Subject: Re: [RFC PATCH 2/3] topology: support node_numa_mem() for determining the fallback node

From: Nishanth Aravamudan <[email-masked]>
Date: Tue, 18 Feb 2014 14:22:42 -0800

On 18.02.2014 [15:49:22 -0600], Christoph Lameter wrote:
> On Tue, 18 Feb 2014, Nishanth Aravamudan wrote:
> 
> > We use the topology provided by the hypervisor, it does actually reflect
> > where CPUs and memory are, and their corresponding performance/NUMA
> > characteristics.
> 
> And so there are actually nodes without memory that have processors?

Virtually (topologically as indicated to Linux), yes. Physically, I
don't think they are, but they might be exhausted, which is we get sort
of odd-appearing NUMA configurations.

> Can the hypervisor or the linux arch code be convinced to ignore nodes
> without memory or assign a sane default node to processors?

I think this happens quite often, so I don't know that we want to ignore
the performance impact of the underlying NUMA configuration. I guess we
could special-case memoryless/cpuless configurations somewhat, but I
don't think there's any reason to do that if we can make memoryless-node
support work in-kernel?

> > > Ok then also move the memory of the local node somewhere?
> >
> > This happens below the OS, we don't control the hypervisor's decisions.
> > I'm not sure if that's what you are suggesting.
> 
> You could also do this from the powerpc arch code by sanitizing the
> processor / node information that is then used by Linux.

I see what you're saying now, thanks!

-Nish