Tightly-coupled multiprocessor systems contain multiple CPUs that are connected at the bus level. These CPUs may have access to a central shared memory (or may participate in a memory hierarchy with both local and shared memory. Chip multiprocessors, also known as multi-core computing, involves more than one processor placed on a single chip and can be thought of the most extreme form of tightly-coupled
multiprocessing. Mainframe systems with multiple processors are often tightly-coupled.
Tightly-coupled systems perform better and are physically smaller than loosely-coupled systems, but have historically required greater initial investments and may depreciate rapidly. Tightly-coupled systems tend to
be much more energy efficient than clusters. This is because considerable economies can be realized by designing components to work together from the beginning in tightly-coupled systems
Loosely-coupled multiprocessor systems (often referred to as clusters) are based on multiple standalone single or dual processor commodity computers interconnected via a high speed communication system
(Gigabit Ethernet is common). A Linux Beowulf cluster is an example of a loosely-coupled system. Nodes in a loosely-coupled system are usually inexpensive commodity computers and can be recycled as independent
machines upon retirement from the cluster.
