Polymorphic ASIC : For Video Decoding
Adarsha Rao, S J
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Video applications are becoming ubiquitous in recent times due to an explosion in the number of devices with video capture and display capabilities. Traditionally, video applications are implemented on a variety of devices with each device targeting a speciﬁc application. However, the advances in technology have created a need to support multiple applications from a single device like a smart phone or tablet. Such convergence of applications necessitates support for interoperability among various applications, scalable performance meet the requirements of different applications and a high degree of reconﬁgurability to accommodate rapid evolution in applications features. In addition, low power consumption requirement is also very stringent for many video applications. The conventional custom hardware implementations of video applications deliver high performance at low power consumption while the recent MPSoC implementations enable high degree of interoperability and are useful to support application evolution. In this thesis, we combine the best features of custom hardware and MPSoC approaches to design a Polymorphic ASIC. A Polymorphic ASIC is an integrated circuit designed to meet the requirements of several applications belonging to a particular domain. A polymorphic ASIC consists of a fabric of computation, storage and communication resources, using which applications are composed dynamically. Although different video applications differ widely in the internal de-tails of operation, at the heart of almost every video application is a video codec (encoder and decoder). The requirements of scalability, high performance and low power consumption are very stringent for video decoding. Therefore this thesis focuses mainly on the architectural design of a Polymorphic ASIC for video decoding. We present an uniﬁed software and hardware architecture (USHA) for Polymorphic ASIC. USHA is a tiled architecture which uses loosely coupled processor and hardware tiles that are software programmable and hardware reconﬁgurable respectively. The distinctive feature of Polymorphic ASIC is the static partitioning of the application and dynamic mapping of ap-plication processes onto the computational tiles. Depending on the application scenarios, a process may be mapped onto one of the hardware or processor tiles. Polymorphic ASIC incor-porates a network–on–chip (NoC) to achieve ﬂexible communication across different tiles. Formulation of a programming framework for Polymorphic ASIC requires an implementation model that captures the structure of video decoder applications as well as the properties of the Polymorphic ASIC architecture. We derive an implementation model based on a combination of parametric polyhedral process networks, stream based functions and windowed dataﬂow models of computation. The implementation model leads to a process network oriented compilation ﬂow that achieves realization agnostic application partitioning and enables seamless migration across uniprocessor, multi–processor, semi hardware and full hardware conﬁgurations of a video decoder. The thesis also presents an application QoS aware scheduler that selects a decoder conﬁguration that best meets the application performance requirements, thereby enabling dynamic performance scaling. The memory hierarchy of Polymorphic ASIC makes use of an application speciﬁc cache. Through a combined analysis of miss rate and external memory bandwidth, we show that the degradation in decoder performance due to memory stall cycles depends on the properties of the video being decoded as well as the behavior of the external memory interface. Based on this observation, we present the design of a reconﬁgurable 2–D cache architecture which can adjust its parameters in accordance with the characteristics of the video stream being decoded. We validate the Polymorphic ASIC using a proof–of–concept implementation on an FPGA. The performance of H.264 decoder on Polymorphic ASIC is evaluated for uniprocessor, multi processor, hardware accelerated and full hardware conﬁgurations. The scaling in performance delivered by these conﬁgurations shows that the Polymorphic ASIC enables the application to achieve super linear speedups . The experimental results show that different implementations of a H.264 video decoder on the Polymorphic ASIC can deliver performance comparable to a wide spectrum of devices ranging from embedded processor like ARM 9 to MPSoCs like IBM Cell. We also present the energy consumption of various conﬁgurations of video decoders on Polymorphic ASIC and an application to conﬁguration mapping aimed at minimizing the overall energy consumption of a Polymorphic ASIC.
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