SIMD

: Single instruction, multiple data (SIMD), is a class of parallel computers in Flynn's taxonomy. It describes computers with multiple processing elements that perform the same operation on multiple data simultaneously. Thus, such machines exploit data level parallelism.

History

The first era of SIMD machines was characterized by supercomputers such as the Thinking Machines CM-1 and CM-2. These machines had many limited functionality processors that would work in parallel. For example, each of 64,000 processors in a Thinking Machines CM-2 would execute the same instruction at the same time so that you could do 64,000 multiplications on 64,000 pairs of numbers at a time.

Supercomputing moved away from the SIMD approach when inexpensive scalar MIMD approaches based on commodity processors such as the Intel i860 XP became more powerful, and interest in SIMD waned. Later, personal computers became common, and became powerful enough to support real-time gaming. This created a mass demand for a particular type of computing power, and microprocessor vendors turned to SIMD to meet the demand. Sun Microsystems introduced SIMD integer instructions in its "VIS" instruction set extensions in 1995, in its UltraSPARC I microprocessor. The first widely-deployed SIMD for gaming was Intel's MMX extensions to the x86 architecture. IBM and Motorola then added AltiVec to the POWER architecture, and there have been several extensions to the SIMD instruction sets for both architectures. All of these developments have been oriented toward support for real-time graphics, and are therefore oriented toward vectors of two, three, or four dimensions. When new SIMD architectures need to be distinguished from older ones, the newer architectures are then considered "short-vector" architectures. A modern supercomputer is almost always a cluster of MIMD machines, each of which implements (short-vector) SIMD instructions. A modern desktop computer is often a multiprocessor MIMD machine where each processor can execute short-vector SIMD instructions.

DSPs

A separate class of processors exists for this sort of task, commonly referred to as Digital Signal Processors, or DSPs. The main difference between DSP and other SIMD-capable CPUs is that the DSPs are self-contained processors with their own instruction set, while SIMD-extensions rely on the general-purpose portions of the CPU to handle the program details, and the SIMD instructions handle the data manipulation only. DSPs also tend to include instructions to handle specific types of data, sound or video for instance, while SIMD systems are considerably of more generic purpose. DSPs generally operate in Scratchpad RAM driven by DMA transfers initiated from the host system and are unable to access external memory.

Some DSPs include SIMD instruction sets. The inclusion of SIMD units in general purpose processors has supplanted the use of DSP chips in computer systems, though they continue to be used in embedded applications. A sliding scale exists - the Cell's SPUs and Ageia's PhysX Physics Processing Unit could be considered half way between CPUs and DSPs, in that they are optimized for numeric tasks and operate in local store, but they can autonomously control their own transfers thus are in effect true CPUs.

Advantages

An application that may take advantage of SIMD is one where the same value is being added (or subtracted) to a large number of data points, a common operation in many multimedia applications. One example would be changing the brightness of an image. Each pixel of an image consists of three values for the brightness of the red (R), green (G) and blue (B) portions of the color. To change the brightness, the R, G and B values are read from memory, a value is added (or subtracted) from them, and the resulting values are written back out to memory.

With a SIMD processor there are two improvements to this process. For one the data is understood to be in blocks, and a number of values can be loaded all at once. Instead of a series of instructions saying "get this pixel, now get the next pixel", a SIMD processor will have a single instruction that effectively says "get lots of pixels" ("lots" is a number that varies from design to design). For a variety of reasons, this can take much less time than "getting" each pixel individually, as with traditional CPU design.

Another advantage is that SIMD systems typically include only those instructions that can be applied to all of the data in one operation. In other words, if the SIMD system works by loading up eight data points at once, the add operation being applied to the data will happen to all eight values at the same time. Although the same is true for any super-scalar processor design, the level of parallelism in a SIMD system is typically much higher.

Disadvantages

Not all algorithms can be vectorized. For example, a flow-control-heavy task like code parsing wouldn't benefit from SIMD.

It also has large register files which increases power consumption and chip area.

Currently, implementing an algorithm with SIMD instructions usually requires human labor; most compilers don't generate SIMD instructions from a typical C program, for instance. Vectorization in compilers is an active area of computer science research. (Compare vector processing.)

Programming with particular SIMD instruction sets can involve numerous low-level challenges.

*SSE (Streaming SIMD Extension) has restrictions on data alignment; programmers familiar with the x86 architecture may not expect this.

*Gathering data into SIMD registers and scattering it to the correct destination locations is tricky and can be inefficient.

*Specific instructions like rotations or three-operand addition aren't in some SIMD instruction sets.

*Instruction sets are architecture-specific: old processors and non-x86 processors lack SSE entirely, for instance, so programmers must provide non-vectorized implementations (or different vectorized implementations) for them.

*The early MMX instruction set shared a register file with the floating-point stack, which caused inefficiencies when mixing floating-point and MMX code. However, SSE2 corrects this.

Chronology

The first use of SIMD instructions was in vector supercomputers of the early 1970s such as the CDC Star-100 and the Texas Instruments ASC. Vector processing was especially popularized by Cray in the 1970s and 1980s.

Later machines used a much larger number of relatively simple processors in a massively parallel processing-style configuration. Some examples of this type of machine included:

ILLIAC IV, circa 1974

ICL Distributed Array Processor (DAP), circa 1974

Burroughs Scientific Processor, circa 1976

Geometric-Arithmetic Parallel Processor, from Martin Marietta, starting in 1981, continued at Lockheed Martin, then at Teranex and Silicon Optix

Massively Parallel Processor (MPP), from NASA/Goddard Space Flight Center, circa 1983-1991

Connection Machine, models 1 and 2 (CM-1 and CM-2), from Thinking Machines Corporation, circa 1985

MasPar MP-1 and MP-2, circa 1987-1996

Zephyr DC computer from Wavetracer, circa 1991

Xplor, from Pyxsys, Inc., circa 2001.

There were many others from that era too.

Hardware

Small-scale (64 or 128 bits) SIMD has become popular on general-purpose CPUs in the early 1990s and continuing through 1997 and later with Motion Video Instructions (MVI) for Alpha. SIMD instructions can be found, to one degree or another, on most CPUs, including the IBM's AltiVec and SPE for PowerPC, HP's PA-RISC Multimedia Acceleration eXtensions (MAX), Intel's MMX and iwMMXt, SSE, SSE2, SSE3 and SSSE3, AMD's 3DNow!, ARC's ARC Video subsystem, SPARC's VIS and VIS2, Sun's MAJC, ARM's NEON technology, MIPS' MDMX (MaDMaX) and MIPS-3D. The IBM, Sony, Toshiba co-developed Cell Processor's SPU's instruction set is heavily SIMD based. NXP founded by Philips developed several SIMD processors named Xetal. The Xetal has 320 16bit processor elements especially designed for vision tasks.

Modern graphics processing units (GPUs) are often wide SIMD implementations, capable of branches, loads, and stores on 128 or 256 bits at a time.

Future processors promise greater SIMD capability: Intel's AVX instructions will process 256 bits of data at once, and Intel's Larrabee graphic microarchitecture promises two 512-bit SIMD registers on each of its cores (VPU - Wide Vector Processing Units) [although as of early 2010, the Larrabee project was canceled at Intel].

Software

SIMD instructions are widely used to process 3D graphics, although modern graphics cards with embedded SIMD have largely taken over this task from the CPU. Some systems also include permute functions that re-pack elements inside vectors, making them particularly useful for data processing and compression. They are also used in cryptography. The trend of general-purpose computing on GPUs (GPGPU) may lead to wider use of SIMD in the future.

Adoption of SIMD systems in personal computer software was at first slow, due to a number of problems. One was that many of the early SIMD instruction sets tended to slow overall performance of the system due to the re-use of existing floating point registers. Other systems, like MMX and 3DNow!, offered support for data types that were not interesting to a wide audience and had expensive context switching instructions to switch between using the FPU and MMX registers. Compilers also often lacked support requiring programmers to resort to assembly language coding.

SIMD on x86 had a slow start. The introduction of 3DNow! by AMD and SSE by Intel confused matters somewhat, but today the system seems to have settled down (after AMD adopted SSE) and newer compilers should result in more SIMD-enabled software. Intel and AMD now both provide optimized math libraries that use SIMD instructions, and open source alternatives like libSIMD and SIMDx86 have started to appear.

Apple Computer had somewhat more success, even though they entered the SIMD market later than the rest. AltiVec offered a rich system and can be programmed using increasingly sophisticated compilers from Motorola, IBM and GNU, therefore assembly language programming is rarely needed. Additionally, many of the systems that would benefit from SIMD were supplied by Apple itself, for example iTunes and QuickTime. However, in 2006, Apple computers moved to Intel x86 processors. Apple's APIs and development tools (XCode) were rewritten to use SSE2 and SSE3 instead of AltiVec. Apple was the dominant purchaser of PowerPC chips from IBM and Freescale Semiconductor and even though they abandoned the platform, further development of AltiVec is continued in several Power Architecture designs from Freescale, IBM.

SIMD within a register, or SWAR, is a range of techniques and tricks used for performing SIMD in general-purpose registers on hardware that doesn't provide any direct support for SIMD instructions. This can be used to exploit parallelism in certain algorithms even on hardware that does not support SIMD directly.

Commercial applications

Though it has generally proven difficult to find sustainable commercial applications for SIMD-only processors, one that has had some measure of success is the GAPP, which was developed by Lockheed Martin and taken to the commercial sector by their spin-off Teranex. The GAPP's recent incarnations have become a powerful tool in real-time video processing applications like conversion between various video standards and frame rates (NTSC to/from PAL, NTSC to/from HDTV formats, etc.), deinterlacing, image noise reduction, adaptive video compression, and image enhancement.

A more ubiquitous application for SIMD is found in video games: nearly every modern video game console since 1998 has incorporated a SIMD processor somewhere in its architecture. The PlayStation 2 was unusual in that one of its vector-float units could function as an autonomous DSP executing its own instruction stream, or as a coprocessor driven by ordinary CPU instructions. 3D graphics applications tend to lend themselves well to SIMD processing as they rely heavily on operations with 4-dimensional vectors. Microsoft's Direct3D 9.0 now chooses at runtime processor-specific implementations of its own math operations, including the use of SIMD-capable instructions.

One of the recent processors to use vector processing is the Cell Processor developed by IBM in cooperation with Toshiba and Sony. It uses a number of SIMD processors (each with independent RAM and controlled by a general purpose CPU) and is geared towards the huge datasets required by 3D and video processing applications.

A recent advancement by Ziilabs was the production of an SIMD type processor which can be used on mobile devices, such as media players and mobile phones.

Larger scale commercial SIMD processors are available from ClearSpeed Technology, Ltd. and Stream Processors, Inc. ClearSpeed's CSX600 (2004) has 96 cores each with 2 double-precision floating point units while the CSX700 (2008) has 192. Stream Processors is headed by computer architect Bill Dally. Their Storm-1 processor (2007) contains 80 SIMD cores controlled by a MIPS CPU.

References

External links

SIMD architectures (2000)

Cracking Open The Pentium 3 (1999)

Short Vector Extensions in Commercial Microprocessor

Article about Optimizing the Rendering Pipeline of Animated Models Using the Intel Streaming SIMD Extensions

Category:Acronyms Category:Computing acronyms Category:Flynn's Taxonomy Category:Digital signal processing Category:Parallel computing Category:Classes of computers