Lecture 17 Chapter 8: Main Memory (cont) Chapter 9: Virtual Memory

1. Lecture 17Chapter 8: Main Memory (cont)Chapter 9: Virtual Memory

2. Binding of Instructions and Data to Memory • Address binding of instructions and data to memory addresses can happen at three different stages • Compile time: • If memory location known a priori, absolute codecan be generated; • must recompile code if starting location changes • Load time: • Must generate relocatable code if memory location is not known at compile time • Execution time: • Binding delayed until run time if the process can be moved during its execution from one memory segment to another. • Need hardware support for address maps (e.g., base and limitregisters)

3. Dynamic Loading • Routine is not loaded until it is called • Better memory-space utilization; • unused routine is never loaded • Useful when large amounts of code are needed to handle infrequently occurring cases • No special support from the operating system is required • implemented through program design

4. 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 • System also known as shared libraries • versions

5. Swapping • A process can be swapped temporarily out of memory to a backing store, and then brought back into memory for continued execution • Backing store– fast disk large enough to accommodate copies of all memory images for all users; • Roll out, roll in– swapping variant used for priority-based scheduling algorithms; • Swapped process should be idle • I/O problem • Major part of swap time is transfer time • Modified versions of swapping are found on many systems

6. Contiguous Allocation • Main memory usually into two partitions: • Resident operating system, • User processes • Relocation registers used to protect user processes from each other, and from changing operating-system code and data • Base register contains value of smallest physical address • Limit register contains range of logical addresses • MMU maps logical address dynamically

7. Contiguous Allocation (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

8. Dynamic Storage-Allocation Problem • First-fit: Allocate the first hole that is big enough • Best-fit: Allocate the smallest hole that is big enough; must search entire list, unless ordered by size • Produces the smallest leftover hole • Worst-fit: Allocate the largest hole; must also search entire list • Produces the largest leftover hole • First-fit and best-fit better than worst-fit in terms of speed and storage utilization. • First-fit generally fastest. How to satisfy a request of size n from a list of free holes

9. Fragmentation • External Fragmentation– total memory space exists to satisfy a request, but it is not contiguous • e.g. first-fit can on average use 2/3 of memory. • Internal Fragmentation– allocated memory may be slightly larger than requested memory; this size difference is memory internal to a partition, but not being used • Reduce external fragmentation by compaction • Shuffle memory contents to place all free memory together in one large block • Compaction is possible only if relocation is dynamic, and is done at execution time

10. Paging • Logical address space of a process can be noncontiguous; • process is allocated physical memory whenever the latter is available • Divide physical memory into fixed-sized blocks called frames • size is power of 2, between 512 bytes and 8,192 bytes • Divide logical memory into blocks of same size called pages • Keep track of all free frames • To run a program of size n pages, • need to find n free frames and load program • Set up a page table to translate logical to physical addresses • Internal fragmentation

11. Paging Hardware Page number Page offset

12. Implementation of Page Table • Associative memory – parallel search • Address translation (p, d) • If p is in associative register, get frame # out • Otherwise get frame # from page table in memory

13. Effective Access Time • Associative Lookup =  time unit • Assume memory cycle time is 1 microsecond • Hit ratio – percentage of times that a page number is found in the associative registers; ratio related to number of associative registers • Hit ratio =  • Effective Access Time(EAT) EAT = (1 + )  + (2 + )(1 – ) = 2 +  – 

15. Memory Protection • implemented by associating protection bit with each frame • Read-only, read-write, etc. • Valid-invalidbit attached to each entry in the page table: • “valid” indicates that the associated page is in the process’ logical address space

16. Shared Pages • Shared code • One copy of read-only (reentrant) code shared among processes • Non-self-modifying code • Shared code must appear in same location in the logical address space of all processes • Private code and data • Each process keeps a separate copy of the code and data

17. Structure of the Page Table • Hierarchical Paging • Hashed Page Tables • Inverted Page Tables

18. Hierarchical Page Tables • Break up the logical address space into multiple page tables • A simple technique is a two-level page table

19. Two-Level Paging Example • A logical address (on 32-bit machine with 1K page size) is divided into: • a page number consisting of 22 bits • a page offset consisting of 10 bits • Since the page table is paged, the page number is further divided into: • a 12-bit page number • a 10-bit page offset • Thus, a logical address is as follows:where pi is an index into the outer page table, and p2 is the displacement within the page of the outer page table page number page offset p2 pi d 10 10 12

20. Three-level Paging Scheme Very large table ! Still very large !

21. Hashed Page Tables • Common in address spaces > 32 bits • The virtual page number is hashed into a page table • contains a chain of elements hashing to the same location • Virtual page numbers are compared in this chain searching for a match • If a match is found, the corresponding physical frame is extracted

22. Inverted Page Table • One entry for each real page of memory • Entry consists of the virtual address of the page stored in that real memory location, with information about the process that owns the page • Decreases memory needed to store each page table, • but increases time needed to search the table • Use hash table to limit the search to one (or at most a few) page-table entries

23. Segmentation • Memory-management scheme that supports user view of memory • A program is a collection of segments • A segment is a logical unit such as: • main program • procedure • function • method • object • local variables • global variables • common block • stack • symbol table • arrays

24. 1 4 2 3 Logical View of Segmentation 1 2 3 4 user space physical memory space

25. Segmentation Architecture • Segments vary in length, • memory allocation is a dynamic storage-allocation problem • Logical address consists of a two tuple: <segment-number, offset>, • Segment table– maps two-dimensional physical addresses • each table entry has: • base– contains the starting physical address where the segments reside in memory • limit– specifies the length of the segment • Segment-table base register (STBR)points to the segment table’s location in memory • Segment-table length register (STLR)indicates number of segments used by a program; segment number s is legal if s < STLR

26. Segmentation Hardware

27. Example of Segmentation

28. Paging & Segmentation

30. Virtual Memory (Chapter 9) • Virtual memory– separation of user logical memory from physical memory. • Only part of the program needs to be in memory for execution • Logical address space can therefore be much larger than physical address space • Allows address spaces to be shared by several processes • Allows for more efficient process creation • Virtual memory can be implemented via: • Demand paging • Demand segmentation

31. Virtual Memory That is Larger Than Physical Memory 

32. Virtual-address Space

33. Demand Paging • Bring a page into memory only when it is needed • Less I/O needed • Less memory needed • Faster response • More users • Page is needed  reference to it • invalid reference  abort • not-in-memory  bring to memory • Lazy swapper– never swaps a page into memory unless page will be needed • Swapper that deals with pages is a pager

34. Transfer of a Paged Memory to Contiguous Disk Space

35. Valid-Invalid Bit • With each page table entry a valid–invalid bit is associated • v in-memory,i  not-in-memory

36. Page Fault • If there is a reference to a page, • first reference to that page will trap to operating system: page fault

37. Performance of Demand Paging • Page Fault Rate 0  p  1.0 • if p = 0 no page faults • if p = 1, every reference is a fault • Effective Access Time (EAT) EAT = (1 – p) x memory access + p (page fault overhead + swap page out + swap page in + restart overhead )

38. Demand Paging Example • Memory access time = 200 nanoseconds • Average page-fault service time = 8 milliseconds • EAT = (1 – p) x 200 + p (8 milliseconds) = (1 – p x 200 + p x 8,000,000 = 200 + p x 7,999,800 • If one access out of 1,000 causes a page fault, then EAT = 8.2 microseconds. This is a slowdown by a factor of 40!!

39. Goals of Virtual Memory • Make programmers job easier • Can write code without knowing how much DRAM is there • Only need to know general memory architecture • (e.g., 32-bit address space) • Don’t have to do overlays • Enable Multiprogramming • Keep several programs running concurrently • Together, these programs may need more DRAM than we have. • Keep only the actively used stuff in DRAM. • Share when possible • When one program does I/O switch CPU to another.

40. Same concepts, New Terminology

41. Same concepts, Different Implementation

42. Copy-on-Write • Copy-on-Write (COW) allows both parent and child processes to initially share the same pages in memory • If either process modifies a shared page, only then is the page copied • allows more efficient process creation as only modified pages are copied • Free pages are allocated from a pool of zeroed-out pages

43. Page Replacement • What happens if there is no free frame? • Page replacement • find some page in memory but not really in use, swap it out • performance: want an algorithm which will result in minimum number of page faults • Same page may be brought into memory several times • Prevent over-allocation of memory by modifying page-fault service routine to include page replacement • Page replacement completes separation between logical memory and physical memory • large virtual memory can be provided on a smaller physical memory • Use modify (dirty) bit to reduce overhead of page transfers • only modified pages are written to disk

44. Need For Page Replacement

45. Basic Page Replacement • Find the location of the desired page on disk • Find a free frame: - If there is a free frame, use it - If there is no free frame, use a page replacement algorithm to select a victimframe • Bring the desired page into the (newly) free frame; - update page & frame tables • Restart the process

46. Page Replacement Algorithms • Want lowest page-fault rate • Evaluate algorithm by running it on a particular string of memory references (reference string) • computing the number of page faults on that string

47. Global vs. Local Allocation • Global replacement– process selects a replacement frame from the set of all frames; one process can take a frame from another • Local replacement– each process selects from only its own set of allocated frames

48. Thrashing • If a process does not have “enough” pages, • the page-fault rate is very high • This leads to low CPU utilization • operating system thinks that it needs to increase the degree of multiprogramming • another process added to the system • Thrashing a process is busy swapping pages in and out

49. Memory-Mapped Files • Memory-mapped file I/O allows file I/O to be treated as routine memory access by mappinga disk block to a page in memory • A file is initially read using demand paging. • A page-sized portion of the file is read from the file system into a physical page. • Subsequent reads/writes to/from the file are treated as ordinary memory accesses. • Simplifies file access by treating file I/O through memory rather than read()write() system calls • Also allows several processes to map the same file allowing the pages in memory to be shared

50. Memory Mapped Files