Lecture 10 Memory Management CSE 120 Principles of Operating Systems guest starring Amin Vahdat Project 2 Due 11 14 Memory Management Next few lectures are going to cover memory management Goals of memory management Mechanisms To provide a convenient abstraction for programming To allocate scarce memory resources among competing processes to maximize performance with minimal overhead Physical and virtual addressing 1 Techniques Partitioning paging segmentation 1 Page table management TLBs VM tricks 2 Policies Page replacement algorithms 3 CSE 120 Lecture 10 Memory Management 2 Lecture Overview Virtual memory warm and fuzzy Survey techniques for implementing virtual memory Fixed and variable partitioning Paging Segmentation Focus on hardware support and lookup procedure Next lecture we ll go into sharing protection efficient implementations and other VM tricks and features CSE 120 Lecture 10 Memory Management 3 Virtual Memory OS provides Virtual Memory VM as the abstraction for managing memory Indirection allows moving programs around in memory Allows processes to address more or less memory than physically installed in the machine Virtual memory enables a program to execute with less than its complete data in physical memory Many programs do not need all of their code and data at once or ever no need to allocate memory for it OS adjusts amount of memory allocated based upon behavior Requires hardware support for efficient implementation Let s go back to the beginning CSE 120 Lecture 10 Memory Management 4 In the beginning Rewind to the days of batch programming Programs use physical addresses directly OS loads job runs it unloads it Multiprogramming changes all of this Want multiple processes in memory at once Overlap I O and CPU of multiple jobs Can do it a number of ways Fixed and variable partitioning paging segmentation Requirements Need protection restrict which addresses jobs can use Fast translation lookups need to be fast Fast change updating memory hardware on context switch CSE 120 Lecture 10 Memory Management 5 Virtual Addresses To make it easier to manage the memory of processes running in the system we re going to make them use virtual addresses logical addresses Virtual addresses are independent of the actual physical location of the data referenced OS determines location of data in physical memory Instructions executed by the CPU issue virtual addresses Virtual addresses are translated by hardware into physical addresses with help from OS The set of virtual addresses that can be used by a process comprises its virtual address space Many ways to do this translation Start with old simple ways progress to current techniques CSE 120 Lecture 10 Memory Management 6 Fixed Partitions Physical memory is broken up into fixed partitions Advantages Hardware requirements base register Physical address virtual address base register Base register loaded by OS when it switches to a process Size of each partition is the same and fixed How do we provide protection Easy to implement fast context switch Problems Internal fragmentation memory in a partition not used by a process is not available to other processes Partition size one size does not fit all very large processes CSE 120 Lecture 10 Memory Management 7 Fixed Partitions Physical Memory Base Register P1 P4 s Base P2 P3 Virtual Address Offset P4 P5 CSE 120 Lecture 10 Memory Management 8 Variable Partitions Natural extension physical memory is broken up into variable sized partitions Advantages Hardware requirements base register and limit register Physical address virtual address base register Why do we need the limit register Protection If physical address base limit then exception fault No internal fragmentation allocate just enough for process Problems External fragmentation job loading and unloading produces empty holes scattered throughout memory CSE 120 Lecture 10 Memory Management 9 Variable Partitions Base Register P3 s Base P1 Limit Register P2 P3 s Limit Virtual Address Offset Yes P3 No Protection Fault CSE 120 Lecture 10 Memory Management 10 Paging Paging solves the external fragmentation problem by using fixed sized units in both physical and virtual memory Physical Memory Virtual Memory Page 1 Page 2 Page 3 Page N CSE 120 Lecture 10 Memory Management 11 User Process Perspective Users and processes view memory as one contiguous address space from 0 through N Virtual address space VAS In reality pages are scattered throughout physical storage The mapping is invisible to the program Protection is provided because a program cannot reference memory outside of its VAS The address 0x1000 maps to different physical addresses in different processes CSE 120 Lecture 10 Memory Management 12 Paging Translating addresses Virtual address has two parts virtual page number and offset Virtual page number VPN is an index into a page table Page table determines page frame number PFN Physical address is PFN offset Page tables Map virtual page number VPN to page frame number PFN VPN is the index into the table that determines PFN One page table entry PTE per page in virtual address space Or one PTE per VPN CSE 120 Lecture 10 Memory Management 13 Page Lookups Physical Memory Virtual Address Page number Offset Page Table Physical Address Page frame Offset Page frame CSE 120 Lecture 10 Memory Management 14 Paging Example Pages are 4K Virtual address is 0x7468 Virtual page is 0x7 offset is 0x468 Page table entry 0x7 contains 0x2 VPN is 20 bits 220 VPNs offset is 12 bits Page frame base is 0x2 0x1000 4K 0x2000 Seventh virtual page is at address 0x2000 3rd physical page Physical address 0x2000 0x468 0x2468 CSE 120 Lecture 10 Memory Management 15 Page Table Entries PTEs 1 1 1 2 M R V Prot 20 Page Frame Number Page table entries control mapping The Modify bit says whether or not the page has been written It is set when a write to the page occurs The Reference bit says whether the page has been accessed It is set when a read or write to the page occurs The Valid bit says whether or not the PTE can be used It is checked each time the virtual address is used The Protection bits say what operations are allowed on page Read write execute The page frame number PFN determines physical page CSE 120 Lecture 10 Memory Management 16 Paging Advantages Easy to allocate memory Memory comes from a free list of fixed size chunks Allocating a page is just removing it from the list External fragmentation not a problem Easy to swap out chunks of a program All
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