Flexible reference trace reduction for VM simulations

The unmanageably large size of reference traces has spurred the development of sophisticated trace reduction techniques. In this article we present two new algorithms for trace reduction: Safely Allowed Drop (SAD) and Optimal LRU Reduction (OLR). Both achieve high reduction factors and guarantee exact simulations for common replacement policies and for memories larger than a user-defined threshold. In particular, simulation on OLR-reduced traces is accurate for the LRU replacement algorithm, while simulation on SAD-reduced traces is accurate for the LRU and OPT algorithms. Both policies can easily be modified and extended to maintain timing information, thus allowing for exact simulation of the Working Set and VMIN policies. OLR also satisfies an optimality property: for a given original trace and chosen memory size, it produces the shortest possible reduced trace that has the same LRU behavior as the original for a memory of at least the chosen size. We present a proof of this optimality of OLR, and show that SAD, while not optimal, yields nearly optimal performance in practice.Our approach has multiple applications, especially in simulating virtual memory systems; many page replacement algorithms are similar to LRU in that more recently referenced pages are likely to be resident. For several replacement algorithms in the literature, SAD- and OLR-reduced traces yield exact simulations. For many other algorithms, our trace reduction eliminates information that matters little: we present extensive measurements to show that the error for simulations of the clock and segq (segmented queue) replacement policies (the most common LRU approximations) is under 3% for the vast majority of memory sizes. In nearly all cases, the error is much smaller than that incurred by the well-known stack deletion technique.SAD and OLR have many desirable properties. In practice, they achieve reduction factors up to several orders of magnitude. The reduction translates to both storage savings and simulation speedups. Both techniques require little memory and perform a single forward traversal of the original trace, making them suitable for online trace reduction. Neither requires that the simulator be modified to accept the reduced trace.