There are several Windows NT features of particular interest to enterprise application developers. Symmetric Multiprocessing and Multithreading Multiprocessing systems have multiple CPUs and can execute more than one set of machine instructions at a time. The exact performance gain varies among SMP architectures.
Most applications run as a single process, with a single flow of control.
These programs can work on one task at a time, but they won't take advantage of SMP systems. For maximum SMP performance, applications need to be developed with multiple threads . Each thread is a separate flow of control through the program. It can be scheduled independently of other threads, and each application thread can accept, process, and return data independently of other application or system threads. As each incoming HTTP or FTP request arrives, IIS spawns a new thread to handle it, and the new thread is automatically assigned to a processor.