Trap-driven memory simulation with Tapeworm II

Abstract

Trap-driven simulation is a new approach for analyzing the performance of memory-system components such as caches and translation-lookaside buffers (TLBs). Unlike the more traditional trace-driven approach to simulating memory systems, trap-driven simulation uses the hardware of a host machine to drive simulations with operating-system kernel traps instead of with address traces. As a workload runs, a trap-driven simulator dynamically modifies access to memory in such a way as to make memory traps correspond exactly to misses in a simulated cache structure. Because traps are handled inside the kernel of the host operating system, a trap-driven simulator can monitor all components of multitask workloads including the operating system itself. Compared to trace-driven simulators, a trap-driven simulator causes relatively little slowdown to the host system because traps occur only in the infrequent case of simulated cache misses. Unfortunately, because they require special forms of hardware support to cause memory-access traps, trap-driven simulators are difficult to port, and they are not as flexible as trace-driven simulators in the types of memory configurations that they can model. Several researchers have recently begun to use trap-driven techniques in their studies of memory-system design tradeoffs, but little is known about how the speed and accuracy of the technique varies with the type of simulations conducted, or about the nature of its drawbacks with respect to portability and flexibility. In this article, we use a prototype trap-driven simulator, named Tapeworm II, to explore these issues. We expose both the strengths and the weaknesses of trap-driven simulation with respect to speed, accuracy, completeness, portability, flexibility, ease-of-use, and memory overhead. Although the results are drawn from a specific implementation of trap-driven simulation, we believe that many of our results from Tapeworm hold true for trap-driven simulation in general.