Increasing CPU resources?

[RAlltuck]

Under the Virtual Machine Propertities/Resources tab of one of my VMs I had a CPU limit of 2600, (Reservation: 0, Share: Normal). The VM is also set up with 1 vCPU. After monitoring the VM and seeing the CPU usage peak out at 100% several times I decided to raise the CPU limit to 5200. I shut the VM down, increased the CPU Limit setting to 5200 and re-started the VM. I am still seeing CPU peaks to 100% and the VM Performance monitoring indicates that CPU resources are still at about 2600 when that 100% peak occurs. The host has about 21 GHz of CPU resources total and all the VMs combined do not use more than 7 GHz, so there is not a problem of having enough CPU resources for the VM in question.

What do I do to get my VM to use more than the initial 2600 MHz CPU given it. I do need to also reboot the host?

Thanks for any and all help.

[oreeh · Accepted solution]

I assume your hosts CPUs are 2.6GHz models.

A VM can only utilize as much MHz as the physical CPUs "provide".

Therefore a single vCPU VM in your case can only utilize a max. of 2600MHz and a dual CPU vSMP VM could utilize 5200MHz.

I admit the CPU resources slider is a bit misleading.

[RAlltuck]

Thanks for the info. I have heard many times that a single vCPU was best for a VM and also that people had seen increased performance on the VM when down sizing from multiple vCPUs to a single vCPU. Am I wrong on that?

[oreeh]

You aren't wrong.

The reason is that it is easier for the ESX scheduler to "find" a free core than to "find" multiple free cores.

It is pretty normal that a VM sometimes utilizes all of its CPU resources - nothing different from a physical system.

When a physical server sometimes uses all its CPU resources you don't plug an additional CPU in it - the same goes for VMs.

Message was edited by: oreeh

Besides: most applications aren't able to utilize a second CPU anyway

[RParker]

> Besides: most applications aren't able to utilize a second CPU anyway

Exactly. There is a difference between SMP programs and Multi threaded, I think this is where people often get confused.

[oreeh]

Another problem are the CPU manufacturers telling every user they need at least a dual core CPU to get the work done (especially when an old Pentium-III already would be overkill).