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Should you be able to use more VCPUs than available?

Asked by Adrian Turjak

In my most recent devstack install I was able to create and run more instances (1 VCPU each) than the VM had actual CPUs. Hypervisor shows me using 6 of 2 VCPUs which seems broken, but all instances seem to be running. I created a second clean devstack install and tried it again. Both cases was able to use more VCPUs than the machine actually had.

Is this a feature, have I misconfigured something, or is this actually a bug?

Might be specific to my environment, but I can't be sure. Would be good if someone can confirm.

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Best fetahi (fetahi) said : 		#1

It is called oversubscription. Since your VMs are likely not to use the VCPUs (or RAM) they are allocated, it is common to run VMs consuming more VCPUs than you have on your hardware. This behavior can be disabled by putting the following parameters your nova.conf
cpu_allocation_ratio=1.0
ram_allocation_ratio=1.0

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Adrian Turjak (adriant-y) said : 		#2

Thanks fetahi, that solved my question.