Configuring Processors and Parallel Processing
Systems that use multiprocessors can take advantage of the enhancements provided by SQL Server for parallel and symmetric multiprocessing. You can control how and when processors are used by SQL Server as well as when queries are processed in parallel.
Optimizing CPU Usage
Multitasking is an important part of the operating system. Often the operating system will need to move threads of execution among different processors. On a system with a light load, moving threads of execution allows the server to improve performance by balancing the workload. On a system with a heavy load, however, this shuffling of threads can reduce performance because processor cache has to be reloaded repeatedly.
SQL Server ...