Operating Systems. Memory Management. Memory management

Transcription

1 Oerating Systems Memory management Memory Management List of Toics 1. Memory Management 2. Memory In Systems Design 3. Binding Times 4. Introduction to Memory Management 5. Raw Memory Model 6. Single Contiguous Memory 7. Relocation - Why and How 8. Overlay Management Toics (continued) 9. Protection 10. Fixed Partitions 11. Nonuniform Sized Fixed Partitions 12. Uniformly Sized Fixed Partitions 13. Simle Paging 14. Benefits of Simle Paging 15. Page Tables 16. Translation Lookaside Buffers 17. Hierarchical Address Caching 1

2 Toics (continued) 18. Dynamic Partitions 19. Fragmentation 20. Internal Fragmentation 21. External Fragmentation 22. Coalescing Holes 23. Comaction 24. Dynamic Partition Placement 25. Simle Segmentation 26. Memory Layout of A C Program Toics (continued) 27. malloc 28. sbrk 29. Memory Hierarchy Memory In Systems design CPU memory I/O System Figure 1: memory Connects CPU and Periherals 2

3 Binding Times source rogram comiler or assembler comile time object model Other object modules linkage editor load module load time system library loader dynamically loaded system library in-memory binary memory image execution time (run time) 3

4 Introduction to Memory Management Memory management issues: 1. Relocation 2. Protection 3. Sharing 4. Logical Organization 5. Physical Organization Consider a series of solutions starting with the most rimitive first Raw Memory Model The raw memory rovides no services and gives the rogrammer comlete control. 0 Figure 3: Raw Machine Model 1 MB Single Contiguous Memory Primitive oerating systems (such as MS- DOS and CP/M) rovide some interfaces to the hardware but not much else in the way of services. 4

5 Single Contiguous Memory (continued) fence register 0 Monitor Figure 4: Single Contiguous Memory 32K Relocation - Why and How Relocation refers to the ability to store a rogram at an arbitrary base memory address. Actual memory locations have hysical or absolute addresses, user rogram's access these locations using logical addresses. Relocation - Why and How (continued) Base register CPU Logical address Physical address memory Figure 5: Address Translation 5

6 Overlay Management Overlays have gone out of fashion with cheaer memory, users (and comilers) determine which code to swa in and out. Figure 6: Overlay Management Examle [2] symbol table common routines 20K 30K 70K overlay driver 10K ass 1 ass 2 80K Protection It is undesirable to ermit user rograms (accidentally or intentionally) to accesses memory outside of their artition. 6

7 Protection (continued) 0 Monitor fence register Figure 7: Protection in Resident Monitor Model 32K Fixed Partitions Fixed artitioning refers to memory being slit into contiguous non overlaing regions of recomuted sizes. Fixed sized artitions make the selection of a artition for a job easy. Nonuniform Sized Fixed Partitions Fixed Partitions may have differing sizes. 7

8 Figure 8: Nonuniform fixed Partitions [3] Oerating system New Process Uniformly Sized Fixed Partitions Memory is frequently artitioned into uniformly sized regions. Figure 9: Uniform Fixed Partition Allocation [3] Oerating system 512 K 512 K 512 K 512 K 512 K 512 K 512 K 512 K 8

9 Simle Paging Paging rovides relocation, and slits memory into fixed length artitions called frames Page frame 0 Page frame 1 Page frame 2 Page frame 3 Figure 10: Simle Paging [1] Page frame 4 Page frame 5 Page frame 6 Page frame 7 Figure 10: Simle Paging (continued) [1] 9

10 Page frame number Page frame size Range of real storage addresses Benefits of Simle Paging Simle aging allowsdiscontiguous storage for memory objects exceeding the age frame size.. Contiguous virtual storage locations. Virtual memory Address maing mechanism Real storage 10

11 Page Tables One simle mechanism is to allocate some real memory sace for a table, and hash age addresses using the high order address bits as ointers into the age table. There are 2 real memory accesses er virtual memory access. Page Tables (continued) b b+ b b Page # Dislacement Page Ma Table d } Virtual address v=(,d) 1 1 d } Real address Translation Lookaside Buffers Translation lookaside buffers (TLB) eliminate one hysical memory reference using secial associative memory, which addressed by its contents in O(1) arallel search time. 11

12 Associative Memory Looku Associative ma Page # Dislacement d } Virtual address V=(,d) 1 1 d Real } Address r Frame # Dislacement Hierarchical Address Caching Rather than lacing all addresses in the TLB recently/frequently used addresses are stored in associative memory, with misses being serviced by the age table. Hierarchical Address Caching (continued) Page table origin register Address of Page table b + Performed only if no match in associative ma b+ 1 Try this first Only if no match in associative ma Virtual address Page # v=(,d) Dislacement d Partial associative Ma (most active ages 1 only Frame # 1 Only if match in associative ma Dislacement d Real address Direct ma (all ages) 12

13 Dynamic Partitions Dynamically artitioned memory allows lacement of relocatable code in variable size contigous memory regions. Dynamic Partitions (continued) user needs I H G F E D C B A 9K. 18K. 11K. 32K. 14K. 25K. 10K. 20K. 15K. Oerating system A 15K Free Oerating Oerating Oerating system system system A 15K A 15K A 15K B 20K B 20K B 20K C 10K C 10K Free Free D 25K Free Fragmentation Fragmentation makes available memory useless by breaking it into discontiguous ieces too small to use. 13

14 Fragmentation (continued) There are two categories of memory fragmentation: 1. Internal Fragmentation --- A fixed artition contains more memory than required by the user, and some is wasted. 2. External Fragmentation --- Results from the holes left by dynamic artitions. Internal Fragmentation Internal fragmentation occurs when fixed size artitions are too large. Page 2 Logical Address Page#=1,Offset= Page 1 Page Internal fragmentation Figure 16: Internal Fragmentation [3] 14

15 External Fragmentation External fragmentation haens when dynamic artitions are released. The fragments are frequently called holes. oerating system A B C D E B finishes and frees its storage. oerating system A Hole C D E D finishes and frees its storage. oerating system A Hole C Hole E Hole Hole Hole Coalescing Holes Adjacent holes in dynamic artitions should be coalesced into a single larger hole. 15

16 oerating system oerating system oerating system other users 2K Hole 5K user A other users A finishes and frees its storage other users 2K Hole 5K Hole other users Oerating system combines adjacent holes to form a single larger hole. other users 7K Hole other users Comaction If the amount of memory available in the holes is large enough to service a request, the holes may made contiguous by comacting storage. oerating systems Free Free oerating systems Free Oerating system laces all in use blocks together leaving free storage as a single, large hole. Free 16

17 Dynamic Partition Placement (a) First-Fit Strategy Place job in first storage hole on free storage lace list in which it will fit (ket in storage address order, or something in random order.) Free Storage List Start Address Length a c e g 16K 14K 5K 30K Request for 13K. 0 a b c d e f g h Oerating systems 16K hole 14K hole 5K hole 30K hole (b) Best-Fit Strategy Place job in the smallest ossible hole in which it will fit. 17

18 e c a g (Ket in ascending order by hole size.) Free Storage List Start Address Length 5K 14K 16K 30K Request for 13K. 0 a b c d e f g h Oerating systems 16K hole 14K hole 5K hole 30K hole (c) Worst-Fit Strategy Place job in the largest ossible hole in which it will fit. g a c e (Ket in descending order by hole size.) Free Storage List Start Address Length 30K 16K 14K 5K Request for 13K. 0 a b c d e f g h Oerating systems 16K hole 14K hole 5K hole 30K hole 18

19 Simle Segmentation Segmentation rovides relocation, and suorts contiguous variable length artitions. Segmentation often rovides rotection (counterexamle Intel 8086). limit base Logical CPU address < yes + memory no Tra; address error Memory Layout of A C Programs Traditional Unix/C memory images of rograms use segments. 19

20 high address stack Command-line arguments and environment variables low address hea uninitialized data (bss) Initialized data text initialized to zero by exec read from rogram file by exec malloc Programmers often want to allocate data objects which ersist beyond the function call creating them (e.g. constructors in OOP). In C and C++ the malloc oerator maintains a linked list of data objects, in the user rogram's Data segment (on the Hea). user data In Use List user data user data Key Free List Memory management Information user data user data user data 20

21 sbrk A user rogram can exhaust its default hea sace allocation. The Unix sbrk system call increases data segment allocation at run time. alication code Memory allocation function malloc user rocess sbrk system call kernel Memory Hierarchy s want to: 1. Increase their address sace, using slow cheaer memory to extend their more exensive faster memory. 2. Increase the seed at which they can the extended memory by using small amounts of exensive fast memory. 21

22 Memory Hierarchy Registers Cache Main Memory Magnetic Disk (Cache) Backus Otical Juke Box Remote Access 22