Hierarchical Verification for Increasing Performance in Reliable Processors

Dynamic verification using the checker processor introduces severe degradation in performance unless the checker is as fast as the main processor core. Without widening the checker’s bandwidth, we propose an active verification management (AVM) approach that utilizes a checker hierarchy. Before an instruction is verified at the checker processor, a filter checker marks a correctness non-criticality indicator (CNI) bit to indicate how likely its result is to be unimportant for reliability. AVM uses the CNI information to realize a congestion avoidance policy. Both reactive and proactive congestion avoidance policies are proposed to mitigate the performance degradation caused by the checker’s congestion. Based on a simplified queueing model, we evaluate the proposed AVM analytically. Our experimental results show that AVM has the potential to solve the verification congestion problem when perfect fault coverage is not needed. With no AVM, congestion at the checker badly affects performance, to the tune of 57%, when compared to that of a non-fault-tolerant processor. With good marking by AVM, the performance of a reliable processor approaches 95% of that of a processor with no verification. Although instructions can be skipped on a random basis, such an approach reduces the fault coverage. A filter checker with a marking policy correlated with the correctness non-criticality metric, on the other hand, significantly reduces the soft error rate. Finally, we also present results showing the trade-off between performance and reliability.