Symmetric multiprocessing
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Symmetric multiprocessing, or SMP, is a multiprocessor computer architecture where two or more identical processors are connected to a single shared main memory. Most common multiprocessor systems today use an SMP architecture.
SMP systems allow any processor to work on any task no matter where the data for that task is located in memory; with proper operating system support, SMP systems can easily move tasks between processors to balance the workload efficiently.
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[edit] Alternatives
SMP is only one style of multiprocessor machine; others include NUMA which dedicates different memory banks to different processors. This allows processors to access memory in parallel, which can dramatically improve memory throughput if 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 more expensive, meaning that balancing a workload is 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 or asymmetrical multiprocessing (ASMP), in which separate specialized processors are used for specific tasks; and computer clustered multiprocessing (e.g. Beowulf), in which not all memory is available to all processors. The former is not widely used or supported (though the high-powered 3D chipsets in modern videocards could be considered a form of asymmetric multiprocessing), while the latter is used fairly extensively to build very large supercomputers. In this discussion a single processor is denoted as a uni processor (UN).
[edit] Advantages and disadvantages
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 written in such a manner that they cannot 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 were predominant in the home computer market as of 2007. Due to the nature of the different programming methods, it would generally require two separate projects 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 compilers and some distributed computing projects, one will see an improvement by a factor of (nearly) the number of additional processors.
Support for SMP must be built into the operating system. Otherwise, the additional processors remain idle and the system functions as a uniprocessor system.
In cases where many jobs are being processed in an SMP environment, administrators often experience a loss of hardware efficiency. Software programs have been developed to schedule jobs so that the processor utilization reaches its maximum potential. Good software packages can achieve this maximum potential by scheduling each CPU separately, as well as being able to integrate multiple SMP machines and clusters.
Access to RAM is serialized; this and cache coherency issues causes performance to lag slightly behind the number of additional processors in the system.
[edit] Entry-level systems
Entry-level servers and workstations with two processors dominate the SMP market today. The most popular entry-level SMP systems use the x86 instruction set architecture and are based on Intel’s Xeon, Pentium D and Core Duo processors or AMD’s Athlon64 X2, Quad FX or Opteron 200 and 2000 series processors. Other readily available non-x86 processor choices in the same market are the Sun Microsystems UltraSPARC, Fujitsu SPARC64, SGI MIPS, Intel Itanium, Hewlett Packard PA-RISC, Hewlett-Packard (formerly Compaq formerly Digital Equipment Corporation) DEC Alpha, IBM POWER and Apple Computer PowerPC (specifically G4 and G5 series, as well as earlier PowerPC 604 and 604e series) processors. In all cases, these systems are available in uniprocessor versions as well.
Earlier SMP systems used motherboards that have two or more CPU sockets. More recently, microprocessor manufacturers introduced CPU devices with two or more processors in one device, for example, the POWER, UltraSPARC, Opteron, Athlon, Core 2 Duo, and Xeon all have multi-core variants. Athlon and Core 2 Duo multiprocessors are socket-compatible with uniprocessor variants, so an expensive dual socket motherboard is no longer needed to implement an entry-level SMP machine.
[edit] Mid-level systems
SMP was first implemented on the Burroughs B5500 in 1961. It was implemented later on other mainframes. Mid-level servers, using between four and eight processors, can be found using the Intel Xeon MP, AMD Opteron 800 and 8000 series and the above-mentioned UltraSPARC, SPARC64, MIPS, Itanium, PA-RISC, Alpha and POWER processors. High-end systems, with sixteen or more processors, are also available with all of the above processors.
With the exception of a few rare 80486 systems, the x86 SMP market began with the Intel Pentium technology supporting up to two processors. The Intel Pentium Pro expanded SMP support with up to four processors natively, and systems with as many as two thousand Pentium Pro processors were built. Later, special versions of the Intel Pentium II, and Intel Pentium III processors allowed dual CPU systems. In 2001 AMD released their Athlon MP, or MultiProcessor CPU, together with the 760MP motherboard chipset as their first offering in the dual processor marketplace. This was followed by the Intel Pentium II Xeon and Intel Pentium III Xeon processors could be used with up to four processors in a system natively. Although several much larger systems were built, they were all limited by the physical memory addressing limitation of 64 GiB. With the introduction of 64-bit memory addressing on the AMD64 Opteron in 2003 and EM64T Xeon in 2005, systems are able to address much larger amounts of memory; their addressable limitation of 16 EB is not expected to be reached in the foreseeable future.
[edit] See also
[edit] SMP capable operating systems
- Burroughs (Unisys) MCP
- QNX real-time operating system
- Mac OS 9, Mac OS X
- UNIX-based systems such as BSD, Sun Solaris, AIX, HP-UX, IRIX, and Linux-based systems
- Plan 9
- BeOS and derivatives
- SkyOS
- OS/2 (since 2.11[1])
- OpenVMS (since VMS 5.0[2])
- VxWorks
- Microsoft Windows NT/2000/XP/2003/Vista
[edit] References
- ^ http://www.edm2.com/index.php/OS/2's_Symmetrical_Multiprocessing_Demystified
- ^ http://www.byte.com/art/9611/sec5/art1.htm
