Symmetric multiprocessing

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In computing, symmetric multiprocessing or SMP involves a multiprocessor computer hardware architecture where two or more identical processors are connected to a single shared main memory and are controlled by a single OS instance. Most common multiprocessor systems today use an SMP architecture. In the case of multi-core processors, the SMP architecture applies to the cores, treating them as separate processors. Processors may be interconnected using buses, crossbar switches or on-chip mesh networks. The bottleneck in the scalability of SMP using buses or crossbar switches is the bandwidth and power consumption of the interconnect among the various processors, the memory, and the disk arrays. Mesh architectures avoid these bottlenecks, and provide nearly linear scalability to much higher processor counts at the sacrifice of programmability[2]:

SMP systems allow any processor to work on any task no matter where the data for that task are located in memory, provided that each task in the system is not in execution on two or more processors at the same time; with proper operating system support, SMP systems can easily move tasks between processors to balance the workload efficiently.

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SMP represents one of the earliest styles of multiprocessor machine architectures, typically used for building smaller computers with up to 8 processors. Larger computer systems might use newer architectures such as NUMA (Non-Uniform Memory Access), which dedicates different memory banks to different processors. In a NUMA architecture, processors may access local memory quickly and remote memory more slowly. This can dramatically improve memory throughput as long as the data is localized to specific processes (and thus processors). On the downside, NUMA makes the cost of moving data from one processor to another, as in workload balancing, more expensive. The benefits of NUMA are limited to particular workloads, notably on servers where the data is often associated strongly with certain tasks or users.

Other systems include asymmetric multiprocessing (ASMP), which uses separate specialized processors for specific tasks (which increases complexity), and computer clustered multiprocessing (such as Beowulf), in which not all memory is available to all processors.

Examples of ASMP include many media processor chips that are a relatively slow base processor assisted by a number of hardware accelerator cores. High-powered 3D chipsets in modern videocards could be considered a form of asymmetric multiprocessing. Clustering techniques are used fairly extensively to build very large supercomputers. In this discussion, a single processor is denoted as a uni processor (UP).

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