Setting the number of cores per CPU in a virtual machine
This article provides information on setting number of cores per CPU in a virtual machine.
Some operating system SKUs (Stocking Keeping Unit) are hard-limited to run on a fixed number of CPUs. For example, Windows Server 2003 Standard Edition is limited to run on up to 4 CPUs. If you install this operating system on an 8-socket physical box, it runs on only 4 of the CPUs. The operating system takes advantage of multi-core CPUs so if your CPUs are dual core, Windows Server 2003 Standard Edition runs on up to 8 cores, and if you have quad-core CPUs, it runs on up to 16 cores, and so on.
Virtual CPUs (vCPU) in VMware virtual machines appear to the operating system as single core CPUs. So, just like in the example above, if you create a virtual machine with 8 vCPUs (which you can do with vSphere) the operating system sees 8 single core CPUs. If the operating system is Windows 2003 Standard Edition (limited to 4 CPUs) it only runs on 4 vCPUs.
Note: Remember that 1 vCPU maps onto a physical core not a physical CPU, so the virtual machine is actually getting to run on 4 cores.
Considering that 1 vCPU is equal to 1 CPU is an assumption for the sake of simplification, since vCPUs are scheduled on logical CPUs which are hardware execution contexts. These tasks can take a while in the case of a single core CPU, CPUs that have only 1 thread per core, or could be just a thread in the case of a CPU that has hyper-threading.
Consider this scenario:
In the physical world you can run Windows 2003 Standard Edition on up to 8 cores (using a 2-socket quad-core box) but in a virtual machine they can only run on 4 cores because VMware tells the operating system that each CPU has only 1 core per socket.
VMware now has a setting which provides you control over the number of cores per CPU in a virtual machine.
This new setting, which you can add to the virtual machine configuration (.vmx) file, lets you set the number of cores per virtual socket in the virtual machine.
To implement this feature:
- Power off the virtual machine.
- Right-click on the virtual machine and click Edit Settings.
- Click Hardware and select CPUs.
- Choose the number of virtual processors.
- Click the Options tab.
- Click General, in the Advanced options section.
- Click Configuration Parameters.
- Include cpuid.coresPerSocket in the Name column.
- Enter a value (try 2, 4, or 8) in the Value column. Note: Ensure that the number of vCPUs is divisible by the number of cpuid.coresPerSocket in the virtual machine. That is, when you divide the number of vCPUs by the number of cpuid.coresPerSocket, it must return an integer value. For example, if your virtual machine is created with 8 vCPUs, coresPerSocket can only be 1, 2, 4, or 8. The virtual machine now appears to the operating system as having multi-core CPUs with the number of cores per CPU given by the value that you provided in step 9.
- Click OK.
- Power on the virtual machine.
Create an 8 vCPU virtual machine and set cpuid.coresPerSocket = 2. Window Server 2003 Standard Edition running in this virtual machine now uses all 8 vCPUs. Under the covers, Windows sees 4 dual-core CPUs. The virtual machine is actually running on 8 physical cores.
- Only values of 1, 2, 4, 8 for the cpuid.coresPerSocket are supported for the multi-core vCPU feature in ESX 4.x.
- In ESX 4.0, if multi-core vCPU is used, hot-plug vCPU is not permitted, even if it is available in the UI.
- Only HV 7 virtual machines support the multi-core vCPU feature.
Important: When using cpuid.coresPerSocket, you should always ensure that you are in compliance with the requirements of your operating system EULA (Regarding the number of physical CPUs on which the operating system is actually running).
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