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

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Date:	Tue, 16 Sep 2014 09:05:15 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Chuck Ebbert <cebbert.lkml@...il.com>
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"


* Chuck Ebbert <cebbert.lkml@...il.com> wrote:

> 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.

Fair enough, I only went by the description.

Thanks,

	Ingo
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