Unblinding the OS to Optimize User-Perceived Flash SSD Latency

In this paper, we present a flash solid-state drive (SSD) optimization that provides hints of SSD internal behaviors, such as device I/O time and buffer activities, to the OS in order to mitigate the impact of I/O completion scheduling delays. The hints enable the OS to make reliable latency predictions of each I/O request so that the OS can make accurate scheduling decisions when to yield or block (busy wait) the CPU, ultimately improving user-perceived I/O performance. This was achieved by implementing latency predictors supported with an SSD I/O behavior tracker within the SSD that tracks I/O behavior at the level of internal resources, such as DRAM buffers or NAND chips. Evaluations with an SSD prototype based on a Xilinx Zynq-7000 FPGA and MLC flash chips showed that our optimizations enabled the OS to mask the scheduling delays without severely impacting system parallelism compared to prior I/O completion methods.