Date of Award

1-1-2009

Degree Name

Doctor of Philosophy

Department

Electrical and Computer Engineering

First Advisor

Zhang,Wei

Abstract

Time predictability is one of the most important design considerations for real-time systems. In this dissertation, time predictability of the instruction cache is studied on both single core processors and multi-core processors. It is observed that many features in modern microprocessor architecture such as cache memories and branch prediction are in favor of average-case performance, which can significantly compromise the time predictability and make accurate worst-case performance analysis extremely difficult if not impossible. Therefore, the time predictability of VLIW (Very Long Instruction Word) processors and its compiler support is studied. The impediments to time predictability for VLIW processors are analyzed and compiler-based techniques to address these problems with minimal modifications to the VLIW hardware design are proposed. Specifically, the VLIW compiler is enhanced to support full if conversion, hyperblock scheduling, and intra-block nop insertion to enable efficient WCET (Worst Case Execution Time) analysis for VLIW processors. Our time-predictable processor incorporates the instruction caches which can mitigate the latency of fetching instructions that hit in the cache. For instruction missing from the cache, instruction prefetching is a useful technique to boost the average-case performance. However, it is unclear whether or not instruction prefetching can benefit the worst-case performance as well. Thus, the impact of instruction prefetching on the worst-case performance of instruction caches is studied. Extension of the static cache simulation technique is applied to model and compute the worst-case instruction cache performance with prefetching. It is shown that instruction prefetching can be reasonably bound, however, the time variation of computing is increased by instruction prefetching. As the technology advances, it is projected that multi-core chips will be increasingly adopted by microprocessor industry. For real-time systems to safely harness the potential of multi-core computing, designers must be able to accurately obtain the worst-case execution time (WCET) of applications running on multi-core platforms, which is very challenging due to the possible runtime inter-core interferences in using shared resources such as the shared L2 caches. As the first step toward time-predictable multi-core computing, this dissertation presents novel approaches to bounding the worst-case performance for threads running on multi-core processors with shared L2 instruction caches. CF (Control Flow) based approach. This approach computes the worst-case instruction access interferences between different threads based on the program control flow information of each thread, which can be statically analyzed. Extended ILP (Integer Linear Programming) based approach. This approach uses constraint programming to model the worst-case instruction access interferences between different threads. In the context of timing analysis in many core architecture, static approaches may also face the scalability issue. Thus, it is important and challenging to design time predictable caches in multi-core architecture. We propose an approach to leverage the prioritized shared L2 caches to improve time predictability for real-time threads running on multi-core processors. The prioritized shared L2 caches give higher priority to real-time threads while allowing low-priority threads to use shared L2 cache space that is available. Detailed implementation and experimental results discussion are presented in this dissertation.

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