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add ILLIAC IV reference. nyer nyer to NVIDIA who claimed they were first. sorry, ILLIAC beat ya by 50 years :) Tags: Mobile edit Mobile web edit Advanced mobile edit |
added screenshot from ARPA-funded "unlimited distribution" document, July 15 1971, just for fits and giggles. the image is also used in an academic paper Tags: Mobile edit Mobile web edit Advanced mobile edit |
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Such machines exploit [[Data parallelism|data level parallelism]], but not [[Concurrent computing|concurrency]]: there are simultaneous (parallel) computations, but each unit performs exactly the same instruction at any given moment (just with different data). A simple example is to add many pairs of numbers together, all of the SIMD units are performing an addition, but each one has different pairs of values to add. SIMD is especially applicable to common tasks such as adjusting the contrast in a [[digital image]] or adjusting the volume of [[digital audio]]. Most modern [[central processing unit]] (CPU) designs include SIMD instructions to improve the performance of [[multimedia]] use. In recent CPUs, SIMD units are tightly coupled with cache hierarchies and prefetch mechanisms, which minimize latency during large block operations. For instance, AVX-512-enabled processors can prefetch entire cache lines and apply fused multiply-add operations (FMA) in a single SIMD cycle.
[[Image:ILLIAC_IV.jpg|thumb|[[ILLIAC IV]] Array overview, from ARPA-funded Introductory description by Steward Denenberg, July 15 1971.<ref>https://apps.dtic.mil/sti/tr/pdf/ADA954882.pdf</ref>]]
SIMD has three different subcategories in [[Flynn's taxonomy#Single instruction stream, multiple data streams (SIMD)|Flynn's 1972 Taxonomy]], one of which is [[single instruction, multiple threads]] (SIMT). SIMT should not be confused with [[Thread (computing)|software threads]] or [[Multithreading (computer architecture)|hardware threads]], both of which are task time-sharing (time-slicing). SIMT is true simultaneous parallel hardware-level execution, such as in the [[ILLIAC IV]]. A key distinction in SIMT is the presence of control flow mechanisms like warps ([[Nvidia]] terminology) or wavefronts (Advanced Micro Devices ([[AMD]]) terminology). These allow divergence and convergence of threads, even under shared instruction streams, thereby offering slightly more flexibility than classical SIMD.{{clarify|reason=Is classical SIMD one of the subcategories in Flynn's 1972 paper? If so, which subcategory?|date=July 2025}}
Each hardware element (PU, or PE in [[ILLIAC IV]] terminology) working on individual data item sometimes also referred as SIMD lane or channel. Modern [[graphics processing unit]]s (GPUs) are often wide SIMD (typically >16 data lanes or channel) implementations.{{cn|date=July 2024}} Some newer GPUs go beyond simple SIMD and integrate mixed-precision SIMD pipelines, which allow concurrent execution of [[8-bit computing|8-bit]], [[16-bit computing|16-bit]], and [[32-bit computing|32-bit]] operations in different lanes. This is critical for applications like AI inference, where mixed precision boosts throughput.
Additionally, SIMD can exist in both fixed and scalable vector forms. Fixed-width SIMD units operate on a constant number of data points per instruction, while scalable designs, like RISC-V Vector or ARM's SVE, allow the number of data elements to vary depending on the hardware implementation. This improves forward compatibility across generations of processors.
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