在基於非揮發性隨機存取記憶體系統上提供改善作業系統效能的機制

Abstract

硬碟效能長久以來一直是電腦系統中一個主要瓶頸，根據Moore’s Law，處理器的效能以每年60%的速度增進；而硬碟的效能僅以每年10%的速度增進，因此在處理器和硬碟之間產生了極大的效能差距。非揮發性記憶體(Non-volatile RAM)是最近幾年新興的一項記憶體技術，其具有非揮發性的特性可以有效改善系統效能。由於傳統緩衝快取以及確保檔案系統資料一致性的設計都是建立在揮發性記憶體上，因此我們在非揮發性記憶體性統上提出三種機制，來進一步增加緩衝快取的效能以及減少確保資料一致性的負擔。第一，我們在VFS和一般檔案系統之間加入一層Ram-based檔案系統來暫時的存放新建立檔案，其目的為延遲檔案分配給檔案系統的時間以避免檔案零碎的情形以及減少不必要的IO。第二，我們修改原本以檔案為單位的寫回策略，而進一步的考慮寫回的區塊在磁碟上的相對應位置，讓較連續且較鄰近的資料一起寫回，以減少寫回的時間；另外，我們還確保不要寫回最近剛被更新過的頁面。最後，我們提供了一個簡單機制可同時確保檔案系統的一致性而不需要額外的磁碟IO負擔。實驗結果顯示我們的機制之效能優於Ext2大約76%；優於Ext3大約94%。

Disk performance has become the major bottleneck of most computer systems for a long time. Following the Moore’s Law, the performance of processors improves by about 60% per year. However, the performance improvement of disks is only about 10%, resulting in an increasing performance gap between processors and disks. Non-volatile memory is an emerging technology in recent years and the characteristic of non-volatile can improve the performance of system. Because the traditional buffer cache management and guaranteeing file system consistent are based on volatile memory, we propose three mechanisms on non-volatile system to improve further the performance of buffer cache and reduce the overhead of consistency. First, we put Ram-based file system between VFS and file system to temporarily store all new files. Its main purpose is delay allocation for all new files to avoid the file fragmentation and reduce the redundant IO traffic. Second, we modify the original file-by-file write back policy and write back the contiguous or neighbor dirty blocks to reduce the seek time and rotation delay of the disk. Besides, we do not write back the recently-updated dirty pages. Last, we propose a simple mechanism that can guarantee the file system consistent without any overhead of disk IO. The experimental results show that the performance of our mechanisms is superior to Ext2 about 76%, and Ext3 about 94%.