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In an ideal world, five processors would do five times the work of one processor. But we live in a world of contention for shared resources, of disk and memory bottlenecks, single-threaded applications, multithreaded applications that require synchronous processing, and poorly coordinated processors. In our world, five processors can be five times as much work for the systems administrator!
Fortunately, Windows NT 4.0 is designed to make the most of multiprocessor configurations. Multiple processors enable multiple threads to execute simultaneously, with different threads of the same process running on each processor. The Windows NT 4.0 microkernel implements symmetric multiprocessing (SMP), wherein any processes—including those of the operating system—can run on any available processor, and the threads of a single process can run on different processors at the same time.
The most common bottlenecks on multiprocessor systems arise when all processors contend for the same operating system or hardware resource. If this resource is in short supply, the system can't benefit from the additional processors.
Shared memory is the Achilles' Heel of multiprocessor systems: Although it enables the threads of a single process to be executed on different processors, it makes multiprocessor systems highly vulnerable to memory shortages, to the design of the cache controller, and to differences in cache management strategies.