Virtual Memory cont d.; File System Interface. 03/30/2007 CSCI 315 Operating Systems Design 1

Transcription

1 Virtual Memory cont d.; File System Interface 03/30/2007 CSCI 315 Operating Systems Design 1 1

2 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. 03/30/2007 CSCI 315 Operating Systems Design 2 2

3 Thrashing Why does paging work? Locality model Process migrates from one locality to another. Localities may overlap. Why does thrashing occur? Σ size of locality > total memory size 03/30/2007 CSCI 315 Operating Systems Design 3 3

4 Locality in Memory-Reference Pattern 03/30/2007 CSCI 315 Operating Systems Design 4 4

5 Working-Set Model working-set window a fixed number of page references. WSS i (working set of Process P i ) = total number of pages referenced in the most recent (varies in time) if too small will not encompass entire locality. if too large will encompass several localities. if = will encompass entire program. D = Σ WSS i total demand frames if D > mthrashing Policy if D > m, then suspend one of the processes. 03/30/2007 CSCI 315 Operating Systems Design 5 5

6 Working-set model 03/30/2007 CSCI 315 Operating Systems Design 6 6

7 Keeping Track of the Working Set Approximate with interval timer + a reference bit Example: = 10,000 Timer interrupts after every 5000 time units. Keep in memory 2 bits for each page. Whenever a timer interrupts copy and sets the values of all reference bits to 0. If one of the bits in memory = 1 page in working set. Why is this not completely accurate? Improvement = 10 bits and interrupt every 1000 time units. 03/30/2007 CSCI 315 Operating Systems Design 7 7

8 Page-Fault Frequency Scheme Establish acceptable page-fault rate. If actual rate too low, process loses frame. If actual rate too high, process gains frame. 03/30/2007 CSCI 315 Operating Systems Design 8 8

9 Memory-mapped Files Memory mapping a file can be accomplished by mapping a disk block to one or more pages in memory. A page-sized portion of the file is read from the file system into a physical page. Subsequent read() and write() operations are handled as memory (not disk) accesses. Writing to the file in memory is not necessarily synchronous to the file on disk. The file can be committed back to disk when it s closed. 03/30/2007 CSCI 315 Operating Systems Design 9 9

10 Memory-mapped Files process A virtual memory process B virtual memory disk file 03/30/2007 CSCI 315 Operating Systems Design 10 10

11 Prepaging Prepaging: In order to avoid the initial number of page faults, the system can bring into memory all the pages that will be needed all at once. This can also be applied when a swapped-out process is restarted. The smart thing to do is to remember the working set of the process. One question that arises is whether all the pages brought in will actually be used Is the cost of prepaging less than the cost of servicing each individual page fault? 03/30/2007 CSCI 315 Operating Systems Design 11 11

12 File System Topics File Concept Access Methods Directory Structure File System Mounting File Sharing Protection 03/30/2007 CSCI 315 Operating Systems Design 12 12

13 File Concept Contiguous logical address space. File types: Data: numeric. character. binary. Program (executable). 03/30/2007 CSCI 315 Operating Systems Design 13 13

14 File Structure None: just a sequence of words or bytes. Simple record structure: Lines, Fixed length, Variable length. Complex Structures: Formatted document, Relocatable load file. Can simulate last two with first method by inserting appropriate control characters. Who decides: Operating system, Program. 03/30/2007 CSCI 315 Operating Systems Design 14 14

15 File Attributes Name only information kept in human-readable form. Type needed for systems that support different types. Location pointer to file location on device. Size current file size. Protection controls who can do reading, writing, executing. Time, date, and user identification data for protection, security, and usage monitoring. Information about files is kept in the directory structure, which is maintained on the disk. 03/30/2007 CSCI 315 Operating Systems Design 15 15

16 File Operations Create. Write. Read. Seek. Delete. Truncate (reset size to 0, keep current attributes). Open(F i ) search the directory structure on disk for entry F i, and move the content of entry to memory. Close (F i ) move the content of entry F i in memory to directory structure on disk. 03/30/2007 CSCI 315 Operating Systems Design 16 16

17 File Types: Name and Extension 03/30/2007 CSCI 315 Operating Systems Design 17 17

18 Access Methods Sequential Access read next write next reset no read after last write (rewrite) Direct Access read n write n position to n read next write next rewrite n n = relative block number 03/30/2007 CSCI 315 Operating Systems Design 18 18

19 Sequential-access File 03/30/2007 CSCI 315 Operating Systems Design 19 19

20 Simulation of Sequential Access on a Direct-access File 03/30/2007 CSCI 315 Operating Systems Design 20 20

21 Example of Index and Relative Files 03/30/2007 CSCI 315 Operating Systems Design 21 21

22 Directory Structure Directory: a symbol table that translates file names into directory entries. ping emacs ifconfig mount fdisk find Both the directory structure and the files reside on disk. Backups of these two structures are kept on tapes. 03/30/2007 CSCI 315 Operating Systems Design 22 22

23 Partitions and Directories (File system organization) 03/30/2007 CSCI 315 Operating Systems Design 23 23

24 Operations on Directories Search for a file. Create a file. Delete a file. List a directory. Rename a file. Traverse the file system. 03/30/2007 CSCI 315 Operating Systems Design 24 24

25 Goals of Directory Logical Organization Efficiency locating a file quickly. Naming convenient to users. Two users can have same name for different files. The same file can have several different names. Grouping logical grouping of files by properties, (e.g., all Java programs, all games, ) 03/30/2007 CSCI 315 Operating Systems Design 25 25

26 Single-Level Directory A single directory for all users. Drawbacks: Naming problem Grouping problem 03/30/2007 CSCI 315 Operating Systems Design 26 26

27 Two-Level Directory A separate directory for each user. Path name. Can have the same file name for different user. Efficient searching. No grouping capability. 03/30/2007 CSCI 315 Operating Systems Design 27 27

28 Tree-Structured Directories 03/30/2007 CSCI 315 Operating Systems Design 28 28

29 Tree-Structured Directories (Cont.) Efficient searching. Grouping Capability. Current directory (working directory): cd /spell/mail/prog, type list. 03/30/2007 CSCI 315 Operating Systems Design 29 29

30 Tree-Structured Directories (Cont.) Absolute or relative path name. Creating a new file is done in current directory by default. Delete a file rm <file-name> Creating a new subdirectory is done in current directory. mkdir <dir-name> Example: if in current directory /mail mkdir count mail prog copy prt exp count Deleting mail deleting the entire subtree rooted by mail. 03/30/2007 CSCI 315 Operating Systems Design 30 30

31 Acyclic-Graph Directories Have shared subdirectories and files. 03/30/2007 CSCI 315 Operating Systems Design 31 31

32 Acyclic-Graph Directories (Cont.) Two different names (aliasing). If dict deletes listdangling pointer. Solutions: Backpointers, so we can delete all pointers. Variable size records a problem. Backpointers using a daisy chain organization. Entry-hold-count solution. 03/30/2007 CSCI 315 Operating Systems Design 32 32

33 General Graph Directory 03/30/2007 CSCI 315 Operating Systems Design 33 33

34 General Graph Directory (Cont.) How do we guarantee no cycles? Allow only links to file not subdirectories. Garbage collection. Every time a new link is added use a cycle detection algorithm to determine whether it is OK. 03/30/2007 CSCI 315 Operating Systems Design 34 34

35 File System Mounting A file system (partition) must be mounted before it can be accessed. A unmounted file system needs to be attached to a mount point before it can be accessed. existing unmounted 03/30/2007 CSCI 315 Operating Systems Design 35 35

36 File Sharing Sharing of files on multi-user systems is desirable. Sharing may be done through a protection scheme. On distributed systems, files may be shared across a network. Network File System (NFS) is a common distributed filesharing method. 03/30/2007 CSCI 315 Operating Systems Design 36 36

37 Protection File owner/creator should be able to control: what can be done, by whom. Types of access: Read, Write, Execute, Append, Delete, List. 03/30/2007 CSCI 315 Operating Systems Design 37 37

38 Access Lists and Groups Mode of access: read, write, execute Three classes of users RWX a) owner access RWX b) group access RWX c) public access Ask manager to create a group (unique name), say G, and add some users to the group. For a particular file (say game) or subdirectory, define an appropriate access. owner group public chmod 761 game Associate a group with a file: chgrp G game 03/30/2007 CSCI 315 Operating Systems Design 38 38