Processor Utilization Concern

[caster]

Hello,

I am planning on doing VSphere for a new ERP installation. The vendor is requesting two quad core cpu's for a physical box. When I discuss this with them and tell them my goal to virtualize, they advise me that the virtualized machine would need four cores allocated to it. I was planning on getting a server with two quad core cpus.

My concerns are when the four cores aren’t in use they will be available to all the VM's?

Doesn’t allocating 1/2 the capacity of the server really defeat virtualization?

Thanks

[AntonVZhbankov]

Measure real load, all discussions before real numbers are a waste of time.

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MCSA, MCTS, VCP, VMware vExpert '2009

http://blog.vadmin.ru

[amvmware]

In my experience software vendors over spec the hardware requirements as they can't be bothered to test different hardware scenarios.

If you already have the app installed then you might want to do some analysis before virtualising it - or see if the vendor can provide you with a reference customer that has virtualised the app and see how it performed for them in a virtual world and what the utilisation statistics were like.

[Dave_Mishchenko]

They should be able to give you some better starting specs to work with if they can so easily drop from 8 CPU core to 4. Unless you expect a heavy CPU load generally you should start with 1 vCPU and increase from that only if you need to.

But in regards to your question about using up 1/2 of the server. When ESX has to schedule a CPU cycle for a VM, it will find an available core from among the 8 in your host. So a VM could move around if a server is busy. With ESX 2 and up to 3.5 U 3, if you had a 4 vCPU VM ESX would have to find 4 core that would be free pretty much at the same time. So you could end up with performance issues if you have a number of multi vCPU VMs running as they are harder to schedule when the server is under a higher CPU load. With ESX 3.5 U4 and continuing in ESX 4 improvements have been made to CPU scheduler to relax how close the 4 vCPUs would have to execute on physical cores enabling easier scheduling Also, with the improvement if a guest doesn't require all of it's CPU cores then ESXi won't have to schedule any physical CPU cores for idle vCPUs.

What happens if your VMs has a low CPU usage or is idle? ESX is free to use the CPU cores for other VMs. So you would not loose 1/2 the capacity of your host if you had a single 4 vCPU VM running. Depending on the load of your VMs it's realistic to get 5 - 8 vCPUs per physical CPU core.

Dave

VMware Communities User Moderator

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