My responses below assume each VM in their examples have 1 vCPU.
Consider the following examples. n Two CPU-bound virtual machines run on a host with 8GHz of aggregate CPU capacity. Their CPU shares are set to Normal and get 4GHz each.
VM1: "Normal" CPU shares (equal to 1000)
VM2: "Normal" CPU shares (equal to 1000)
So in this case 8Ghz (total amount of host)/2 (shares set to Normal) = 4Ghz, it's correct?
Yes, when VM1 and VM2 are in CPU contention with one another, they have an equal entitlement to be allocated CPU time.
There are 2000 total shares (VM1 + VM2) and each VM has 50% of those shares.
If they are the only 2 VMs running, changing both their CPU shares to "High" or both to "Low" would still mean they have an equal entitlement (50% of the total shares)
A third CPU-bound virtual machine is powered on. Its CPU shares value is set to High
VM1: "Normal" CPU shares (equal to 1000)
VM2: "Normal" CPU shares (equal to 1000)
VM3: "High" CPU shares (equal to 2000)
In this previous example I don't understand very well. Can help me to understand?
VM3 has twice the entitlement of VM1 and of VM2. VM1 and VM2 still have an equal entitlement against one another.
There are now 4000 total shares. VM1 has 25% of them, VM2 has 25% of them, VM3 has 50% of them. The same would be true if VM3 was "Normal" and both VM1 and VM2 were "Low".
In simple terms, shares are a RELATIVE allocation method, and only have any meaning at all when there is resource contention.
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