HT in a VMware environment

[Samikouk]

I am after some information here regarding Hyper Threading. We have a mixed ESX 4 and ESXi 4.1 in one cluster with hyper threading enabled on some but not all hosts. By migrating a machine from an HT enabled non-HT enabled host would vmkernel virtulise HT on a non-HT host?

I would be grateful from some insight to this.

Many thanks

[weinstein5]

The vmkernel treats hyperthreads and cores the same way as a Logical CPIU (LCPU) - it will schedule the vCPU to a LCPU whether it is a hyperthread or a physical core,

[Samikouk]

but in the hyperthreading world the core will be split into virtually two cores and hence you would get half the power of an actual core, would that still stand if you vMotion the VM from an HT to non-HT host?

[vijayar]

Hi,

When you migrate a VM from a HT to a non HT enable host there is no change happening on the host or VM level as this layer is being virtualized by VMKernel.

Regards,

Vijay .A .R

Sent on my BlackBerry® from Vodafone

[Samikouk]

Thank you for both your responses.

Am I wrong in saying 2 vCPU in HT = 1 vCPU in non-HT host? Taking into account that vmkernel is virtualising this layer.

[vijayar]

Yes, there would not be any change in how it schedules the CPU to the virtual machine.

Regards,

Vijay .A .R

Sent on my BlackBerry® from Vodafone

[a_p_ · Accepted solution]

You are right, but don't worry. The Hypervisor/scheduler knows to differentiate between real cores and HyperThreading.

André

[weinstein5]

Not really - it is a misconception that ht is one half of a full CPU - there is a performance hit but nothing as bad as 50%

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