Re: Empty memory nodes in VNUMA

[Boris Ostrovsky <boris.ostrovsky@oracle.com>]

On 08/14/2015 11:35 AM, Wei Liu wrote:
> On Fri, Aug 14, 2015 at 11:19:53AM -0400, Boris Ostrovsky wrote:
>> What is the purpose of 'nr_vmemranges < nr_vnodes' test in
>> xc_domain_setvnuma()? Can't we have nodes with no memory?
>>
>> If that's the case, this check will still miss configurations when a node
>> spans multiple memory ranges.
>>
>> For example, this fails:
>>
>> vcpus = 4
>> vnuma = [ [ "pnode=0","size=2048","vcpus=0-1" ],
>>            [ "pnode=1","size=1800" ],
>>            [ "pnode=2","size=0","vcpus=2-3" ],
>>            [ "pnode=3","size=100" ] ]
>>
>> but this
>>
>> vcpus = 4
>> vnuma = [ [ "pnode=0","size=2048","vcpus=0-1" ],
>>            [ "pnode=1","size=1801" ],
>>            [ "pnode=2","size=0","vcpus=2-3" ],
>>            [ "pnode=3","size=100" ] ]
>>
>> does not: because of MMIO hole this will cause a second 1MB range to be
>> created on node 1 (in libxl__vnuma_build_vmemrange_hvm()).
>>
>> Can we drop this check?
>>
> I would say yes. There is certainly such hardware that some NUMA node
> has 0 memory.
>
> Note that this function was written in the early day of vNUMA (4.5).  I
> think that function has the assumption that each vnode has one
> vmemrange.
>
> Try removing that check and see what happens?

I already did --- you think I'd ask without first trying? ;-) Yes, it 
works fine --- one a couple of tests that I tried.

Do you want me to send a patch?

-boris

>
> But, how far are you willing to go? Do you want system with no
> vmemranges at all (that means, no memory at all)? If so that
> nr_vmemranges == 0 check should also be removed. Just kidding...
>
> Wei.
>
>> -boris