linux-kernel - Re: [RFC Patch V1 00/30] Enable memoryless node on x86 platforms

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Date:	Fri, 11 Jul 2014 13:02:14 -0700
From:	Dave Hansen <dave.hansen@...el.com>
To:	Greg KH <gregkh@...uxfoundation.org>,
	Jiang Liu <jiang.liu@...ux.intel.com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>,
	David Rientjes <rientjes@...gle.com>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	"Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
	Tony Luck <tony.luck@...el.com>, linux-mm@...ck.org,
	linux-hotplug@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC Patch V1 00/30] Enable memoryless node on x86 platforms

On 07/11/2014 08:33 AM, Greg KH wrote:
> On Fri, Jul 11, 2014 at 10:29:56AM +0200, Peter Zijlstra wrote:
>> > On Fri, Jul 11, 2014 at 03:37:17PM +0800, Jiang Liu wrote:
>>> > > Any comments are welcomed!
>> > 
>> > Why would anybody _ever_ have a memoryless node? That's ridiculous.
> I'm with Peter here, why would this be a situation that we should even
> support?  Are there machines out there shipping like this?

This is orthogonal to the problem Jiang Liu is solving, but...

The IBM guys have been hitting the CPU-less and memoryless node issues
forever, but that's mostly because their (traditional) hypervisor had
good NUMA support and ran multi-node guests.

I've never seen it in practice on x86 mostly because the hypervisors
don't have good NUMA support. I honestly think this is something x86 is
going to have to handle eventually anyway.  It's essentially a resource
fragmentation problem, and there are going to be times where a guest
needs to be spun up and hypervisor has nodes with either no spare memory
or no spare CPUs.

The hypervisor has 3 choices in this case:
1. Lie about the NUMA layout
2. Waste the resources
3. Tell the guest how it's actually arranged

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