Re: [Qemu-devel] [Question] inconsistent memory amount statistics

[David Hildenbrand <david@redhat.com>]

On 22.06.2018 16:11, Igor Mammedov wrote:
> On Fri, 22 Jun 2018 09:41:15 +0200
> David Hildenbrand <david@redhat.com> wrote:
> 
>> Starting qemu with and querying some outputs:
>>
>> [...]
>> -m 4G,maxmem=20G,slots=2 \
>> -numa node,nodeid=0,cpus=0-1 -numa node,nodeid=1,cpus=2-3 \
>> [...]
>> -device virtio-balloon \
>> -object memory-backend-ram,id=mem0,size=8G \
>> -device pc-dimm,id=dimm0,memdev=mem0 \
>> -object memory-backend-ram,id=mem1,size=8G \
>> -device nvdimm,id=dimm1,memdev=mem1,node=1
>>
>> (qemu) info numa
>> info numa
>> 2 nodes
>> node 0 cpus: 0 1
>> node 0 size: 10240 MB
>> node 0 plugged: 0 MB
>> node 1 cpus: 2 3
>> node 1 size: 10240 MB
>> node 1 plugged: 0 MB
>>
>>
>> (qemu) info memory_size_summary
>> info memory_size_summary
>> base memory: 4294967296
>> plugged memory: 17179869184
>>
>> (qemu) info memory-devices
>> info memory-devices
>> Memory device [dimm]: "dimm0"
>>   addr: 0x140000000
>>   slot: 0
>>   node: 0
>>   size: 8589934592
>>   memdev: /objects/mem0
>>   hotplugged: false
>>   hotpluggable: true
>> Memory device [nvdimm]: "dimm1"
>>   addr: 0x340000000
>>   slot: 1
>>   node: 1
>>   size: 8589934592
>>   memdev: /objects/mem1
>>   hotplugged: false
>>   hotpluggable: true
>>
>>
>> (qemu) info balloon
>> info balloon
>> balloon: actual=12288
>>
>>
>> 1. "info numa"
>> - considers both, pc-dimm and nvdimm
>> - "-device ..." are considered as "!plugged" although it could be
>>   theoretically "unplugged"
> I'd think that this part is broken there should be no difference
> -device vs device_add here (the only difference here is cold/hot-plugged).

I agree, it is "coldplugged" and should therefore be considered as
"plugged". I'll prepare a patch, then we can discuss.

> 
>> - device_add devices are considered as "plugged"
>  
>>
>> 2. "info memory_size_summary"
>> - considers both, pc-dimm and nvdimm
>> - "-device ..." are considered as "plugged"
>> - device_add devices are considered as "plugged"
>>
>> 3. "info balloon"
>> - does not consider nvdimm devices to calculate "actual"
>> -- actual = get_current_ram_size() - inflated
>> -- get_current_ram_size() does not consider nvdimm
>>
>> So we have some inconsistency in regards of
>> 1. What is considered memory and what not (pc-dimm vs nvdimm)
>> 2. What is considered plugged memory (-device vs. device_add)
>>
>>
>> Is this what we expect? I think we should make up our mind
>>
>> a) what "plugged" means
>> b) which stats should consider "nvdimm" and which not.
>>
>> I would have guessed that "nvdimms" might be memory devices but should
>> never count towards memory statistics ("not actually ram" - they might
>> be OK).
> well, it depends. 
> nvdimm might be actually battery powered RAM with nvram backend.

I agree that it depends. So I wonder if virtio-balloon should be fixed
too, to consider nvdimms. (the complete balloon interface is broken, one
does not specify how much memory to inflate/deflate, but how much memory
the guest should have. One of the reasons why I dislike global balloon
drivers. They don't play nicely along with memory devices (that are e.g.
got hotplugged, or nvdimm). But that is a different story :) ).

>  
>> Especially "info memory_size_summary" ... "plugged-memory - amount of
>> memory that was hot-plugged" - this seems to be wrong. And I wonder if
>> we should exclude nvdimm from that.
> These summary interfaces look broken to me as what is memory might be
> unclear. It would be better if we just provide primitives to query/list
> individual and let user do the math the way he/she prefers.

We have such an interface for memory devices (info memory-devices), but
not for initial memory. That is at least now accessible via "info
memory_size_summary".

> 
> Wrt QEMU's CLI interface /VM it emulates/ it's all RAM,
> so if we report some statistics we probably should include nvdimm as well.

I agree.

-- 

Thanks,

David / dhildenb