POFGSP: Priority-Based Out-of-Order Scheduling and Fine-Grain Status Polling for SSD Performance Improvement

With the development of flash technology, the increasing throughput gap between nand flash memory (NFM) arrays and the I/O interface has become a performance bottleneck for NFM-based solid-state drives (SSDs). Multilevel parallelism techniques have been employed on modern SSDs to meet the challenge of increasing demands for bandwidth in I/O-intensive workloads. However, conventional parallel methods only monitor the status of ways, resulting in the “idle bubble”—idle time of the dies cannot execute subsequent operations until all the dies in the way complete command execution. This issue limits the resource utilization and performance of SSDs. To minimize the idle bubble, we propose priority-based out-of-order scheduling and fine-grain status polling (POFGSP). The priority-based out-of-order scheduling relaxes constraints on command execution order and schedules commands with the same execution time to be executed in parallel. Therefore, the scheduler reduces these idle bubbles caused by differences in command execution times. Moreover, the fine-grain status polling approach polls the die-level status during the interface’s idle time, reducing idle bubbles with accurate status. Compared to state-of-the-art schedulers, our POFGSP approach can reduce request response time by 35.6% under real-world cloud block storage workloads and improve the SSD system’s maximum bandwidth by 8.7%–74.9%.