Re: [Qemu-devel] qemu-ppc and NUMA topology

[Nishanth Aravamudan <nacc@linux.vnet.ibm.com>]

On 19.05.2014 [15:37:52 -0700], Nishanth Aravamudan wrote:
> Hi Alexey,
> 
> I've been looking at hw/ppc/spapr.c::spapr_populate_memory() and ran
> into a few questions:
> 
> 1) The values from 1 to nb_numa_nodes are used as indices into the
> node_mem array, but that is not populated, necessarily, linearly.
> vl.c::add_node() uses the nodeid parameter as the index into node_mem,
> if it is specified.
> 
> 2) The node ID is based upon the index into the array, but it seems like
> it should actually be based upon the nodeid specified, if any. That is,
> we set the value at index 4 (which is statically the reference point in
> 'ibm,associativity-reference-points') of 'ibm,associativty' for each
> 'ibm,memory@....' node to the index we are currently at. But as
> mentioned in 1) above that index isn't necessarily currently the nodeid
> specified on the command-line.
> 
> What this all means, is that if I specify something like:
> 
> -numa node,nodeid=1,cpus=0-7,mem=2048 -numa
> node,nodeid=5,cpus=8-15,mem=0 -numa node,nodeid=9,mem=2048
> 
> Linux sees:
> 
> numactl --hardware
> available: 3 nodes (0-2)
> node 0 cpus: 8 9 10 11 12 13 14 15
> node 0 size: 0 MB
> node 0 free: 0 MB
> node 1 cpus: 0 1 2 3 4 5 6 7
> node 1 size: 2024 MB
> node 1 free: 1560 MB
> node 2 cpus:
> node 2 size: 0 MB
> node 2 free: 0 MB
> 
> Maybe we don't really care about this, but I just noticed it when trying
> to reproduce some really weird topologies from PowerVM.

Upon further investigation into node_mem, it seems like this assumption
is present throughout the qemu code, e.g, the qemu monitor 'info numa'
command. Will just document it for myself as a weird way to make
memoryless nodes show up :)

Thanks,
Nish