Generating Representative Test Sequences from Real Workload for Minimizing DRAM Verification Overhead

Abstract

Dynamic Random Access Memory (DRAM) standards have evolved for higher bandwidth, larger capacity, and lower power consumption, so their specifications have become complicated to satisfy the design goals. These complex implementations have significantly increased the test time overhead for design verification; thus, a tremendous amount of command sequences are used. However, since the sequences generated by real machines or memory simulators are the results of scheduling for high performance, they result in low test coverage with repetitive patterns. Eventually, various workloads should be applied to increase the coverage, but this approach incurs significant test time overhead. A few preliminary studies have been proposed to generate predefined or random sequences to cover various test cases or increase test coverage. However, they have limitations in representing various memory behaviors of real workloads. In this article, we define a performance metric for estimating the test coverage when using command sequences. Then, our experiment shows that the coverage of a real machine and a simulator is low and similar. Also, the coverage patterns are almost the same in all tested benchmarks. To alleviate the problem, we propose a test-oriented command scheduling algorithm that increases the test coverage while preserving the memory behaviors of workloads and reducing the test time overhead by extracting representative sequences based on the similarity between command sequences. For the sequence extraction and the coverage estimation, our test sequences are embedded into vectors using bag-of-Ngrams. Compared to the simulator, our algorithm achieves 2.94x higher coverage while reducing the test overhead to 7.57%.