Energy-efficient mobile GPU systems

The design of mobile GPUs is all about saving energy. Smartphones and tablets are battery-operated and thus any type of rendering needs to use as little energy as possible. Furthermore, smartphones do not include sophisticated cooling systems due to their small size, making heat dissipation a primary concern. Improving the energy-efficiency of mobile GPUs will be absolutely necessary to achieve the performance required to satisfy consumer expectations, while maintaining operating time per battery charge and keeping the GPU in its thermal limits. The first step in optimizing energy consumption is to identify the sources of energy drain. Previous studies have demonstrated that the register file is one of the main sources of energy consumption in a GPU. As graphics workloads are highly data- and memory-parallel, GPUs rely on massive multithreading to hide the memory latency and keep the functional units busy. However, aggressive multithreading requires a huge register file to keep the registers of thousands of simultaneous threads. Such a big register file exceeds the power budget typically available for an embedded graphics processors and, hence, more energy-efficient memory latency tolerance techniques are necessary. On the other hand, prior research showed that the off-chip accesses to system memory are one of the most expensive operations in terms of energy in a mobile GPU. Therefore, optimizing memory bandwidth usage is a primary concern in mobile GPU design. Many bandwidth saving techniques, such as texture compression or ARM's transaction elimination, have been proposed in both industry and academia. The purpose of this thesis is to study the characteristics of mobile graphics processors and mobile workloads in order to propose different energy saving techniques specifically tailored for the low-power segment. Firstly, we focus on energy-efficient memory latency tolerance. We analyze several techniques such as multithreading and prefetching and conclude that they are effective but not energy-efficient. Next, we propose an architecture for the fragment processors of a mobile GPU that is based on the decoupled access/execute paradigm. The results obtained by using a cycle-accurate mobile GPU simulator and several commercial Android games show that the decoupled architecture combined with a small degree of multithreading provides the most energy efficient solution for hiding memory latency. More specifically, the decoupled access/execute-like design with just 4 SIMD threads/processor is able to achieve 97% of the performance of a larger GPU with 16 SIMD threads/processor, while providing 20.5% energy savings on average. Secondly, we focus on optimizing memory bandwidth in a mobile GPU. We analyze the bandwidth usage in a set of commercial Android games and find that most of the bandwidth is employed for fetching textures, and also that consecutive frames share most of the texture dataset as they tend to be very similar. However, the GPU cannot capture inter-frame texture re-use due to the big size of the texture dataset for one frame. Based on this analysis, we propose Parallel Frame Rendering (PFR), a technique that overlaps the processing of multiple frames in order to exploit inter-frame texture re-use and save bandwidth. By processing multiple frames in parallel textures are fetched once every two frames instead of being fetched in a frame basis as in conventional GPUs. PFR provides 23.8% memory bandwidth savings on average in our set of Android games, that result in 12% speedup and 20.1% energy savings. Finally, we improve PFR by introducing a hardware memoization system on top. We analyze the redundancy in mobile games and find that more than 38% of the Fragment Program executions are redundant on average. We thus propose a task-level hardware-based memoization system that provides 15% speedup and 12% energy savings on average over a PFR-enabled GPU.El diseño de las GPUs (Graphics Procesing Units) móviles se centra fundamentalmente en el ahorro energético. Los smartphones y las tabletas son dispositivos alimentados mediante baterías y, por lo tanto, cualquier tipo de renderizado debe utilizar la menor cantidad de energía posible. Mejorar la eficiencia energética de las GPUs móviles será absolutamente necesario para alcanzar el rendimiento requirido para satisfacer las expectativas de los usuarios, sin reducir el tiempo de vida de la batería. El primer paso para optimizar el consumo energético consiste en identificar qué componentes son los principales consumidores de la batería. Estudios anteriores han identificado al banco de registros y a los accessos a memoria principal como las mayores fuentes de consumo energético en una GPU. El propósito de esta tesis es estudiar las características de los procesadores gráficos móviles y de las aplicaciones móviles con el objetivo de proponer distintas técnicas de ahorro energético. En primer lugar, la investigación se centra en desarrollar métodos energéticamente eficientes para ocultar la latencia de la memoria principal. El resultado de la investigación es una arquitectura desacoplada para los Fragment Processors de la GPU. Los resultados experimentales utilizando un simulador de ciclo y distintos juegos de Android muestran que una arquitectura desacoplada, combinada con un nivel de multithreading moderado, proporciona la solución más eficiente desde el punto de vista energético para ocultar la latencia de la memoria prinicipal. Más específicamente, la arquitectura desacoplada con sólo 4 SIMD threads/processor es capaz de alcanzar el 97% del rendimiento de una GPU más grande con 16 SIMD threads/processor, al tiempo que se reduce el consumo energético en un 20.5%. En segundo lugar, el trabajo de investigación se centró en optimizar el ancho de banda en una GPU móvil. Se realizó un estudio del uso del ancho de banda en distintos juegos de Android y se observó que la mayor parte del ancho de banda se utiliza para leer texturas. Además, se observó que frames consecutivos comparten una gran parte de las texturas. Sin embargo, la GPU no puede capturar el reuso de texturas entre frames dado que el tamaño de las texturas utilizadas por un frame es mucho mayor que la caché de segundo nivel. Basándose en este análisis, se desarrolló Parallel Frame Rendering (PFR), una técnica que solapa el procesado de multiples frames consecutivos con el objetivo de explotar el reuso de texturas entre frames y ahorrar así ancho de bando. Al procesar múltiples frames en paralelo las texturas se leen de memoria principal una vez cada dos frames en lugar de leerse en cada frame como sucede en una GPU convencional. PFR proporciona un ahorro del 23.8% en ancho de banda en promedio para distintos juegos de Android, este ahorro de ancho de banda redunda en un incremento del rendimiento del 12% y un ahorro energético del 20.1%. Por último, se mejoró PFR introduciendo un sistema hardware capaz de evitar cómputos redundantes. Un análisis de distintos juegos de Android reveló que más de un 38% de las ejecuciones del Fragment Program eran redundantes en promedio. Así pues, se propuso un sistema hardware capaz de identificar y eliminar parte de los cómputos y accessos a memoria redundantes, dicho sistema proporciona un incremento del rendimiento del 15% y un ahorro energético del 12% en promedio con respecto a una GPU móvil basada en PFR

Shader optimization and specialization

In the field of real-time graphics for computer games, performance has a significant effect on the player’s enjoyment and immersion. Graphics processing units (GPUs) are hardware accelerators that run small parallelized shader programs to speed up computationally expensive rendering calculations. This thesis examines optimizing shader programs and explores ways in which data patterns on both the CPU and GPU can be analyzed to automatically speed up rendering in games. Initially, the effect of traditional compiler optimizations on shader source-code was explored. Techniques such as loop unrolling or arithmetic reassociation provided speed-ups on several devices, but different GPU hardware responded differently to each set of optimizations. Analyzing execution traces from numerous popular PC games revealed that much of the data passed from CPU-based API calls to GPU-based shaders is either unused, or remains constant. A system was developed to capture this constant data and fold it into the shaders’ source-code. Re-running the game’s rendering code using these specialized shader variants resulted in performance improvements in several commercial games without impacting their visual quality

Enhancing a Neurosurgical Imaging System with a PC-based Video Processing Solution

This work presents a PC-based prototype video processing application developed to be used with a specific neurosurgical imaging device, the OPMI® PenteroTM operating microscope, in the Department of Neurosurgery of Helsinki University Central Hospital at Töölö, Helsinki. The motivation for implementing the software was the lack of some clinically important features in the imaging system provided by the microscope. The imaging system is used as an online diagnostic aid during surgery. The microscope has two internal video cameras; one for regular white light imaging and one for near-infrared fluorescence imaging, used for indocyanine green videoangiography. The footage of the microscope’s current imaging mode is accessed via the composite auxiliary output of the device. The microscope also has an external high resolution white light video camera, accessed via a composite output of a separate video hub. The PC was chosen as the video processing platform for its unparalleled combination of prototyping and high-throughput video processing capabilities. A thorough analysis of the platform and efficient video processing methods was conducted in the thesis and the results were used in the design of the imaging station. The features found feasible during the project were incorporated into a video processing application running on a GNU/Linux distribution Ubuntu. The clinical usefulness of the implemented features was ensured beforehand by consulting the neurosurgeons using the original system. The most significant shortcomings of the original imaging system were mended in this work. The key features of the developed application include: live streaming, simultaneous streaming and recording, and playing back of upto two video streams. The playback mode provides full media player controls, with a frame-by-frame precision rewinding, in an intuitive and responsive interface. A single view and a side-by-side comparison mode are provided for the streams. The former gives more detail, while the latter can be used, for example, for before-after and anatomic-angiographic comparisons.fi=Opinnäytetyö kokotekstinä PDF-muodossa.|en=Thesis fulltext in PDF format.|sv=Lärdomsprov tillgängligt som fulltext i PDF-format

Media gateway utilizando um GPU

Mestrado em Engenharia de Computadores e Telemátic

Hardware Accelerators for Animated Ray Tracing

Future graphics processors are likely to incorporate hardware accelerators for real-time ray tracing, in order to render increasingly complex lighting effects in interactive applications. However, ray tracing poses difficulties when drawing scenes with dynamic content, such as animated characters and objects. In dynamic scenes, the spatial datastructures used to accelerate ray tracing are invalidated on each animation frame, and need to be rapidly updated. Tree update is a complex subtask in its own right, and becomes highly expensive in complex scenes. Both ray tracing and tree update are highly memory-intensive tasks, and rendering systems are increasingly bandwidth-limited, so research on accelerator hardware has focused on architectural techniques to optimize away off-chip memory traffic. Dynamic scene support is further complicated by the recent introduction of compressed trees, which use low-precision numbers for storage and computation. Such compression reduces both the arithmetic and memory bandwidth cost of ray tracing, but adds to the complexity of tree update.This thesis proposes methods to cope with dynamic scenes in hardware-accelerated ray tracing, with focus on reducing traffic to external memory. Firstly, a hardware architecture is designed for linear bounding volume hierarchy construction, an algorithm which is a basic building block in most state-of-the-art software tree builders. The algorithm is rearranged into a streaming form which reduces traffic to one-third of software implementations of the same algorithm. Secondly, an algorithm is proposed for compressing bounding volume hierarchies in a streaming manner as they are output from a hardware builder, instead of performing compression as a postprocessing pass. As a result, with the proposed method, compression reduces the overall cost of tree update rather than increasing it. The last main contribution of this thesis is an evaluation of shallow bounding volume hierarchies, common in software ray tracing, for use in hardware pipelines. These are found to be more energy-efficient than binary hierarchies. The results in this thesis both confirm that dynamic scene support may become a bottleneck in real time ray tracing, and add to the state of the art on tree update in terms of energy-efficiency, as well as the complexity of scenes that can be handled in real time on resource-constrained platforms