Re: [PATCH RFC v2 4/7] libxl/vNUMA: vNUMA libxl functionality.

[George Dunlap <george.dunlap@eu.citrix.com>]

On 09/17/2013 05:38 PM, Ian Campbell wrote:
> On Tue, 2013-09-17 at 17:36 +0100, George Dunlap wrote:
>> On Fri, Sep 13, 2013 at 9:50 AM, Elena Ufimtseva <ufimtseva@gmail.com> wrote:
>>> vNUMA libxl supporting functionality.
>>> libxl supporting functionality for vNUMA includes:
>>> * having vNUMA memory areas sizes, transforms it to
>>> start and end pfns based on domain e820 map;
>>> * contructs vnode_to_pnode map for vNUMA nodes memory
>>> allocation and pass it to Xen; the mechanism considers
>>> automatic NUMA placement in case of presence of hardware
>>> NUMA; In best case scenario all vnodes will be allocated
>>> within one pnode. If the domain spans different pnodes,
>>> the vnodes will be one-by-one placed to pnodes. If such
>>> allocation is impossible due to the memory constraints,
>>> the allocation will use default mechanism; this is worst
>>> case scenario.
>>
>> Why would someone want to make a VM with two vnodes and then put them
>> on the same pnode?  Apart from testing, of course, but our defaults
>> should be for the common case of real users.
>
> If your pool of machines included 1- and 2-node systems might you want
> to do this so that when you migrate the vm to a two node system it can
> make use of it?

Yes, but from the description (and from a brief glance at the code) it 
looks like if it can happen to fit all the vnodes on a single pnode, it 
will do so -- even if the node affinity for the domain spans several nodes.

I think if I was a user, and made a VM with 2 vnodes, and then set its 
node affinity to two pnodes, I would expect the tools to place each 
vnode on one pnode.

At this point it may be bike-shedding, though... at some point we'll 
have the ability to specify node affinity for individual vcpus, at which 
time we can specify a pnode affinity for individual vnodes.  As long as 
we have something not barking mad here, we can revisit it before the 
release (as long as someone writes down to do it).

  -George