EC503 - OPERATING SYSTEMS

1. EC503 - OPERATING SYSTEMS TOPIC 3 - MEMORY MANAGEMENT

2. SUB-TOPICS

3. MEMORY MANAGEMENT

4. What is Memory Management (MM)?

5. Why Manage Memory?

6. Routines • A routine is any sequence of codes that is intended to be called and used repeatedly during the execution of a program / process. • Routines can be divided into two, namely: • Resident Routines • Transient Routines. • A resident routine is a library routine (function) that is linked with an application, which includes initialization routines and callable service stubs. This routine resides in the memory. • A transient routine is a library routine that is loaded at run time, such as Dynamic Link Libraries (DLL) routines for interacting with I/O devices. This routine is only loaded when needed.

7. Terminologies

8. FIXED PARTITION MM • Equal-size partitions: • Any process whose size is less than or equal to the partition size can be loaded into an available partition. • If all partitions are full, the operating system can swap a process out of a partition. • A program may not fit in a partition. • The programmer must design the program with overlays. • Main memory use is inefficient. • Any program, no matter how small, occupies an entire partition. • This is called internal fragmentation.

9. Fixed Partitioning

10. DYNAMIC MM • Partitions are of variable length and number. • Process is allocated exactly as much memory as required. • Eventually get holes in the memory. This is called external fragmentation. • Must use compaction to shift processes so they are contiguous and all free memory is in one block.

11. Dynamic Partitioning

12. SEGMENTATION MM • All segments of all programs do not have to be of the same length. • There is a maximum segment length. • Addressing consist of two parts - a segment number and an offset. • Since segments are not equal, segmentation is similar to dynamic partitioning.

13. Logical Address

14. Paging

15. Logical-to-Physical Address

16. PAGING MM • Partition memory into small equal fixed-size chunks and divide each process into the same size chunks. • The chunks of a process are called pages and chunks of memory are called frames. • Operating system maintains a page table for each process: • Contains the frame location for each page in the process. • Memory address consist of a page number and offset within the page.

17. Assignment of Process Pages

18. Assignment of Process Pages

19. Page Tables

20. VIRTUAL MEMORY

21. What is Virtual Memory (VM)?

22. VM MODELS • VM models can be divided into three: • Demand Paging. • Swapping. • Shared VM.

23. Demand Paging • Bring a page into memory only when it is needed: • Less I/O needed. • Less memory needed. • Faster response. • More users. • The first reference to a page will trap to OS with a page fault. • OS looks at another table to decide: • Invalid reference - abort • Just not in memory.

24. Swapping • Processes 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; must provide direct access to these memory images. • Roll out, roll in - swapping variant used for priority based scheduling algorithms; lower priority process is swapped out, so higher priority process can be loaded and executed. • Major part of swap time is transfer time; total transfer time is directly proportional to the amount of memory swapped. • Modified versions of swapping are found on many systems, i.e. UNIX and Microsoft Windows.

25. Swapping • Memory allocation changes as • processes come into memory • leave memory • Shaded regions are unused memory

26. Swapping • Allocating space for growing data segment • Allocating space for growing stack & data segment

27. Swapping Schematic View

28. Shared Memory • Virtual memory makes it easy for several processes to share memory. • All memory access are made via page tables and each process has its own separate page table. • For two processes sharing a physical page of memory, its physical page frame number must appear in a page table entry in both of their page tables. • The shared physical page does not have to exist at the same place in virtual memory for any or all of the processes sharing it.

29. PAGED VM • Paged VM can be explained by: • Page Tables. • Dynamic Address Translation. • Paging Supervisor.

30. Page Tables • Internal operation of MMU with 16 4 KB pages

31. Page Tables • 32 bit address with 2 page table fields • Two-level page tables

32. Page Tables • Typical page table entry

33. Dynamic Address Translation • The relation between virtual addresses and physical memory addresses given by page table - address translation

34. Paging Supervisor • Is a part of the OS that creates and manages page tables. • If the hardware raises a page fault exception, the paging supervisor will: • Accesses the secondary storage. • Returns the page that has the virtual address the page fault. • Updates the page tables to reflect the physical location of the virtual address. • Tells the translation mechanism to restart the request. • When all physical memory is already in use, the paging supervisor must free a page in primary storage to hold swapped-in page using replacement algorithms.

35. CACHE MEMORY

36. What is Cache?

37. What is Cache Memory (CM)?

38. Cache and Main Memory

39. Cache Operation • CPU requests contents of memory location • Check cache for this data • If present, get from cache (fast) • If not present, read required block from main memory to cache • Then deliver from cache to CPU • Cache includes tags to identify which block of main memory is in each cache slot.

40. Types of CM • CM can be divided into: • CPU cache. • Disk cache. • Web cache. • CPU cache stores copies of data from the most frequently used main memory locations. • Disk cache is used as a buffer between the hard disk and the rest of the computer system.

41. Types of CM • Web cacheis a mechanism for the temporary storage (caching) of web documents, such as HTML pages and images, to reduce bandwidth usage, server load, and perceived lag. • A web cache stores copies of documents passing through it; subsequent requests may be satisfied from the cache if certain conditions are met. • For example, a Google's cache link in its search results provides a way of retrieving information from websites that have recently gone down and a way of retrieving data more quickly than by clicking the direct link.