MemoryMapped Files amp; Unified VM Systm Computer 内存映射文件amp；统一的虚拟机系统的计算机.pptVIP

Memory-Mapped Files Unified VM System Vivek Pai Mechanics Forgot to finish producer-consumer Memory-mapped files unified VM Allocating Memory Old days: manual tuning of sizes Benefits? Drawbacks? Manual Memory Tuning Fixed-size allocations for VM, FS cache Done right, protects programs from each other Backing up filesystem trashes FS cache Large-memory programs don’t compete with disk-bound programs Done poorly? Memory underutilized What Is Main Memory? At some level, a cache for the disk Permanent data written back to fs Temporary data in main memory or swap Main memory is much faster than disk Consider one program that accesses lots of files and uses lots of memory How do you optimize this program? Could you view all accesses as page faults? Consider Ages With Pages What happens if 5 FS pages are really active? What happens if relative demands change over time? Unified VM Systems Now what happens when a page is needed? What happens on disk backup? Did we have the same problem before? Why Mmap? File pages are a lot like VM pages We don’t load all of a process at once – why load all of a file at once? Why copy a file to access it? There’s one good reason Mmap Definition void * mmap( void *addr, size_t len, int prot, int flags, int fildes, off_t off); Addr: where we want to map it Len: how much we want mapped Prot: allow reading, writing, exec Flags: is mapped shared/private/anonymous, fixed/variable location, swap space reserved? Fildes: what file is being mapped Off: start offset in file Mmap Diagram Mmap Implications # of VM regions increases Was never really just code/text/heap/stack Access/protection info on all regions Filesystem no longer sole way to access file Previously, access info via read( ) and write( ) Same file via filesystem and mmap? Mmap Versus Read When read( ) completes All pages in range were loaded at some point A copy of the data in user’s buffers If underlying file changes, no change to data in user’s buffer When mmap( ) completes Ma