Knowledgebase: Cloud Hosting
CPU Priority
Posted by Joe Rebis on 10 February 2012 07:06 AM


What does the CPU Priority feature mean when creating a new machine?

The amount of processing power allocated to your virtual machine (VM) is calculated by the following equation.

CPU Cores x CPU Priority = Total GHz

The amount of CPU resource a VM is given is the CPU priority (you can think of this as its “share percentage”) multiplied by the number of cores allocated to that VM. This is a minimum number – clients can burst over it, up to 100% multiplied by the number of cores. For example, on a hypervisor with 3GHz CPU cores:

• 100% x 1 core = 3GHz (burstable to 3GHz)
• 10% x 2 cores = 600MHZ (burstable to 6GHz)
• 5 % x 4 cores = 600MHz (burstable to 12GHz)

By default,  we allow the burstbility of cloud resources for general flexibilty of all of our customers. For example, users are able to create 5 VMs with 100% CPU priority/1 CPU core on a hypervisor (HV a.k.a. Host Server) with a 4-core CPU. In this example, we would reduce the guaranteed CPU for each VM.


How does it work?

Imagine a hypervisor like a grocery store with only two checkouts (CPU cores). In the afternoon, there are not so many customers (virtual servers) so when someone needs to go to checkout, there is always one available. Later in the night, a queue starts to form and customers have to wait, but some of them push to the front of the queue (100 % priority) whilst all other (1% priority) have to wait patiently for the line ahead to clear.


How come I can build a machine with 1 CPU and 1% priority and it works fine?

Since we allow the CPU to burst over it's allocated amount a VM with only 1% priority would still function fine. However, please keep in mind in a crunch, VMs with 100% CPU priority will take precedence over a lower "priority" VM (e.g. 1%) and may not continue to perform as expected- rather than being migrated to a new Hypervisor with more resources. This is why this is called "priority" rather than CPU Percentage.


Can I save money with using the CPU Priority Setting?

Yes, we allocate approximately $ .02 cents an hour per 100% priority. So, a VM with 50% priority would mean .01 cents an hour saving about $3.65 per month at the time this article was written. Please see our cloud pricing on the front of our website for current pricing.   


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