Q. How can I enable hyper-threading in a hypervisor like vSphere 6?
First, the underlying server or system must support hyper-threading technology, often denoted as HT or HTT. This means the system's processors must be compliant with HTT, and hyper-threading support must be enabled in the system's basic I/O system (BIOS) firmware. The actual label within a BIOS menu structure can vary depending on the firmware vendor, but is often denoted as Enable Logical Processor or Enable Hyper-threading. If there is any question about firmware support, refer to the system's documentation or contact the server vendor for support.
Once enabled at the hardware level, you must also enable hyper-threading at the hypervisor level. VSphere 6 typically enables hyper-threading by default, but it's important for administrators to verify status as part of system configuration. Open the vSphere Web Client navigator and locate the particular host system, select the Manage tab and click Settings. Select Advanced System Settings in the System area and select the VMkernel.Boot.hyperthreading entry. If the value is true, hyper-threading is enabled in vSphere 6.
It's also important to check the way that hypervisors see processors to verify that hyper-threading is properly enabled. VSphere 6 allows administrators to locate the host system in the vSphere Web Client navigator, select the Manage tab and click Settings. Choose Processors to review details about the physical and logical processors present in the system. For example, a quad-core processor with hyper-threading enabled will show four physical cores and eight logical processors.
Be careful when applying CPU affinity to VM workloads. CPU affinity features allow administrators to restrict the available processors that can be assigned to VMs. When constraints are applied improperly or without proper planning, it may be possible to force multiple demanding VMs to use logical processors on the same physical core, causing potential performance degradation of either -- or both -- workloads.
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