Multiprocessing is the use of two or more
central processing units
A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuitry that executes instructions comprising a computer program. The CPU performs basic arithmetic, logic, controlling, an ...
(CPUs) within a single
computer system.
The term also refers to the ability of a system to support more than one processor or the ability to allocate tasks between them. There are many variations on this basic theme, and the definition of multiprocessing can vary with context, mostly as a function of how CPUs are defined (
multiple cores on one
die, multiple dies in one
package, multiple packages in one
system unit
A system is a group of interacting or interrelated elements that act according to a set of rules to form a unified whole. A system, surrounded and influenced by its environment, is described by its boundaries, structure and purpose and expresse ...
, etc.).
According to some on-line dictionaries, a multiprocessor is a computer system having two or more
processing units (multiple processors) each sharing
main memory and peripherals, in order to simultaneously process programs. A 2009 textbook defined multiprocessor system similarly, but noting that the processors may share "some or all of the system’s memory and I/O facilities"; it also gave tightly coupled system as a synonymous term.
At the
operating system
An operating system (OS) is system software that manages computer hardware, software resources, and provides common daemon (computing), services for computer programs.
Time-sharing operating systems scheduler (computing), schedule tasks for ef ...
level, ''multiprocessing'' is sometimes used to refer to the execution of multiple concurrent
processes
A process is a series or set of activities that interact to produce a result; it may occur once-only or be recurrent or periodic.
Things called a process include:
Business and management
*Business process, activities that produce a specific se ...
in a system, with each process running on a separate CPU or core, as opposed to a single process at any one instant.
When used with this definition, multiprocessing is sometimes contrasted with
multitasking, which may use just a single processor but switch it in time slices between tasks (i.e. a
time-sharing system
In computing, time-sharing is the sharing of a computing resource among many users at the same time by means of multiprogramming and multi-tasking.DEC Timesharing (1965), by Peter Clark, The DEC Professional, Volume 1, Number 1
Its emergence a ...
). Multiprocessing however means true parallel execution of multiple processes using more than one processor.
Multiprocessing doesn't necessarily mean that a single process or task uses more than one processor simultaneously; the term
parallel processing is generally used to denote that scenario.
Other authors prefer to refer to the operating system techniques as
multiprogramming and reserve the term ''multiprocessing'' for the hardware aspect of having more than one processor.
The remainder of this article discusses multiprocessing only in this hardware sense.
In
Flynn's taxonomy, multiprocessors as defined above are
MIMD machines.
As the term "multiprocessor" normally refers to tightly coupled systems in which all processors share memory, multiprocessors are not the entire class of MIMD machines, which also contains
message passing multicomputer systems.
Pre-history
Possibly the first expression of the idea of multiprocessing was written by
Luigi Federico Menabrea
Luigi Federico Menabrea (4 September 1809 – 24 May 1896), later made 1st Count Menabrea and 1st Marquess of Valdora, was an Italian general, statesman and mathematician who served as the seventh prime minister of Italy from 1867 to 1869.
B ...
in 1842, about
Charles Babbage's
analytical engine (as translated by
Ada Lovelace): "the machine can be brought into play so as to give several results at the same time, which will greatly abridge the whole amount of the processes."
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
An operating system (OS) is system software that manages computer hardware, software resources, and provides common daemon (computing), services for computer programs.
Time-sharing operating systems scheduler (computing), schedule tasks for ef ...
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
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 have share 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.
An early example of a master/slave multiprocessor system 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 3 microprocessors, an 8-bit
Zilog Z80 CPU running at 4MHz, a 16-bit
Motorola 68000 CPU running at 6MHz and an
Intel 8021 in the keyboard. When the system was booted, the Z-80 was the master and the Xenix boot process initialized the slave 68000, and then transferred control to the 68000, whereupon the CPUs changed roles and the Z-80 became a slave processor that was 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 ran on the 68000 CPU. The Z-80 could 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
microprocessor 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 microprocessor, 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 in fail-safe systems and sometimes applied to describe
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
UMA), or may participate in a memory hierarchy with both local and shared memory (SM)(
NUMA). The
IBM p690
The IBM System p is a high-end line of RISC (Power)/UNIX-based servers. It was the successor of the RS/6000 line, and predecessor of the IBM Power Systems server series.
History
The previous RS/6000 line was originally a line of workstations and ...
Regatta is an example of a high end SMP system.
Intel
Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California, Santa Clara, California. It is the world's largest semiconductor chip manufacturer by revenue, and is one of the devel ...
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
RAM.
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.
Loosely coupled multiprocessor system
Loosely coupled multiprocessor systems (often referred to as
clusters) are based on multiple standalone relatively low processor count
commodity computers interconnected via a high speed communication system (
Gigabit Ethernet is common). A Linux
Beowulf cluster is an example of a
loosely coupled
In computing and systems design, a loosely coupled system is one
# in which components are weakly associated (have breakable relationships) with each other, and thus changes in one component least affect existence or performance of another comp ...
system.
Tightly coupled systems perform better and are physically smaller than loosely coupled systems, but have historically required greater initial investments and may
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.
See also
*
Multiprocessor system architecture
*
Symmetric multiprocessing
*
Asymmetric multiprocessing
*
Multi-core processor
A multi-core processor is a microprocessor on a single integrated circuit with two or more separate processing units, called cores, each of which reads and executes program instructions. The instructions are ordinary CPU instructions (suc ...
*
BMDFM
Binary Modular Dataflow Machine (BMDFM) is a software package that enables running an application in parallel on shared memory symmetric multiprocessing (SMP) computers using the multiple processors to speed up the execution of single application ...
– Binary Modular Dataflow Machine, a SMP MIMD runtime environment
*
Software lockout
*
OpenHMPP
References
{{Parallel computing
Parallel computing
Classes of computers
Computing terminology