Memory Management

2. Memory Management • Logical and Physical Address Spaces • Contiguous Allocation • Paging • Segmentation • Virtual Memory Techniques (Next Lecture) GMU – CS 571

3. Memory Management • Memory density available for constant dollars tends to double every 18 months. • Why bother about memory management? • Parkinson’s Law: “Data expands to fill the space available for storage” • Emerging memory-intensive applications • Memory usage of evolving systems tends to double every 18 months. GMU – CS 571

4. Background • Program must be brought (from disk) into memory and placed within a process for it to be run • Main memory and registers are only storage CPU can access directly • Register access in one CPU clock (or less) • Main memory access can take many cycles • Cache sits between main memory and CPU registers • Protection of memory required to ensure correct operation GMU – CS 571

5. Basic Concepts • Multiple processes typically take space in main memory at the same time • How to adjust the degree of multiprogramming to maximize CPU utilization? • Suppose average process computes only 20% of time it is sitting in memory and does I/O 80% of time. • Probabilistic view: CPU Utilization = 1 – xnwhere x is the average ratio of I/O time to process lifetime in memory, and n is the degree of multiprogramming. GMU – CS 571

6. Multiprogramming • CPU utilization as a function of the number of processes in memory and their degree of I/O. Degree of multiprogramming GMU – CS 571

7. Basic Concepts • Multiple processes typically take space in main memory at the same time • Address binding • Allocation of space in main memory – contiguous vs. non-contiguous GMU – CS 571

8. 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 the memory location is known a priori, absolute code can 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 limit registers). GMU – CS 571

9. 1024 PROGRAM PROGRAM JUMP X JUMP 1824 X 1824 LOAD Y LOAD 2034 DATA DATA Y 2034 Address Binding (Cont.) 0 PROGRAM JUMP 800 800 LOAD 1010 DATA 1010 GMU – CS 571

10. Logical vs. Physical Address Space • The concept of a logical address space that is bound to a separate physicaladdress space is central to proper memory management. • Logical address – generated by the CPU; also referred to as virtual address. • Physical address – address seen by the memory unit. • Logical and physical addresses are the same in compile-time and load-time address-binding schemes; they differ in execution-time address-binding scheme. GMU – CS 571

11. Memory-Management Unit (MMU) • Hardware device that maps logical address to physical address for execution time binding. • In a simple MMU scheme, the value in the relocation (or, base) 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. GMU – CS 571

12. Dynamic relocation using a relocation register GMU – CS 571

13. Use of Relocation and Limit Registers Hardware Support for Relocation and Limit Registers GMU – CS 571

14. Contiguous Allocation • Processes (that are eligible for the CPU) are placed to the main memory in contiguous fashion • Can all of a program’s data and code be loaded into memory? • Dynamic loading • Dynamic linking • How to partition the memory across multiple processes? • Fixed partitioning • Dynamic partitioning GMU – CS 571

15. Contiguous Allocation (Cont.) • Fixed Partitioning • Divide the memory into fixed-size partitions • One process/partition • The degree of multiprogramming is bounded by the number of partitions. • When a partition is free, a process is selected from the input queue and is loaded into memory • OS keeps track of which partitions are free and which are in use. GMU – CS 571

16. Partition 1 Process 1 Process 1 Partition 2 Process 2 Process 2 Partition 3 Process 3 Process 3 Contiguous Allocation with Fixed-Size Partitions Process 4 GMU – CS 571

17. Contiguous Allocation (Cont.) • Dynamic Partitioning • Partitions are created dynamically, so that each process is loaded into a partition of exactly the size that it will need. • 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 (holes) GMU – CS 571

18. Process 1 Process 1 Process 2 Process 2 Process 3 Process 3 Contiguous Allocation with Variable-Size Partitions Main Memory Process 4 GMU – CS 571

19. Dynamic Storage-Allocation Problem How to satisfy a request of size n from a list of free holes? • First-fit: Allocate the first hole that is big enough. • Best-fit: Allocate the smallest hole that is big enough. • Must search the entire list, unless ordered by size. Produces the smallest leftover hole. • Worst-fit: Allocate the largest hole. • Produces the largest leftover hole. • Next-fit: Starts to search from the last allocation made, chooses the first hole that is big enough. • First-fit and best-fit better than worst-fit in terms of storage utilization. GMU – CS 571

20. Fragmentation Problem • Internal Fragmentation – allocated memory may be slightly larger than requested memory; the unused memory is internal to a partition. • Contiguous allocation with fixed-size partitions has the internal fragmentation problem • External Fragmentation – total memory space exists to satisfy a request, but it is not contiguous. • Contiguous allocation with variable-size partitions has the external fragmentation problem • 50-percent rule: Even with an improved version of First Fit, given N allocated blocks, another N/2 blocks will be lost to external fragmentation! (on the average) • Memory Compaction (for execution time address binding) GMU – CS 571

21. Paging • A memory management scheme that allows the physical address space of a process to be non-contiguous. • Divide physical memory into fixed-sized blocks called frames. • Divide logical memory into blocks of same size called pages. • Any page can go to any free frame • To run a program of size n pages, need to find n free frames and load program. • Set up a page table (one per process) to translate logical addresses (generated by process) to physical addresses. • External Fragmentation is eliminated. • Internal fragmentation is a problem. GMU – CS 571

22. 0 1024 2048 3072 Paging Example 0 Frame 0 1024 Frame 1 2048 Frame 2 A’s Page 0 3072 Frame 3 0 Page 0 1024 Page 1 2048 Page 2 3072 A’s Page 3 Page 3 4093 A’s Page 1 Logical Address Space of Process A A’s Page 2 Frame 15 16383 16383 Main Memory Main Memory GMU – CS 571

23. Address Translation Scheme • Observe: The simple limit/relocation register pair mechanism is no longer sufficient. • m-bit logical address generated by CPU is divided into: • Page number(p) – used as an index into a pagetable which contains base address of each page in physical memory. • Page offset(d) – combined with base address to define the physical memory address that is sent to the memory unit. GMU – CS 571

24. Address Translation Architecture GMU – CS 571

25. Address Translation Architecture The length of the page table entry can be bigger than the required number of bits for the physical address For example, the physical address 24 bits, d = 5, then the page table entry is >= 24-5 d = 5 => page size 2^5 Physical address 24 bits => # of frames = (2^24)/2^5 = 2^19 Most memories are byte (8-bit) addressable There are 16 or 32-bit word addressable GMU – CS 571

26. Paging Example Consider some logical addresses: 6 - 0110 13 - 1101 32-byte memory and 4-byte pages GMU – CS 571

27. Free Frames OS needs to manage physical memory and keep track of what frames are available (typically using a ‘frame table’) Before allocation After allocation GMU – CS 571

28. Hardware Support for Page Table • Option: keep page table in registers • Fast • Not realistic if page table is large • Must reload these registers to run the process • Option: Keep the page table in memory • Page-tablebase register (PTBR) points to the page table location. • Page-table length register (PTLR), if it exists, indicates the size of the page table. • In this scheme every data/instruction access requires two memory accesses- one for the page table and one for the data/instruction. GMU – CS 571

29. Associative Memory • The two memory access problem can be solved by the use of a special fast-lookup hardware cache called translation look-aside buffer(TLBs). • TLB is an associative, high speed memory (expensive). Address translation (A´, A´´) • If page A´ is in associative register, get frame # out. • Otherwise get frame # from page table in memory • Typically, TLB contains 64 – 1024 entries  not all pages are in the TLB. Page # Frame # GMU – CS 571

30. Paging Hardware With TLB GMU – CS 571

31. Translation Look-Aside Buffer • Works like a cache – when we have a ‘miss’ want to add that info to the TLB • When we want to add a new entry to a full TLB, one entry must be replaced. Many different policies LRU (Least Recently Used), random, … • Some entries can be wired down (non removable) • Some TLB’s store address-space identifiers (ASIDs) to provide address space protection • What if TLB does not support separate ASIDs? TLB must be flushed when a new process is selected. GMU – CS 571

32. Effective Access Time • How does the paging affect the memory access time with or without TLB? • TLB Hit ratio – percentage of times that a page number is found in TLB. Example: Assume that in the absence of paging, effective memory access time is 100 nanoseconds (computed through the cache hit ratio and cache memory/main memory cycle times). • Assume that Associative Lookup is 20 nanoseconds • Assume TLB Hit ratio is 80% • Effective Access Time (EAT) EAT = 0.8 * 120 + 0.2 * 220 = 140 nanoseconds GMU – CS 571

33. Hierarchical Page Tables • Having page tables with more than one million entries is not uncommon in modern architectures. • Break up the logical address space into multiple page tables. • A simple technique is a two-level page table (forward-mapped page table in Pentium II.). GMU – CS 571

34. Two-Level Page-Table Scheme GMU – CS 571

35. Two-Level Paging Example • A logical address (on 32-bit machine with 4KByte page size) is divided into: • a page number consisting of 20 bits • a page offset consisting of 12 bits • Since the page table is paged, the page number is further divided into: • a 10-bit page number • a 10-bit page offset • Thus, a logical address is as follows:where p1 is an index into the outer page table, and p2 is the displacement within the page of the outer page table. GMU – CS 571

36. Address-Translation Scheme • Address-translation scheme for a two-level 32-bit paging architecture GMU – CS 571

37. Multiple-Level Paging • For a system with a 64-bit logical space, two-level paging scheme may no longer be appropriate. • SPARC architecture (32 bit addressing) supports a three-level paging scheme, whereas the 32-bit Motorola 68030 architecture supports a four-level paging scheme. GMU – CS 571

38. Three-level Paging Scheme GMU – CS 571

39. Other Paging Strategies For large logical address paces, hashed page tables and inverted page tables can be used. Hashed Page Tables: • Common in address spaces > 32 bits • The virtual page number is hashed into a page table. This 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. GMU – CS 571

40. Hashed Page Table GMU – CS 571

41. Other Paging Strategies • 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 that page • Decreases memory needed to store each page table, but increases time needed to search the table when a page reference occurs • Use hash table to limit the search to one — or at most a few — page-table entries GMU – CS 571

42. Inverted Page Table Architecture GMU – CS 571

43. Shared Pages • Shared code • One copy of read-only (reentrant) code shared among processes (i.e., text editors, compilers, window systems). • Particularly important for time-sharing environments. • Private code and data • Each process keeps a separate copy of the code and data. GMU – CS 571

44. Shared Pages (Example) GMU – CS 571

45. 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, object, local variables, global variables, common block, stack, symbol table, arrays GMU – CS 571

46. Logical View of Segmentation 1 4 1 2 3 2 4 3 Virtual address space of a process physical memory GMU – CS 571

47. Segmentation Architecture • Logical address consists of a pair: <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 process • segment number s is legal if s < STLR. GMU – CS 571

48. Segmentation Hardware GMU – CS 571

49. Example of Segmentation GMU – CS 571

50. Segmentation: Protection and Sharing • Each segment represents a semantically defined portion of the program. • All entries in a segment are likely to be used in the same way. • Segment-table entry will contain the protection info. • Sharing • Segments are shared when entries in the segment tables of two different processes point to the same physical location. • Memory Allocation • Similar to the paging case except that the segments are of variable length. • external fragmentation • first fit/best fit GMU – CS 571