Reducing Frequency in Real-Time Systems via Speculation and Fall-Back Recovery

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dc.contributor.advisor Dr. Eric Rotenberg, Committee Chair en_US
dc.contributor.advisor Dr. Gregory T. Byrd, Committee Member en_US
dc.contributor.advisor Dr. Frank Mueller, Committee Member en_US
dc.contributor.advisor Dr. Alexander G. Dean, Committee Member en_US Anantaraman, Aravindh Venkataseshadri en_US 2010-04-02T18:12:20Z 2010-04-02T18:12:20Z 2003-04-22 en_US
dc.identifier.other etd-04182003-144951 en_US
dc.description.abstract In real-time systems, safe operation requires that tasks complete before their deadlines. Static worst-case timing analysis is used to derive an upper bound on the number of cycles for a task, and this is the basis for a safe frequency that ensures timely completion in any scenario. Unfortunately, it is difficult to tightly bound the number of cycles for a complex task executing on a complex pipeline, and so the safe frequency tends to be over-inflated. Power efficiency is sacrificed for safety. The situation only worsens as advanced microarchitectural techniques are deployed in embedded systems. High-performance microarchitectural techniques such as caching, branch prediction, and pipelining decrease typical execution times. At the same time, it is difficult to tightly bound the worst-case execution time of complex tasks on highly dynamic substrates. As a result, the gap between worst-case execution time and typical execution time is expected to increase. This thesis explores frequency speculation, a technique for reconciling the power/safety trade-off. Tight but unsafe bounds (derived from past task executions) are the basis for a low speculative frequency. The task is divided into multiple smaller sub-tasks and each sub-task is assigned an interim soft deadline, called a checkpoint. Sub-tasks are attempted at the speculative frequency. Continued safe progress of the task as a whole is confirmed for as long as speculative sub-tasks complete before their checkpoints. If a sub-task exceeds its checkpoint (misprediction), the system falls back to a higher recovery frequency that ensures the overall deadline is met in spite of the interim misprediction. The primary contribution of this thesis is the development of two new frequency speculation algorithms. A drawback of the original frequency speculation algorithm is that a sub-task misprediction is detected only after completing the sub-task. The misprediction can be detected earlier through the use of a watchdog timer that expires at the checkpoint unless the sub-task completes in time to advance it to the next checkpoint. Early detection is superior because recovery can be initiated earlier, in the middle of the mispredicted sub-task. This introduces extra slack that can be used to lower the speculative frequency even further. A new issue that arises with early detection is bounding the amount of work that remains in the mispredicted sub-task after the misprediction is detected. The two new algorithms differ in how the unfinished work is bounded. The first algorithm conservatively bounds the execution time of the unfinished portion using the worst-case execution time of the entire sub-task. The second uses more sophisticated analysis to derive a much tighter bound. Both early-detection algorithms outperform the late-detection algorithm. For tight deadlines, the sophisticated analysis of the second early-detection algorithm truly pays off. It yields 60-70% power savings for six real-time applications from the C-lab suite. en_US
dc.rights I hereby certify that, if appropriate, I have obtained and attached hereto a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dissertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to NC State University or its agents the non-exclusive license to archive and make accessible, under the conditions specified below, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report. en_US
dc.subject dynamic voltage scaling (DVS) en_US
dc.subject frequency speculation en_US
dc.subject worst-case execution time (WCET) en_US
dc.subject worst-case timing analysis en_US
dc.subject real-time systems en_US
dc.subject power and energy management en_US
dc.title Reducing Frequency in Real-Time Systems via Speculation and Fall-Back Recovery en_US MS en_US thesis en_US Computer Engineering en_US

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