SmartCon: Smart Context Switching for Fast Storage Devices

Abstract

Handling of storage IO in modern operating systems assumes that such devices are slow and CPU cycles are valuable. Consequently, to effectively exploit the underlying hardware resources, for example, CPU cycles, storage bandwidth and the like, whenever an IO request is issued to such device, the requesting thread is switched out in favor of another thread that may be ready to execute. Recent advances in nonvolatile storage technologies and multicore CPUs make both of these assumptions increasingly questionable, and an unconditional context switch is no longer desirable. In this article, we propose a novel mechanism called SmartCon, which intelligently decides whether to service a given IO request in interrupt-driven manner or busy-wait-based manner based on not only the device characteristics but also dynamic parameters such as IO latency, CPU utilization, and IO size. We develop an analytic performance model to project the performance of SmartCon for forthcoming devices. We implement SmartCon mechanism on Linux 2.6 and perform detailed evaluation using three different IO devices: Ramdisk, low-end SSD, and high-end SSD. We find that SmartCon yields up to a 39% performance gain over the mainstream block device approach for Ramdisk, and up to a 45% gain for PCIe-based SSD and SATA-based SSDs. We examine the detailed behavior of TLB, L1, L2 cache and show that SmartCon achieves significant improvement in all cache misbehaviors.