[libvirt-users] HugePages - can't start guest that requires them

[G. Richard Bellamy]

*facepalm*

Now that I'm re-reading the documentation it's obvious that <page/>
and @nodeset are for the guest, "This tells the hypervisor that the
guest should have its memory allocated using hugepages instead of the
normal native page size." Pretty clear there.

Thank you SO much for the guidance, I'll return to my tweaking. I'll
report back here with my results.




On Wed, Feb 4, 2015 at 12:17 AM, Michal Privoznik <mprivozn at redhat.com> wrote:
> On 04.02.2015 01:59, G. Richard Bellamy wrote:
>> As I mentioned, I got the instances to launch... but they're only
>> taking HugePages from "Node 0", when I believe my setup should pull
>> from both nodes.
>>
>> [atlas] http://sprunge.us/FSEf
>> [prometheus] http://sprunge.us/PJcR
>
> [pasting interesting nits from both XMLs]
>
> <domain type='kvm' id='2'>
>   <name>atlas</name>
>   <uuid>d9991b1c-2f2d-498a-9d21-51f3cf8e6cd9</uuid>
>   <memory unit='KiB'>16777216</memory>
>   <currentMemory unit='KiB'>16777216</currentMemory>
>   <memoryBacking>
>     <hugepages>
>       <page size='2048' unit='KiB' nodeset='0'/>
>     </hugepages>
>     <nosharepages/>
>   </memoryBacking>
>   <!-- no numa pining -->
> </domain>
>
>
> <domain type='kvm' id='3'>
>   <name>prometheus</name>
>   <uuid>dda7d085-701b-4d0a-96d4-584678104fb3</uuid>
>   <memory unit='KiB'>16777216</memory>
>   <currentMemory unit='KiB'>16777216</currentMemory>
>   <memoryBacking>
>     <hugepages>
>       <page size='2048' unit='KiB' nodeset='2'/>
>     </hugepages>
>     <nosharepages/>
>   </memoryBacking>
>   <!-- again no numa pining -->
> </domain>
>
> So, at start, the @nodeset attribute to <page/> element refers to guest
> numa nodes, not host ones. And since you don't define any numa nodes for
> your guests, it's useless. Side note - I wonder if we should make
> libvirt fail explicitly in this case.
>
> Moreover, you haven't pinned your guests onto any host numa nodes. This
> means it's up to the host kernel and its scheduler where guest will take
> memory from. And subsequently hugepages as well. I think you want to add:
>
>   <numatune>
>     <memory mode='strict' nodeset='0'/>
>   </numatune>
>
> to guest XMLs, where @nodeset refers to host numa nodes and tells where
> the guest should be placed. There are other modes too so please see
> documentation to tune the XML to match your use case perfectly.
>
> Michal