Re: [Qemu-devel] [RFC] x86: Allow to set NUMA distance for different NUMA nodes

[Eric Blake]

On 03/03/2017 07:57 AM, Eduardo Habkost wrote:

>> With this patch, when a user wants to create a guest that contains
>> several vNUMA nodes and also wants to set distance among those nodes,
>> the QEMU command would like:
>>
>> ```
>> -object 
>> memory-backend-ram,size=1G,prealloc=yes,host-nodes=0,policy=bind,id=node0 \
>> -numa 
>> node,nodeid=0,cpus=0,memdev=node0,distance=10,distance=21,distance=31,distance=41
>>  \

> 
> It would be nice to have a more intuitive syntax to represent
> ordered lists in QemuOpts. But this is what we have today.
> 

Markus has the discussion on representing arrays via the command line;
particularly since this array is very tightly coupled to the order in
which values are presented, it may be worth having:

-numa
node,nodeid=0,cpus=0,memdev=nod0,distance.0=10,distance.1=21,distance.2=31,distance.3=41

with the explicit distance.0= suffixes to distance making it more
obvious that we are dealing with an array.

> I think the proposal makes sense. I would like the semantics of the new option
> to be documented at qapi-schema.json and qemu-options.hx.
> 
> I would call the new NumaNodeOptions field "distances", as it is
> a list of distances.

Indeed, Markus is trying (with his work on -blockdev for 2.9) to get the
command line to a point where it is identical to the QMP code, by
reusing qapi-schema.json, so we should very much keep that in mind with
whatever we add to -numa in 2.10.


> but in the future we could support something like:
> 
>   -numa node,nodeid=0,cpus=0,memdev=node0 \
>   -numa node,nodeid=1,cpus=1,memdev=node1 \
>   -numa node,nodeid=2,cpus=2,memdev=node2 \
>   -numa node,nodeid=3,cpus=3,memdev=node3 \
>   -numa 
> distances,distances[0][0]=10,distances[0][1]=21,distances[0][2]=31,distances[0][3]=41,\
>                   
> distances[1][0]=21,distances[1][1]=10,distances[1][2]=21,distances[1][3]=31,\
>                   
> distances[2][0]=31,distances[2][1]=21,distances[2][2]=10,distances[2][3]=21,\
>                   
> distances[3][0]=41,distances[3][1]=31,distances[3][2]=21,distances[3][3]=10

Except that [] requires special shell quoting, so the proposal would be
more like:

-numa distances.0.0=10,distances.0.1=21

Right now, QMP doesn't support 2-D arrays (although this may be a good
reason to introduce support), so that's also something to think about
(not insurmountable, but makes the task more complex).

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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