Re: [Qemu-devel] [PATCH 10/16] dimm: add busy slot check and slot auto-allocation

[Igor Mammedov <imammedo@redhat.com>]

On Fri, 26 Jul 2013 11:26:16 +0200
Paolo Bonzini <pbonzini@redhat.com> wrote:

> Il 26/07/2013 09:38, Igor Mammedov ha scritto:
> > Perhaps denying memory add and suggesting node migration to a node with
> > more memory would be right approach, otherwise user is bound to be hit by
> > cross node penalty.
> 
> Or better, the user can first change the policy from "bind" to
> "preferred", and then hotplug.
> 
> >> > I agree that specifying the policy on every hotplug complicates
> >> > management and may be overkill.  But then, most guests are not NUMA at
> >> > all and you would hardly perceive the difference, you would just have to
> >> > separate
> >> > 
> >> >     set-mem-policy 0 size=2G
> >> >     device_add dimm,slot=0
> > I'm confused, size is inherent property of generic DimmDevice and policies
> > are NUMA specific of node.
> 
> No, the size is not a property of the DimmDevice, it is a property of
> the host object that backs the DimmDevice.  This is like the size is not
> a property of a disk, but rather of the image that backs it.
> 
> Now, there may be a good reason to remove the host/guest distinction in
> the case of memory, but I'm still not sure this is the case.
I don't like split model in this case because it's complicates interface
and confuses user. On bare-metal when user adds DIMM, it definitely has
size property. So when user adds DIMM device like:

     set-mem-policy 0 size=2G,somehostprop=y
     device_add dimm,slot=0

it's not very clear/intuitive to what 'size' relates to. On contrary:

     set-mem-policy 0 somehostprop=y
     device_add dimm,slot=0,size=2G

clearly says that we are adding 2Gb DIMM and allocator of memory that
bakes up DIMM, takes care about host specific details isolating them
from DIMM device model (which resembles baremetal one as much as possible).

> In the case of NUMA policies, I talked to Andrea Arcangeli and indeed
> his suggestion was to have a single policy per guest node (typically
> bind or preferred if guest node size <= host node size, interleave if
> guest node size > host node size).
with current implementation we could inherit DimmBusNumaAware from DimmBus,
and replace allocator to do that, not touching DIMM device model at all.

> 
> However, there are other properties of the memory object (e.g. hugetlbfs
> path) that could be customized for every slot.
Why not keep this mapping in object that allocates memory (DimmBus) and
allow it apply them to allocated memory when DIMM device is being added?

It still would be a split model but DimmDevice interface and implementation
would stay stable and the same (i.e. device_add dimm,id,size,slot,start,node)
whether we use per DIMM host specific policies, NUMA node policies or not care
about them at all.

> Paolo
> 
> > If there is need for per DIMM policies, I'd store NUMA specific policy inside
> > object that implements it (allocates memory), and apply them to DIMM when
> > it's realized.
> > 
> >     set-mem-policy nodeid=X,mem-hostnodes=Z,dev=dimmY
> >     device_add dimm,id=dimmY,size=2G,node=X
> > 
> 
>