Types of multiprocessing

Several categories of multiprocessing (MP) systems exist.

Shared nothing MP
The processors share nothing (each has its own memory, caches, and disks), but they are interconnected. This type of muliprocessing is also called a pure cluster.
Shared disks MP
The advantages of shared disks are that part of a familiar programming model is retained (disk data is addressable and coherent, memory is not), and high availability is much easier than with shared-memory systems. The disadvantages are limited scalability due to bottlenecks in physical and logical access to shared data.
Shared Memory Cluster
All of the processors in a shared memory cluster have their own resources (main memory, disks, I/O) and each processor runs a copy of the operating system.
Shared memory MP
All of the processors are tightly coupled inside the same box with a high-speed bus or a switch. The processors share the same global memory, disks, and I/O devices. Only one copy of the operating system runs across all of the processors, and the operating system must be designed to exploit this architecture (multithreaded operating system).

Parent topic: Symmetrical Multiprocessor concepts and architecture