In online service systems, delay experienced by a user from the service request to the service completion is one of the most critical performance metrics. To improve user delay experience, in this thesis, we investigate a novel aspect of system design: proactive serving, where the system can predict future user request arrivals and allocate its capacity to serve these upcoming requests proactively. This approach is complementary to the conventional capacity boosting mechanism and is motivated by recent industrial and academic advances. In particular, we investigate the fundamentals of proactive serving from a queuing theory perspective. / First, most importantly, we show that under proactive serving the average user delay decreases exponentially (in the prediction window size) for a wide range of queuing models. Furthermore, the delay reduction is robust against prediction errors. We also show that both the variance of user delay and the tail of user delay decrease exponentially under proactive serving, which are also important user delay experience metrics. / We then show that proactive serving is more effective in decreasing user delay than capacity boosting in light workload regime. In particular, the average user delay decays inverse-proportionally in system capacity, but exponentially in the prediction window size in proactive serving. / Finally we demonstrate how to leverage proactive serving in system design from a optimization point of view, e.g., how many resources are dedicated to proactive serving. The results provide useful engineering insights to system designers. / Our trace-driven simulation results demonstrate the practical power of proactive serving: for example, under the YouTube data trace of 1000 different videos, the average user delay can be decreased by 50% when the system predicts 100 seconds ahead. Our results provide useful insights for proactive serving and justify its increasing applications in practical systems. / 對於在線服務系統，由於系統服務造成的用戶延遲是衡量系統性能的重要指標。提高用戶的延遲體驗的傳統方法是提高系統中服務器的性能。在本論文中，我們研究一種新穎的叫做“前瞻性服務”的方法用來提高用戶的延遲體驗。前瞻性服務是指系統在預測用戶需求的基礎上，在用戶產生需求之前，系統已經將服務送到用戶手中。前瞻性服務是傳統方法的有力補充。我們從排隊論的角度研究前瞻性服務對用戶的延遲的提高。 / 首先，對於多种排隊系統，我們證明前瞻性服務能夠指數性降低用戶的平均延遲。而且前瞻性服務對於用戶需求預測的誤差具有魯棒性。我們同時也證明了前瞻性服務能夠指數性降低用戶的延遲方差和尾概率。 / 然後，我們證明前瞻性服務在系统低负载时比傳統方法在降低用戶的平均延遲上更加有效。前瞻性服務能夠指數性降低用戶的平均延遲。而通過提高系統服務器性能，只能反比例降低用戶的平均延遲。 / 最後，我們從優化的角度分析怎樣在系統設計中利用前瞻性服務，給系統設計者提供有用的建議。 / 我們基於實際數據的仿真結果驗證了前瞻性服務在實際系統的作用。例如，基於Youtube數據的仿真表明，如果系統能提前一百秒預測用戶的需求，那麼前瞻性服務能夠降低一半的用戶延遲。 / Zhang, Shaoquan. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2014. / Includes bibliographical references (leaves 103-108). / Abstracts also in Chinese. / Title from PDF title page (viewed on 02, December, 2016). / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only.