Despite the rapid hardware upgrades, a common complaint among smartphone owners is the poor battery life. To many users, being required to charge the smartphone after a single day of moderate usage is unacceptable. Moreover, current smartphones suffer various unpredictable delays during operation, e.g., when launching an app, leading to poor user experience. In this dissertation, we provide solutions that enhance systems on portable devices using information obtained from their users and upper

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... ayers on the I/O path. ACKNOWLEDGMENTS This dissertation would not have been possible without the help of many kind, talented, and hardworking people. I would first like to thank my advisor, Professor Gang Zhou, for all of his insight and guidance over the years.