Optimizing Performance of Persistent Memory File Systems using Virtual Superpages

Existing persistent memory file systems can significantly improve the performance by utilizing the advantages of emerging Persistent Memories (PMs). Especially, they can employ superpages (e.g., 2MB a page) of PMs to alleviate the overhead of locating file data and reduce TLB misses. Unfortunately, superpage also induces two critical problems. First, the data consistency of file systems using superpages causes severe write amplification during overwrite of file data. Second, existing management of superpages may lead to large waste of PM space. In this paper, we propose a Virtual Superpage Mechanism (VSM) to solve the problems by taking advantages of virtual address space. On one hand, VSM adopts multi-grained copy-on-write mechanism to reduce the write amplification while ensuring data consistency. On the other hand, VSM presents zero-copy file data migration mechanism to eliminate the loss of space utilization efficiency caused by superpages. We implement the proposed VSM mechanism in Linux kernel based on PMFS. Compared with the original PMFS and NOVA, the experimental results show that VSM improves 36% and 14% on average for write and read performance, respectively. Meanwhile, VSM can achieve the same space utilization efficiency of file system that uses the normal 4KB pages to organize files.