Tight competitive ratios for parallel disk prefetching and caching

  • Authors:

  • Wing-Kai Hon;Rahul Shah;Peter J. Varman;Jeffrey Scott Vitter

  • Affiliations:

  • National Tsing-Hua University, Hsinchu City, Taiwan Roc;Louisiana State University, Baton Rouge, LA, USA;Rice University, Houston, TX, USA;Purdue University, West Lafayette, IN, USA

  • Venue:
  • Proceedings of the twentieth annual symposium on Parallelism in algorithms and architectures
  • Year:
  • 2008

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Abstract

We consider the natural extension of the well-known single disk caching problem to the parallel disk I/O model (PDM) [17]. The main challenge is to achieve as much parallelism as possible and avoid I/O bottlenecks. We are given a fast memory (cache) of size M memory blocks along with a request sequence Σ =(b1,b2,...,bn) where each block bi resides on one of D disks. In each parallel I/O step, at most one block from each disk can be fetched. The task is to serve Σ in the minimum number of parallel I/Os. Thus, each I/O is analogous to a page fault. The difference here is that during each page fault, up to D blocks can be brought into memory, as long as all of the new blocks entering the memory reside on different disks. The problem has a long history [18, 12, 13, 26]. Note that this problem is non-trivial even if all requests in Σ are unique. This restricted version is called read-once. Despite the progress in the offline version [13, 15] and read-once version [12], the general online problem still remained open. Here, we provide comprehensive results with a full general solution for the problem with asymptotically tight competitive ratios. To exploit parallelism, any parallel disk algorithm needs a certain amount of lookahead into future requests. To provide effective caching, an online algorithm must achieve o(D) competitive ratio. We show a lower bound that states, for lookahead L ≤ M, any online algorithm must be Ω(D)-competitive. For lookahead L greater than M(1+1/ε), where ε is a constant, the tight upper bound of O(√MD/L) on competitive ratio is achieved by our algorithm SKEW. The previous algorithm tLRU [26] was O((MD/L)2/3)-competitive and this was also shown to be tight [26] for an LRU-based strategy. We achieve the tight ratio using a fairly different strategy than LRU. We also show tight results for randomized algorithms against oblivious adversary and give an algorithm achieving better bounds in the resource augmentation model.