vCPUs, Shares and Resource Pools in VC

[emba]

Hi list,

I know, there are many threads about how fast can a vCPU become, limits etc. The overall statement in the groups here is, that a vCPU can never run faster than the speed of the physical core it is assigned to. Sounds reasonable...Now, I was reading about Resource Pools and Shares again, because we plan to use this handy feature to group VMs according to their priorities. The document "Resource Management Guide" states about Shares:

The amount of resources represented by each share changes when a new virtual
machine is powered on. This affects all virtual machines. For example:
- Two virtual machines run on a host with 8GHz. Both are set to Normal and get
4GHz each.
- A third virtual machine is powered on. It is set to High, which means it should
have twice as many shares as the machines set to Normal. The new virtual machine
receives 4GHz and the two other machines get only 2GHz each.

Does this make sense? Are Shares always being applied or only during contention? The document seems to be somewhat misleading...

Thanks in advance

Michael

[Yattong]

Hey Emba,

Yes, Share are only applied when resources are in contention.

Good Luck

[Dave_Mishchenko]

Shares would only be applied if there were contention for resources. Otherwise the vCPUs would be able to run at the full speed of the physical CPU core.

[emba]

Thank you, guys.

So this means the Documentation is wrong? I would like to open an SR and clarify this with VMware Support.

What do you suggest?

[Dave_Mishchenko · Accepted solution]

It doesn't really address what happens when VMs relinquish their CPU shares so it could be better. Here's a great discussion on that topic - see especially the first post by puneetz on the 2nd page - http://communities.vmware.com/thread/73411.

The last post of the thread also has a good link to a presentation - here's the article from it http://www.vmware-tsx.com/download.php?asset_id=36.