Editing Multiprocessing (section)

== Key topics ==

===Processor symmetry===

In a '''multiprocessing''' system, all CPUs may be equal, or some may be reserved for special purposes.  A combination of hardware and [[operating system]] software design considerations determine the symmetry (or lack thereof) in a given system.  For example, hardware or software considerations may require that only one particular CPU respond to all hardware interrupts, whereas all other work in the system may be distributed equally among CPUs; or execution of kernel-mode code may be restricted to only one particular CPU, whereas user-mode code may be executed in any combination of processors.  Multiprocessing systems are often easier to design if such restrictions are imposed, but they tend to be less efficient than systems in which all CPUs are utilized.

Systems that treat all CPUs equally are called [[symmetric multiprocessing]] (SMP) systems.  In systems where all CPUs are not equal, system resources may be divided in a number of ways, including [[asymmetric multiprocessing]] (ASMP), [[non-uniform memory access]] (NUMA) multiprocessing, and [[computer cluster|clustered]] multiprocessing.

====Master/slave multiprocessor system====
In a master/slave multiprocessor system, the master CPU is in control of the computer and the slave CPU(s) performs assigned tasks.  The CPUs can be completely different in terms of speed and architecture.  Some (or all) of the CPUs can share a common bus, each can also have a private bus (for private resources), or they may be isolated except for a common communications pathway.  Likewise, the CPUs can share common RAM and/or have private RAM that the other processor(s) cannot access.  The roles of master and slave can change from one CPU to another.

Two early examples of a mainframe master/slave multiprocessor are the [[Bull Gamma 60]] and the [[Burroughs Large Systems#B5000, B5500, and B5700|Burroughs B5000]].<ref>{{cite manual
 |title        = The Operational Characteristics of the Processors for the Burroughs B5000
 |id           = 5000-21005A
 |version      = Revision A
 |year         = 1963
 |url          = http://www.bitsavers.org/pdf/burroughs/LargeSystems/B5000_5500_5700/5000-21005_B5000_operChar.pdf
 |publisher    = [[Burroughs Corporation|Burroughs]]
 |access-date  = June 27, 2023
 |archive-date = 30 May 2023
 |archive-url  = https://web.archive.org/web/20230530061204/http://www.bitsavers.org/pdf/burroughs/LargeSystems/B5000_5500_5700/5000-21005_B5000_operChar.pdf
 |url-status   = live
}}</ref>

An early example of a master/slave multiprocessor system of microprocessors is the Tandy/Radio Shack [[TRS-80 Model 16]] desktop computer which came out in February 1982 and ran the multi-user/multi-tasking [[Xenix]] operating system, Microsoft's version of UNIX (called TRS-XENIX).  The Model 16 has two microprocessors: an 8-bit [[Zilog Z80]] CPU running at 4&nbsp;MHz, and  a 16-bit [[Motorola 68000]] CPU running at 6&nbsp;MHz. When the system is booted, the Z-80 is the master and the Xenix boot process initializes the slave 68000, and then transfers control to the 68000, whereupon the CPUs change roles and the Z-80 becomes a slave processor responsible for all I/O operations including disk, communications, printer and network, as well as the keyboard and integrated monitor, while the operating system and applications run on the 68000 CPU. The Z-80 can be used to do other tasks.

The earlier [[TRS-80 Model II]], which was released in 1979, could also be considered a multiprocessor system as it had both a Z-80 CPU and an Intel 8021<ref>{{cite book |title=TRS-80 Model II Technical Reference Manual |date=1980 |publisher=Radio Shack |page=135}}</ref> microcontroller in the keyboard.  The 8021 made the Model II the first desktop computer system with a separate detachable lightweight keyboard connected with by a single thin flexible wire, and likely the first keyboard to use a dedicated microcontroller, both attributes that would later be copied years later by Apple and IBM.

===Instruction and data streams===

In multiprocessing, the processors can be used to execute a single sequence of instructions in multiple contexts ([[single instruction, multiple data]] or SIMD, often used in [[vector processing]]), multiple sequences of instructions in a single context ([[multiple instruction, single data]] or MISD, used for [[Redundancy (engineering)|redundancy]] in fail-safe systems and sometimes applied to describe [[Pipeline (computing)|pipelined processors]] or [[hyper-threading]]), or multiple sequences of instructions in multiple contexts ([[multiple instruction, multiple data]] or MIMD).

===Processor coupling===

====Tightly coupled multiprocessor system====
Tightly coupled multiprocessor systems contain multiple CPUs that are connected at the bus level.  These CPUs may have access to a central shared memory (SMP or [[Uniform Memory Access|UMA]]), or may participate in a memory hierarchy with both local and shared memory (SM)([[non-uniform memory access|NUMA]]). The [[IBM p690]] Regatta is an example of a high end SMP system. [[Intel]] [[Xeon]] processors dominated the multiprocessor market for business PCs and were the only major x86 option until the release of [[AMD]]'s [[Opteron]] range of processors in 2004. Both ranges of processors had their own onboard cache but provided access to shared memory; the Xeon processors via a common pipe and the Opteron processors via independent pathways to the system [[Random-access memory|RAM]].

Chip multiprocessors, also known as [[Multi-core (computing)|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.

====Loosely coupled multiprocessor system====
{{main | shared nothing architecture}}
Loosely coupled multiprocessor systems (often referred to as [[Computer cluster|clusters]]) are based on multiple standalone relatively low processor count [[commodity computer]]s interconnected via a high speed communication system ([[Gigabit Ethernet]] is common). A Linux [[Beowulf cluster]] is an example of a [[loose coupling|loosely coupled]] system.

Tightly coupled systems perform better and are physically smaller than loosely coupled systems, but have historically required greater initial investments and may [[depreciation|depreciate]] rapidly; nodes in a loosely coupled system are usually inexpensive commodity computers and can be recycled as independent machines upon retirement from the cluster.

Power consumption is also a consideration. Tightly coupled systems tend to be much more energy-efficient than clusters. This is because a considerable reduction in power consumption can be realized by designing components to work together from the beginning in tightly coupled systems, whereas loosely coupled systems use components that were not necessarily intended specifically for use in such systems.

Loosely coupled systems have the ability to run different operating systems or OS versions on different systems.