A Study Of Swap Cache Based Prefetching To Improve Virtual Memory Performance

With dramatic increase in processor speeds over the last decade, disk latency has become a critical issue in computer systems performance. Disks, being mechanical devices, are orders of magnitude slower than the processor or physical memory. Most Virtual Memory(VM) systems use disk as secondary storage for idle data pages of an application. The working set of pages is kept in memory. When a page requested by the processor is not present in memory, it results in a page fault. On a page fault, the Operating System brings the requested page from the disk into memory. Thus the performance of Virtual Memory systems depends on disk performance. In this project, we aim to reduce the effect of disks on Virtual Memory performance compared to the traditional demand paging system. We study novel techniques of page grouping and prefetching to improve Virtual Memory system performance. We group pages, evicted from memory at about the same time, into a single large block. On a page fault, we prefetch the entire block along with the faulting page. We implement this grouping and prefetching scheme with a swap cache. The swap cache combines a group of pages, evicted from memory, into a superblock. Superblock is the basic unit of I/O operation during paging and swapping. During a disk read, the entire superblock that has the required page is read from the disk directly into memory. We prefetch all pages with memory eviction locality in a single disk read. From this study, we find that swap cache based prefetching significantly reduces the number of read accesses to the disk. Our simulations show that the number of read accesses to the disk reduced by at least 12% for all the six SPEC 2000 benchmark applications used in this study. For some applications, the number of read accesses reduced by as much as 90%. We also find improvement in Virtual Memory I/O performance of many SPEC 2000 benchmark applications. With the swap cache, Virtual Memory performance of five of the six SPEC 2000 benchmark applications improved by at least 25%, with some improving up to 88%.