Adaptive caching for demand prepaging

Abstract

Demand prepaging was long ago proposed as a method for taking advantage of high disk bandwidths and avoiding long disk latencies by fetching, at each page fault, not only the demanded page but also other pages predicted to be used soon. Studies performed more than twenty years ago found that demand prepaging would not be generally beneficial. Those studies failed to examine thoroughly the interaction between prepaging and main memory caching. It is unclear how many main memory page frames should be allocated to cache pages that were prepaged but have not yet been referenced. This issue is critical to the efficacy of any demand prepaging policy.In this paper, we examine prepaged allocation and its interaction with two other important demand prepaging parameters: the degree, which is the number of extra pages that may be fetched at each page fault, and the predictor that selects which pages to prepage. The choices for these two parameters, the reference behavior of the workload, and the main memory size all substantially affect the appropriate choice of prepaged allocation. In some situations, demand prepaging cannot provide benefit, as any allocation to prepaged pages will increase page faults, while in other situations, a good choice of allocation will yield a substantial reduction in page faults. We will present a mechanism that dynamically adapts the prepaged allocation on-line, as well as experimental results that show that this mechanism typically reduces page faults by 10 to 40% and sometimes by more than 50%. In those cases where demand prepaging should not be used, the mechanism correctly allocates no space for prepaged pages and thus does not increase the number of page faults. Finally, we will show that prepaging offers substantial benefits over the simpler solution of sing larger pages, which can substantially increase page faults.