CSC501 Operating Systems Principles. Memory Management

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1 CSC501 Operating Systems Principles Memory Management 1

2 Previous Lectures q Linker & Loader q Today Q Memory Management 2

3 Layer user filesystem inter-machine net. memory mgr (high) device mgr + drivers real-time clock mgr IPC process coord process mgr memory mgr (low) user/kernel boundary Why? hardware

4 Memory Management q Low-level memory management Q Manages memory within kernel address space Q Allocates address spaces for processes Q Treats memory as a single, exhaustible resource Q In layered-os design, it is positioned in the hierarchy below process manager q High-level memory management Q Manages pages within address space Q Divides memory into abstract resources Q Positioned in the hierarchy above device manager 4

5 Low-level Memory Management q Conceptual Uses Q Allocation of process stack v getstk, freestk v Used by OS Q Allocation of heap storage v getmem, freemem v Used by applications and OS 5

6 Possible Allocation Strategies q Single free list Q First-fit: scan free list and allocate first hole that is large enough Q Next-fit: start search from end of last allocation Q Best-fit: find smallest hole that is adequate Q Worst fit: find largest hole q Multiple lists Q By exact size (static / dynamic) q Hierarchy Q Binary size allocation q FIFO cache with above methods 6

7 Example Implementation in Xinu q Single free list Q Ordered by increasing address Q Singly-linked Q Initialized at system startup to all free memory q Allocation policy Q Heap: first-fit Q Stack: last-fit Q Minimizes fragmentation 7

8 Result of Xinu Allocation Policy q Stack allocation from highest free memory q Heap allocation from lowest free memory 8

9 Illustration of Xinu Free List q List in order of address q Related Files: h/mem.h sys/getmem.c sys/freemem.c 9

10 High-level Memory Management q Memory is cheap today, and getting cheaper Q But applications are demanding more and more memory, there is never enough! q Let s take a look at the history

11 In the Beginning (prehistory) q Batch systems Q One program loaded in physical memory Q Runs to completion q If job larger than physical memory, use overlays Q Identify sections of program that v Can run to a result v Can fit into the available memory Q Add statement after result to load a new section Q Like passes in a compiler

12 In the Beginning (multi-programming) q Multiple processes in physical memory at the same time Q Allows fast switching to a ready process Q Divide physical memory into multiple pieces partitioning q Partition requirements Q Protection keep processes from smashing each other Q Fast execution memory accesses can t be slowed by protection mechanisms Q Fast context switch can t take forever to setup mapping of addresses

13 Physical Memory Empty 0x0000FFFF Process 3 Process 2 Physical address space Process 1 0x OS Kernel

14 Loading a Process q Relocate all addresses relative to start of partition Q See Linking and Loading q Memory protection assigned by OS Q Block-by-block to physical memory q Once process starts Q Partition cannot be moved in memory Q Why?

15 Physical Memory 0x0001FFFF Empty Process 3 Physical address space Empty Process 1 0x OS Kernel

16 Problem q What happens when Process 4 comes along and requires space larger than the largest empty partition? v Wait v Complex resource allocation problem for OS v Potential starvation

17 Physical Memory Empty Process 3 Process 4 Empty Process 1 OS Kernel

18 Solution q Logical Address: an address used by the program that is translated by computer into a physical address each time it is used q When the program utters 0x00105C, Q The machine accesses 0x01605C instead

19 Simplest Implementation q Base and Limit registers Q Base added to all addresses Q Limit checked on all memory references q Loaded by OS at each context switch Limit Reg Reloc Reg CPU logical address < no yes + physical address Physical Memory error

20 Physical Memory 0x0003FFFF Physical address space Empty Process 3 Empty Process 1 Limit Base 0x OS Kernel

21 Advantages q No relocation of program addresses at load time Q All addresses relative to zero q Built-in protection provided by Limit Q No physical protection per page or block q Fast execution Q Addition and limit check at hardware speeds within each instruction q Fast context switch Q Need only change base and limit registers q Partition can be suspended and moved at any time Q Process is unaware of change Q Expensive for large processes!

22 Physical Memory 0x0003FFFF Process 4 Physical address space Process 3 Process 1 Limit Base 0x OS Kernel

23 Challenge Memory Allocation q Fixed partitions q Variable partitions

24 Partitioning Strategies Fixed q Fixed Partitions divide memory into equal sized pieces (except for OS) Q Degree of multiprogramming = number of partitions Q Simple policy to implement v All processes must fit into partition space v Find any free partition and load the process q Problem Internal Fragmentation Q Unused memory in a partition that is not available to other processes Question: What is the right partition size?

25 Partitioning Strategies Variable q Memory is dynamically divided into partitions based on process needs Q More complex management problem v Need data structures to do tracking of free and used memory v New process is allocated memory from hole large enough to fit it q Problem External Fragmentation Q Unused memory between partitions that is not too small to be used by any processes

26 Partitioning Strategies Variable OS OS OS process 1 process 1 process 1 process 2 Process 2 Terminates Process 4 Starts process 4 process 3 process 3 process 3 26

27 User s View of a Program Question: Can we have multiple sets of base and limit registers?

28 Logical View of Segmentation user space physical memory space

29 Segmentation q Logical address consists of a pair: <segment-number, offset> q Segment table where each entry has: Q Base: contains the starting physical address where the segments reside in memory. Q Limit: specifies the length of the segment.

30 Segment Lookup Index to segment register table segment # virtual address offset Segment register table limit base R RWX X physical memory segment 0 segment 1 segment 2 <? no yes + Physical Address segment 3 raise protection fault segment 4

31 Segmentation q Common in early minicomputers Q Small amount of additional hardware 4 or 8 segments Q Used effectively in Unix q Good idea that has persisted and supported in current hardware and OSs Q X86 supports segments Q Linux supports segments Question: Do we still have external fragmentation problem? If yes, can we further improve it?

32 Next Lecture q Paging 32

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