Chapter 9. Storage Management
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1 Chapter 9 Storage Management
2 Memory allocation techniques Uniprogramming Fixed-partition multiprogramming Variable-partition multiprogramming Paging Virtual memory
3 Uniprogramming Operating system resides at one end of memory Application at the other end System only executes one job at time Example: Pep/ operating system Disadvantages: Inflexible, CPU time wasted waiting for I/O
4 Fixed-partition multiprogramming Operating system in one fixed reserved partition of memory Multiple processes in fixed partitions of memory Must solve the address problem
5 Figure OS 1 4
6 Logical addresses Logical address is the address generated by the assembler assuming the program begins at address Physical address = logical address + partition address Base register converts from logical to physical Bound register keeps program isolated
7 Figure 9.2 Base register Bound register A CPU true + < Logical Physical Valid address address false physical address Mem Illegal address interrupt
8 Problems with fixedpartitions Scheduling a small job in a large partition because the small partitions are all used Determining the optimal partition in the first place
9 Variable-partition multiprogramming Establish a partition only when a job is loaded into memory The size of the partition can match the size of the job A region available for use by an incoming job is a hole
10 Figure
11 Figure 9.4 Job Size Action J1 12 Start J2 Start J 12 Start J4 4 Start J5 4 Start J1 12 Stop J 4 Start J5 4 Stop J7 Start J Start
12 Figure 9.5 Best-fit algorithm J1 J7 J2 J2 J2 J2 J2 J J J J J J4 J4 J4 J4 J4 J5 J5 J5 J J J (a) J1 to J5 starts. (b) J1 stops. (c) J starts. (d) J5 stops. (e) J7 starts.
13 Figure 9. Compacting main memory J7 J7 J2 J J4 J J2 J J4 J (a) Shifting all jobs to the top. (b) Shifting only J.
14 Figure 9.7 First-fit algorithm J J J J1 J7 J2 J2 J2 J2 J2 J J J J J J4 J4 J4 J4 J4 J5 J5 J5 (a) J1 to J5 starts. (b) J1 stops. (c) J starts. (d) J5 stops. (e) J7 starts.
15 Problems with variable partitions Fragmentation Consolidating holes is time-consuming
16 Paging Rather than coalesce several small holes to fit the program, fragment the program to fit the holes A job is divided into pages Main memory is divided into frames, each one the same size as a page No coalescing of holes is ever required
17 Figure 9. Frame address (hex) Frame number (dec) J2, P1 4 1 J1 J2 J J, P2 2 Page Page Page J1, P1 C Page 1 Page 1 Page Page 2 Page Page J, P 1 Jobs divided into 1-kilobyte pages J2, P J, P J1, P2 1C J1, P 2 1 J, P1 2C 11 Memory divided into 1-kilobyte frames
18 Figure 9.9 Page number Offset bits 1 bits (a) Logical address. Frame number Offset bits 1 bits (b) Physical address. Page tables
19 Figure 9.1 CPU Logical address 5F Page table Physical address 2DF J2, P1 J, P2 J1, P1 J, P J2, P J, P J1, P2 J1, P J, P1 Frame address (hex) 4 C C C Frame numberr (dec) Frame number (bin)
20 Figure 9.11 Internal fragmentation P P1 P2 P
21 Problem with paging To execute a program, the entire program must be loaded into memory Most large programs have many sections of code that never execute Memory is used inefficiently with parts of the program taking up main memory unnecessarily
22 Virtual memory Cycle pages of the program from disk into memory only when they need to be executed The page that is executing in memory together with the pages in memory that have recently been executed is the program s working set As the program progresses, pages enter and leave the working set
23 Figure 9.12 Disk Page tables Main memory Frame table J1 P P1 P2 P P4 P5 P P7 P P9 J2 P P1 J P P1 P2 P J1 Frame Number Loaded 4 5 Y Y Y N N N N N N N Frame Number 1 2 J2 Loaded Y Y J Frame Number Loaded Y Y N N J, P1 J2, P J, P J2, P1 J1, P J1, P1 J1, P Job Dirty Y Y N Y Y N N
24 Page tables One page table for each program Converts logical address to physical address as in paging Loaded bit is 1 if the page is in memory A page fault occurs if the program needs to read or write a page that is not in memory Page is loaded into an empty frame If no empty frames, then a page is replaced
25 Frame tables One frame table with an entry for each frame Dirty bit initialized to when page is first loaded into memory Set to 1 on a STWr to the frame, not on a LDWr from the frame When a page is replaced it is written back to disk only if the dirty bit is set to 1
26 Frame allocation Before a job starts executing, how many frames should be allocated for that job? System can allocate frames proportional to the physical size of the code
27 Page replacement First in, first out (FIFO) On a page fault, select the page to be replaced as the one that first entered the set of loaded pages Least recently used (LRU) On a page fault, select the page to be replaced as the one that was least recently read from or written to
28 F F F F F F F Page references Loaded pages Page fault First In, First Out (FIFO) Figure 9.1
29 F F F F F F Page references Loaded pages Page fault First In, First Out (FIFO) Figure 9.14
30 Bélády s anomoly In general, the greater the number of frames allocated to a program, the fewer the number of page faults In a few cases with FIFO, an increase in the number of frames increases the number of page faults Example page reference sequence, 1, 2,,, 1, 4,, 1, 2,, 4
31 Figure 9.15 Number of faults Number of faults Anomaly Number of frames (a) Expected effect of more frames on the number of page faults Number of frames (b) Bélády s anomaly with the FIFO replacement algorithm.
32 F F F F F F Least Recently Used (LRU) Figure 9.1
33 File management Create a new file Delete a file Rename a file Open a file for editing Read the next data item from the file
34 Figure 9.17 Spindle Read/write head Arm Block Cylinder Track Sector (a) A hard disk drive. (b) A single disk.
35 Contributions to the Seek time disk access time Time for head to reach cylinder Latency Time for start of sector to rotate to head Transmission time Time for sector to pass under head
36 Contiguous Linked Indexed File allocation techniques
37 Figure 9.1 Contiguous allocation Directory File Start Size TermPaper 5 MailingList
38 Figure 9.19 Linked allocation Directory File Start TermPaper MailingList
39 Figure 9.2 Indexed allocation Directory File TermPaper Blocks 5, 11, 12, 14, 2, MailingList 1, 19,
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