Environment Hyper-threading technology (sometimes also called simultaneous multithreading, or SMT) allows a single physical processor core to behave like two logical processors, essentially allowing two independent threads to run simultaneously
If the hardware and BIOS supports hyper-threading, ESXi will use it.
It is always recommended to enable hyper-threading for better performance.
Details on how to enable hyper-threading can be obtained from the VMWare Guide below.
Reference : http://www.vmware.com/pdf/Perf_Best_Practices_vSphere5.0.pdf