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Should you be able to use more VCPUs than available?

asked 2013-12-19 02:51:48 -0500

adriant-y gravatar image

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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answered 2013-12-19 08:42:25 -0500

adriant-y gravatar image

Thanks fetahi, that solved my question.

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answered 2013-12-19 05:28:01 -0500

fetahi gravatar image

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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Asked: 2013-12-19 02:51:48 -0500

Seen: 287 times

Last updated: Dec 19 '13