CSE221 - lec06: Virtual Memory: VAX/VMS & Mach
date
Oct 24, 2024
slug
cse221-lec06
status
Published
tags
System
summary
type
Post
06 - Virtual Memory: VAX/VMS & Mach
Virtual Memory Management in the VAX/VMS Operating System
Goals:
- support different hardware
- handle resource constraints (memory & I/O)
- Virtual Address Space: 4GB
- Physical Address Space: 250KB - 4MB
below is a pic. illustrating VAX/VMS address space:
Paging Mechanisms
memory are devided into fixed-size page frames, address translation is via page tables
Space Efficiency
- process page table is paged (can be swap out)
- keep OS page table in physical memory
TLB
- stores va to pa mapping
- split between user and kernel TLB
Process-local replacement policy
- evict pages from the requested process only
- FIFO policy
Page Caching
- cache pages about to be evicted
- when page fault occures on pages that are in free list or modified list, they can be returned to resident set directly
Clustering
- read / write in batch & prefetch pages
Summary & take-away
- Address Space abstraction
- Hardware support for paging
- Techniques for efficiency
Machine-Independent Virtual Memory Management for Paged Uniprocessor and Multiprocessor Architectures
Goals:
- Machine independent approach to manage virtual memory
- hardware difference:
- single v.s. multi-processor
- non-uniform memory
- paging v.s. segmentation
- support sparse address space
- memory-mapped files
- multi-thread
Components of Mach VM Implementation: Address Map & Resident Page Table
Address Map: A double-linked list that keeps attribute of memory objects.
Resident Page Table: keep address translation information
Components of Mach VM Implementation: Memory Object
- backing store for a range of virtual address (resembles unix files)
- customizable pager
Components of Mach VM Implementation: pmap
- hardware-dependent module
- exact form differ wrt. hardware
Summary & take-away
- Mach VM’s approach to VM: build a machine independent virtual memory representation, and use a machine depedent module to do the translation.
- The idea is simple but the key is to keep the efficiency.