论文标题

带有时间窗口和延迟的缓存

Caching with Time Windows and Delays

论文作者

Gupta, Anupam, Kumar, Amit, Panigrahi, Debmalya

论文摘要

我们考虑了经典加权分页问题的两个概括,其中包含了页面请求延迟服务的概念。第一个是(加权)分页带有时间窗口(pagetw)问题,就像经典的加权分页问题一样,只是每个页面请求在给定的截止日期之前只需要提供。这个问题出现在在线缓存的许多实际应用中,例如Linux内核中的“截止日期” I/O调度程序和按需流媒体。第二个且更一般的问题是(加权)分页带有延迟(分页)问题,其中延迟服务页面请求会导致对目标进行罚款。这个问题概括了缓存问题以允许延迟服务,这是最近在在线算法中吸引的工作线(例如Emek等人Stoc '16,Azar等人的STOC '17,Azar和Touitou Focs '19)。 我们给出$ o(\ log k \ log n)$ - 在$ n $页面上的Pagetw和分类问题的竞争算法,其尺寸为$ k $。对于这两个问题,这都大大提高了$ O(k)$的最佳界限(Azar等人17)。我们还考虑了离线pagetw和分页问题,​​为此,我们给出$ O(1)$近似算法并证明APX硬度。这些是离线问题的第一个结果;即使是NP硬度,我们的工作也不是。我们算法的核心是Pagetw问题的一种新颖的“打击” LP松弛,它克服了自然LP的$ω(k)$完整性差距,以解决该问题。据我们所知,这是用于延迟/截止日期的在线算法的基于LP的算法的第一个示例。

We consider two generalizations of the classical weighted paging problem that incorporate the notion of delayed service of page requests. The first is the (weighted) Paging with Time Windows (PageTW) problem, which is like the classical weighted paging problem except that each page request only needs to be served before a given deadline. This problem arises in many practical applications of online caching, such as the "deadline" I/O scheduler in the Linux kernel and video-on-demand streaming. The second, and more general, problem is the (weighted) Paging with Delay (PageD) problem, where the delay in serving a page request results in a penalty being assessed to the objective. This problem generalizes the caching problem to allow delayed service, a line of work that has recently gained traction in online algorithms (e.g., Emek et al. STOC '16, Azar et al. STOC '17, Azar and Touitou FOCS '19). We give $O(\log k\log n)$-competitive algorithms for both the PageTW and PageD problems on $n$ pages with a cache of size $k$. This significantly improves on the previous best bounds of $O(k)$ for both problems (Azar et al. STOC '17). We also consider the offline PageTW and PageD problems, for which we give $O(1)$ approximation algorithms and prove APX-hardness. These are the first results for the offline problems; even NP-hardness was not known before our work. At the heart of our algorithms is a novel "hitting-set" LP relaxation of the PageTW problem that overcomes the $Ω(k)$ integrality gap of the natural LP for the problem. To the best of our knowledge, this is the first example of an LP-based algorithm for an online algorithm with delays/deadlines.

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