A Hybrid DRAM/PCM Buffer Cache Architecture for Smartphones with QoS Consideration

Flash memory is widely used in mobile phones to store contact information, application files, and other types of data. In an operating system, the buffer cache keeps the I/O blocks in dynamic random access memory (DRAM) to reduce the slow flash accesses. However, in smartphones, we observed two issues which reduce the benefits of the buffer cache. First, a large number of synchronous writes force writing the data from the buffer cache to flash frequently. Second, the large amount of I/O accesses from background applications diminishes the buffer cache efficiency of the foreground application, which degrades the quality-of-service (QoS). In this article, we propose a buffer cache architecture with hybrid DRAM and phase change memory (PCM) memory, which improves the I/O performance and QoS for smartphones. We use a DRAM first-level buffer cache to provide high buffer cache performance and a PCM last-level buffer cache to reduce the impact of frequent synchronous writes. Based on the proposed hierarchical buffer cache architecture, we propose a sub-block management and background flush to reduce the impact of the PCM write limitation and the dirty block write-back overhead, respectively. To improve the QoS, we propose a least-recently-activated first replacement policy (LRA) to keep the data from the applications that are most likely to become the foreground one. The experimental results show that with the proposed mechanisms, our hierarchical buffer cache can improve the I/O response time by 20% compared to the conventional buffer cache. The proposed LRA can improve the foreground application performance by 1.74x compared to the conventional CLOCK policy.