[Engine-devel] CPU topology in the API

[Simon Grinberg]

Hi list,

The current status is that though they look the same, CPU topology for hosts and VMs differ. 

In both you have    
    <topology cores="N" sockets="Y"/>

for hosts: Cores = Total cores on the host, Y=number of sockets 
for VMs: Cores = Cores per socket, Y=number of sockets

This means that for a hosts that has 4 sockets with 8 cores per socket the topology will be presented as:
    <topology cores="32" sockets="4"/>
While for VM with the same requested topology will show:
    <topology cores="8" sockets="4"/>

I think they should not be different to avoid confusion but:

* The information we displayed for the host can't count on the fact that cores are
distributed evenly across sockets, because theoretically a host could contain
several different CPUs, so it can't be displayed as a multiplication. 
* On the other hand changing the VM topology will break the API though it will make it aligned both with hosts and with the 'New VM' dialogue in the UI.

For oVirt 3.x it may be that nothing can be changed however for oVirt it is allowed in theory to break the API (a bit :)) so the options as I see it are:

1. Don't touch, leave the confusion. 
2. Make host align to VMs with the risk that on some hosts this may be a bit misleading - should be rare.
3. Make host topology looks like VM but allow multiple CPU topologies in the CPUs sub collection of the host. 
   (This also requires change in VDSM API)
4. Align VMs to Hosts

I would go for 4 or 2
Current CPU topology for the hosts is a new commit, thus it may be allowed to change it now since no one is using it yet. This works in favour of 2. In any case only 3 discloses all the information in all possible cases.  


Thoughts?


Thanks, 
Simon.