Sunday, July 21, 2013

Overallocating vCPUs

I'm always looking for a way to explain why overallocating vCPUs to a VM is a bad idea. At best, it doesn't help. I've seen some discussion this morning on Twitter about this, so I'm sharing how I explain this to people.

Let's say you're going to dinner alone. When you talk to the hostess, you tell her you'd like a table for four because you're really hungry. You can eat faster at a table for four, right? Like, four times as fast? Of course not.

It's the same with vCPUs. If your workload is based on a single-threaded application, and you give it four vCPUs, that workload is dining alone at a table for four.

The metaphor can be extended to explain why doing this over and over again has ripple effects on the host (or the restaurant in this case). But I'll leave that up to you to think about. After all, you're the hostess.