the interconnect between that caches.
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