VMware vSphere

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  • 1.  Physical Cores vs HyperThreaded Cores

    Posted Aug 22, 2016 06:32 PM

    I have had this question for a while, we have a WebSphere VM that runs 8 vCPU. Now our server is a 2 CPU w/ 10 cores each. totaling 20 cores but when you load ESXi on it, it reports a total of 40 vCPU due to hyper threading. So my question is are my VMs really only running on 4 physical cores or is it actually using 8 physical cores? If the app isn't using hyper threading would it make sense to disable HT on the bios and just use physical cores?



  • 2.  RE: Physical Cores vs HyperThreaded Cores

    Posted Aug 22, 2016 06:53 PM

    The CPU scheduler in the esxi host will figure out the best way to place the 8 vCPUs in your VM. This will generally be on separate physical cores, rather than on separate logical processors on the same core. But it could use the same physical core of this happen to be the most efficient scheduling on a highly utilised host. The OS on the VM will not be able to utilise, and is not aware of, the logical processors.

    Remember that the 2 logical cores (in hyperthreading) use the same physical core but share some cache, and enabling hyperthreading will basically ensure that the physical cores get utilised more efficiently. You might gain  something like 20 - 30% CPU capacity on the host by enabling hyperthreading, so ensure it stays enabled.