Symmetric multiprocessing Guide, Meaning , Facts, Information and Description
Symmetric multiprocessing (SMP) is a multiprocessor computing architecture where all processorss can access, and are equally close to, all random access memory locations.SMP is the most common small computer multiprocessor architecture. However, SMP does not scale well beyond roughly 16 processors, due to the difficulties of designing hardware that will share the memory pool between processors without becoming a system bottleneck. Realistic applications hit serious performance limits at around 8 processors. It is generally assumed that in SMP each additional processor yields an 85% increase in system throughput compared to the previous processor such that additional processors give diminishing returns. For the average user systems with more than 4 processors do not provide significant increases in apparent processing power, with 2 processors giving by far the largest increase in apparent power. (See Amdahl's law).
SMP has many uses in science, industry, and business where software is usually custom programmed for multithreaded processing. However, most consumer products such as word processors and computer games are not written in such a manner that they can gain large benefits from SMP systems. For games this is usually because writing a program to increase performance on SMP systems will produce a performance loss on uniprocessor systems, which comprise the largest percentage of the market. Due to the nature of the different programming methods, it would generally require a separate project to support both uniprocessor and SMP systems with maximum performance. Programs running on SMP systems do, however, experience a performance increase even when they have been written for uniprocessor systems. This is because hardware interrupts that usually suspend program execution while the kernel handles them can run on an idle processor instead. The effect in most applications (e.g. games) is not so much a performance increase as the appearance that the program is running much more smoothly. In some applications, particularly software compiles and some distributed computing projects, one will see an improvement by a factor of (nearly) the number of additional processors.
This of course assumes that the OS supports SMP; if it does not, then the additional processors remain idle and the system functions as a uni-processor system.
Currently, the SMP consumer market consists of the Intel Pentium 4-based Xeon DP and the AMD Opteron 2XX series. (Each of these processors can be used only in dual configurations, where two CPUs work together in one system.) For the mid-range market there is the Intel Pentium 4-based Xeon MP and the AMD Opteron 8XX series, which support systems that have more than two CPUs.
Other alternative SMP platforms include the high-end Intel Itanium and Itanium 2, and the low-end AMD Athlon MP processor. All of the aforementioned chips support SMP. The Opteron chip scales to 8 CPUs, while the Itanium 2 scales to 64 CPUs in platforms such as the SGI Altix series.
Older generations of Intel x86 chips were also used in SMP configurations, especially the high-end Pentium Pro. They are still popular on the second-hand market, among developers and other advanced users. SMP x86 systems before the arrival of the Intel Pentium were rare, so multiprocessor Intel 80486 motherboards may be considered to be a collector item.
Apple Computer has for several years been aggressively pursuing multiprocessor systems, with its dual-processor PowerPC G4 and G5 systems. The interplay between RISC and SMP, with a multiprocessor-aware OS such as Mac OS X, have meant significant performance gains for the Apple platform.
The logical progression onward from SMP systems is the NUMA architecture, whereby the requirement that all processors be equally close to all memory locations is relaxed.
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