Hyber Threading - HT

A thread is a unit of execution that runs on a CPU that can only execute one thread at a time. However, a CPU is very good at switching between threads very quickly to share its power, giving the illusion of simultaneous execution.

Hyper-Threading is a technology found only on Intel processors, that tries to make more efficient use of the CPU by duplicating the architectural state to provide two logical CPUs.
This allows more than 1 thread to be scheduled and enables simultaneous execution when the threads require different processor functions. The effect of this is that you will see two logical CPUs to which the operating system can assign different threads. However, it is only the architectural state that is duplicated, not the physical execution engine. Figure shows a 4 dual-core system with Hyper-Threading enabled, giving 16 logical processors.

Hyper-Threading is only ever going to give you a maximum of 1.3 times nonHyper-Threaded performance and in practice it will be closer to 1.1 to 1.15 times. This knowledge helps put any decision about enabling it back into perspective.

Disable Hyper-Threading unless you have the time to test it thoroughly. Don’t use it on Windows 2000 as it’s not Hyper-Threading aware.

SQL Server license Sacket level not at core CPU

Multi-core effectively means more than one CPU on the same chip so when you buy a dual-core processor you’re actually buying two CPU cores, Windows will see two CPUs, and you should treat it for capacity planning purposes like two single-core CPUs.

It gets even better from a licensing perspective because Microsoft per-processor licensing is per socket, not per core. Socket refers to the physical socket that you plug the processor into.


This licensing model gives Microsoft a competitive advantage over its competitors in the database market, where others charge customers a license fee per core. This is great news for SQL Server customers because a server with four dual-core CPUs will perform comparably with an eight single-core server but at half the license cost.
Image from book

Multi-core processors are only licensed per socket so if you can get an 8-core processor you’ll only need a Windows and SQL Server license for 1 socket!