Run Placement Policies for Concurrent Mergesorts Using Parallel Prefetching

We study the performance of various run placement policies on disks for the merge phase of concurrent mergesorts using parallel prefetching. The initial sorted runs (input) of a merge and its final sorted run (output) are stored on multiple disks but each run resides only on a single disk. In this paper, we examine through detailed simulations three different run placement policies and the impact of buffer thrashing. The results show that, with buffer thrashing avoidance, the best performance can be achieved by a run placement policy that uses a proper subset of the disks dedicated for writing the output runs while the rest of the disks are used for prefetching the input runs in parallel. However, the proper number of write disks is workload dependent, and if not carefully chosen, it can adversely affect the system performance. In practice, a reasonably good performance can be achieved by a run placement policy that does not place the output run of a merge on any of the disks that store its own input runs but allows the output run to share the same disk with some of the input runs of other merges.