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

[Igor Mammedov <imammedo@redhat.com>]

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).

> - 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.
 
> 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.

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.