Adaptive and Speculative Slack Simulations of CMPs on CMPs

Current trends signal an imminent crisis in the simulation of future CMPs (Chip Multiprocessors). Future micro-architectures will offer more and more thread contexts to execute parallel programs, but the execution speed of each thread will not improve at the same pace. CMPs with 10´s or even 100´s of cores are envisioned. Simulating these future CMP sefficiently without compromising accuracy is a challenge. Slack simulation is a general parallel simulation paradigm which provides flexible trade-offs between simulation accuracy and speed. Simulation threads do not synchronize after every target core cycle as in cycle-by-cycle simulation. Rather a maximum slack (the slack bound) is enforced between the clocks of all simulated cores. A slack simulation may become inaccurate because of simulation violations. Such violations occur when a resource is accessed by two cores in different order in the simulation and in the target system. We introduce and demonstrate techniques to detect violations, to adapt the simulation slack to maintain a target violation rate, and to checkpoint and rollback a slack simulation when violations are detected. We show some simulation performance/accuracy data for a set of five Splash benchmarks in the context of an 8-core CMP with a snooping cache coherence protocol simulated on Slack Sim, our universal slack simulation platform.