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Memory Management

Memory Management. 4.1 Basic memory management 4.2 Swapping 4.3 Virtual memory 4.4 Page replacement algorithms 4.5 Modeling page replacement algorithms 4.6 Design issues for paging systems 4.7 Implementation issues 4.8 Segmentation. Chapter 4. Memory Management .

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Memory Management

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  1. Memory Management 4.1 Basic memory management 4.2 Swapping 4.3 Virtual memory 4.4 Page replacement algorithms 4.5 Modeling page replacement algorithms 4.6 Design issues for paging systems 4.7 Implementation issues 4.8 Segmentation Chapter 4

  2. Memory Management • Ideally programmers want memory that is • large • fast • non volatile • Memory hierarchy • small amount of fast, expensive memory – cache • some medium-speed, medium price main memory • gigabytes of slow, cheap disk storage • Memory manager handles the memory hierarchy

  3. Requirements of MM • Relocation: cannot be sure where program will be loaded in memory • Protection: avoiding unwanted interference by other processes • Efficient use of CPU and main memory • Sharing: data shared by cooperating processes

  4. CPU Utilization Degree of multiprogramming

  5. Multiprogramming with Fixed Partitions • Fixed memory partitions a) separate input queues for each partition b) single input queue

  6. Multiprogramming with Fixed Partitions • Memory is allocated according to some algorithm, e.g. using best fit • Strength: easy implementation • Weakness: inefficient use of memory because of internal fragmentation (partitions may not be full); limited number of active processes

  7. Swapping or Dynamic Partitioning Memory allocation changes by swapping processes in and out Shaded regions are unused memory - external fragmentation

  8. Problem with growing segments • Allocating space for growing data segment • Allocating space for growing stack & data segment

  9. Virtual Memory • Problem: some programs are too big for main memory; large programs in memory limit the degree of multiprogramming • Solution: keep only those parts of the programs in main memory that are currently in use • Basic idea: a map between program-generated addresses (virtual address space) and main memory • Main techniques: paging and segmentation

  10. Paging (1) The position and function of the MMU

  11. Paging (2) The relation betweenvirtual addressesand physical memory addres-ses given bypage table

  12. Page Tables (1) Internal operation of MMU with 16 4 KB pages

  13. Page Tables (2) Second-level page tables • 32 bit address with 2 page table fields • Two-level page tables Top-level page table

  14. Page Tables (3) Typical page table entry

  15. TLBs – Translation Lookaside Buffers A TLB to speed up paging

  16. Inverted Page Tables Comparison of a traditional page table with an inverted page table

  17. Page Replacement • Page fault: referencing a page that is not in main memory • Page fault forces choice • which page must be removed to make room for incoming page • Modified page must first be saved • unmodified just overwritten • Better not to choose an often used page • will probably need to be brought back in soon

  18. Page Fault Handling (1) • Hardware traps to kernel • General registers saved • OS chooses page frame to free • If selected frame is dirty, writes it to disk

  19. Page Fault Handling (2) • OS brings scheduled new page in from disk • Page tables updated • Faulting instruction backed up to when it began • Faulting process scheduled • Registers restored • Program continues

  20. Optimal Page Replacement Algorithm • Replace page needed at the farthest point in future • Optimal but unrealizable • Estimate by … • logging page use on previous runs of process • although this is impractical

  21. Not Recently Used Page Replacement Algorithm • Each page has Reference bit, Modified bit • bits are set when page is referenced, modified • Pages are classified • not referenced, not modified • not referenced, modified • referenced, not modified • referenced, modified • NRU removes page at random • from lowest numbered non empty class • Macintosh v.m. uses a variant of NRU

  22. FIFO Page Replacement Algorithm • Maintain a linked list of all pages • in order they came into memory • Page at beginning of list (the oldest page) is replaced • Disadvantage • page in memory the longest may be often used

  23. Second Chance Page Replacement Algorithm • Operation of a second chance • pages sorted in FIFO order • Page list if fault occurs at time 20, A has R bit set(numbers above pages are loading times)

  24. The Clock Page Replacement Algorithm

  25. Least Recently Used (LRU) • Assume pages used recently will used again soon • throw out page that has been unused for longest time • Must keep a linked list of pages • most recently used at front, least at rear • update this list every memory reference !! • Alternatively, keep counter in each page table entry indicating the time of last reference • choose page with lowest value counter

  26. Implementation of LRU LRU using a matrix – pages referenced in order 0,1,2,3,2,1,0,3,2,3

  27. Simulating LRU in Software • The aging algorithm simulates LRU in software

  28. The Working Set • Working set: the set of pages currently used by the process – Changes over time. • Locality of reference: during any phase of execution, the process references only a relatively small fraction of its pages. • Thrashing: a program causing page faults at every few instructions.

  29. The Working Set Page Replacement Algorithm The working set algorithm

  30. The WSClock Page Replacement Algorithm Operation of the WSClock algorithm

  31. Review of Page Replacement Algorithms

  32. Modeling Page Replacement AlgorithmsBelady's Anomaly a) FIFO with 3 page frames b) FIFO with 4 page frames • P's show which page references show page faults

  33. Design Issues for Paging SystemsLocal versus Global Allocation Policies (1) a) Original configuration b) Local page replacement c) Global page replacement

  34. Local versus Global Allocation Policies (2) Page fault rate as a function of the number of page frames assigned

  35. Load Control • Despite good designs, system may still thrash • When PFF algorithm indicates • some processes need more memory • but no processes need less • Solution :Reduce number of processes competing for memory • swap one or more to disk, divide up pages they held • reconsider degree of multiprogramming

  36. Page Size Small page size • Advantages • less internal fragmentation • better fit for various data structures, code sections • less unused program in memory • Disadvantages • programs need many pages, larger page tables

  37. Cleaning Policy • Need for a background process, paging daemon • periodically inspects state of memory • When too few frames are free • selects pages to evict using a replacement algorithm • It can use same circular list (clock) as regular page replacement algorithm

  38. Segmentation (1) • One-dimensional address space with growing tables • One table may bump into another

  39. Segmentation (2) Allows each table to grow or shrink, independently

  40. Segmentation (3) Comparison of paging and segmentation

  41. Implementation of Pure Segmentation (a)-(d) Development of external fragmentation (e) Compaction

  42. Segmentation with Paging: MULTICS (1) • Descriptor segment points to page tables • Segment descriptor – numbers are field lengths

  43. Segmentation with Paging: MULTICS (2) A 34-bit MULTICS virtual address

  44. Segmentation with Paging: MULTICS (3) Conversion of a 2-part MULTICS address into a main memory address

  45. Segmentation with Paging: MULTICS (4) Simplified version of the MULTICS TLB

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