CSE325 Principles of Operating Systems. Memory. David P. Duggan. March 6, 2010

Transcription

1 CSE325 Principles of Operating Systems Memory David P. Duggan March 6, 2010

2 Where is Memory? Characteristics? Issues/challenges? 3/6/12 CSE325 - Main Memory 2

3 Outline Memory management requirements Memory hierarchy Memory partitioning Best-fit, worst-fit, first-fit, next-fit Dynamic linking, address relocation, swapping 3/6/12 CSE325 - Main Memory 3

4 The External View of the Memory Manager Application Program exec() shmalloc() sbrk() getrlimit() VirtualAlloc() VMQuery() VirtualLock() VirtualFree() ZeroMemory() UNIX File Mgr Device Mgr Memory Mgr Process Mgr File Mgr Device Mgr Memory Mgr Process Mgr Windows Hardware 3/6/12 4

5 Requirements Memory Manager Minimize executable memory access time Maximize executable memory size Executable memory must be cost-effective Today s memory manager: Allocates primary memory to processes Maps process address space to primary memory Minimizes access time using cost-effective memory configuration May use static or dynamic techniques 3/6/12 CSE325 - Main Memory 5

6 Storage Hierarchies 3/6/12 CSE325 - Main Memory 6

7 The Basic Memory Hierarchy CPU Registers Primary Memory (Executable Memory) e.g. RAM Secondary Memory e.g. Disk or Tape 3/6/12 CSE325 - Main Memory 7

8 Exploiting the Hierarchy Upward moves are (usually) copy operations Require allocation in upper memory Image exists in both higher & lower memories Updates are first applied to upper memory Downward move is (usually) destructive Update image in lower memory Destroy image in upper memory Place frequently-used info high, infrequently-used info low in the hierarchy Reconfigure as process changes phases 3/6/12 CSE325 - Main Memory 8

9 Address Space vs Primary Memory Process Address Space Hardware Primary Memory Mapped to objects other than memory 3/6/12 CSE325 - Main Memory 9

10 Creating an Executable Program Source code Library code Other objects C Reloc Object code Link Edit Secondary memory Primary memory Compile time: Translate elements Link time: Combine elements Loader Load time: Allocate primary memory Adjust addresses in address space Copy address space from secondary to primary memory Process address space 10

11 Dynamic Linking Linking postponed until execution time Small piece of code, stub, used to locate the appropriate memory-resident library routine Stub replaces itself with the address of the routine, and executes the routine Operating system needed to check if routine is in processes memory address Dynamic linking is particularly useful for libraries 3/6/12 CSE325 - Main Memory 11

12 A Sample Code Segment static int gvar; int proc_a(int arg){ gvar = 7; put_record(gvar); } 3/6/12 CSE325 - Main Memory 12

13 The Relocatable Object module Code Segment Relative Address Generated Code entry proc_a 0220 load =7, R store R1, push call put_record 0400 External reference table 0404 put_record External definition table 0540 proc_a (symbol table) 0799 (last location in the code segment) Data Segment Relative Address Generated variable space 0036 [Space for gvar variable] 0049 (last location in the data segment) static int gvar; int proc_a(int arg){ gvar = 7; put_record(gvar); } 13

14 The Linked Program Code Segment Relative Address Generated Code 0000 (Other modules) 1008 entry proc_a 1220 load =7, R store R1, push call (End of proc_a) (Other modules) 2334 entry put_record 2670 (optional symbol table) 2999 (last location in the code segment) Data Segment Relative Address Generated variable space 0136 [Space for gvar variable] 1000 (last location in the data segment) 3/6/12 CSE325 - Main Memory 14

15 The Program Loaded at Location 4000 Relative Address Generated Code 0000 (Other process s programs) 4000 (Other modules) 5008 entry proc_a 5036 [Space for gvar variable] 5220 load =7, R store R1, push call (End of proc_a) (Other modules) 6334 entry put_record 6670 (optional symbol table) 6999 (last location in the code segment) 7000 (first location in the data segment) 7136 [Space for gvar variable] 8000 (Other process s programs) 15

16 Dynamic Relocation Hardware device that maps virtual to physical address The value in the relocation register is added to every address generated by a user process at the time it is sent to memory The user program deals with logical addresses; it never sees the real physical addresses 16

17 Memory Partitioning P i Unused In Use Issue: Need a mechanism/ policy for loading P i s address space into primary memory Operating System Process 3 Process 0 Process 2 Process 1 3/6/12 CSE325 - Main Memory 17

18 Memory Partitioning (Cont.) Multiple-partition allocation Hole block of available memory; holes of various size are scattered throughout memory When a process arrives, it is allocated memory from a hole large enough to accommodate it Operating system maintains information about: a) allocated partitions b) free partitions (hole) OS" OS" OS" OS" process 5" process 5" process 5" process 5" process 9" process 9" process 8" process 10" process 2" process 2" process 2" process 2" 3/6/12 CSE325 - Main Memory 18

19 Fixed-Partition Memory Mechanism Operating System Operating System P i needs n i units Region 0 N Region 0 N 0 n i P i Region 1 N Region 1 N 1 Region 2 Region 3 N N Region 2 Region 3 N 2 N 3 3/6/12 CSE325 - Main Memory 19

20 Fixed-Partition Memory Best-Fit Operating System Allocate the smallest hole that is big enough. Region 0 N 0 Internal Fragmentation Region 1 P i Region 2 N 1 N 2 Pros and Cons? Region 3 N 3 3/6/12 CSE325 - Main Memory 20

21 Fixed-Partition Memory Worst-Fit Operating System Allocate the largest hole. P i Region 0 N 0 Region 1 Region 2 Region 3 N 1 N 2 N 3 Pros and Cons? 3/6/12 CSE325 - Main Memory 21

22 Fixed-Partition Memory First-Fit Operating System P i Region 0 N 0 Allocate the first hole that is big enough. Region 1 N 1 Pros and Cons? Region 2 Region 3 N 2 N 3 3/6/12 CSE325 - Main Memory 22

23 Fixed-Partition Memory Next-Fit Operating System Region 0 N 0 Allocate the first hole that is big enough starting from the last placement. Pros and Cons? P i Region 1 P i+1 Region 2 Region 3 N 1 N 2 N 3 Fixed-Partition strategies were used in batch multiprogramming systems (why?) Not good for timesharing systems 3/6/12 CSE325 - Main Memory 23

24 Variable Partition Memory Operating System Operating System Operating System Operating System Process 0 Process 0 Process 0 Process 1 Process 6 Process 6 Process 2 Process 2 Process 2 Process 3 Process 4 Process 5 Process 4 Process 5 Process 4 Loader adjusts every address in every absolute module when placed in memory External fragmentation Compaction moves program in memory 24

25 Buddy System Entire space available is treated as a single block of 2 U If a request of size s such that 2 U-1 < s <= 2 U, entire block is allocated Otherwise block is split into two equal buddies The process continues until the smallest block greater than or equal to s is generated 3/6/12 CSE325 - Main Memory 25

26 3/6/12 CSE325 - Main Memory 26

27 Program and Process Address Spaces Relocatable Program Address Space Process Address Space 0 Runtime binding User Process Address Space Primary Memory 3GB 4GB Supervisor Process Address Space 3/6/12 CSE325 - Main Memory 27

28 Dynamic Memory Allocation Could use dynamically allocated memory Process wants to change the size of its address space Smaller Creates an external fragment Larger May have to move/relocate the program Allocate holes in memory according to Best- /Worst- / First- /Next-fit 3/6/12 CSE325 - Main Memory 28

29 Moving an Executable Image Executable Program Loader Executable Image Loader Executable Image 3/6/12 CSE325 - Main Memory 29

30 A base and a limit register define a logical address space Mapping Protection Relocation register Limit register 3/6/12 CSE325 - Main Memory 30

31 Logical Addresses reference to a memory location independent of the current mapping of data to memory translation must be made to the physical address Relative address expressed as a location relative to some known point Physical the absolute address or actual location in main memory 3/6/12 CSE325 - Main Memory 31

32 Dynamic Address Relocation CPU Relative Address 0x02010 Relocation Register 0x x12010 load R1, 0x02010 MAR Program loaded at 0x10000 Relocation Register = 0x10000 Program loaded at 0x04000 Relocation Register = 0x04000 We never have to change the load module addresses! 32

33 HW address protection with base and limit registers 3/6/12 CSE325 - Main Memory 33

34 Special Case: Swapping Special case of dynamic memory allocation Suppose there is high demand for executable memory Equitable policy might be to time-multiplex processes into the memory (also space-mux) Means that process can have its address space unloaded when it still needs memory Usually only happens when it is blocked 3/6/12 CSE325 - Main Memory 34

35 Swapping Image for P i Swap P i out Swap P j in Image for P j 3/6/12 CSE325 - Main Memory 35

36 Memory Mgmt Strategies Fixed-Partition used only in batch systems Variable-Partition used everywhere (except in virtual memory) Swapping systems Popularized in timesharing Relies on dynamic address relocation Dynamic Loading (Virtual Memory) Exploit the memory hierarchy Paging Segmentation 3/6/12 CSE325 - Main Memory 36