1. Silberschatz, Galvin and Gagne  2002 13.1 Operating System Concepts Chapter 13+14: I/O Systems and Mass- Storage Structure I/O Hardware Application I/O Interface Kernel I/O Subsystem Disk Structure Disk Scheduling Disk Management Swap-Space Management

2. Silberschatz, Galvin and Gagne  2002 13.2 Operating System Concepts I/O Hardware Incredible variety of I/O devices Common concepts  Port  Bus (daisy chain or shared direct access)  Controller (host adapter) I/O instructions control devices Devices have addresses, used by  Direct I/O instructions  Memory-mapped I/O

3. Silberschatz, Galvin and Gagne  2002 13.3 Operating System Concepts A Typical PC Bus Structure

4. Silberschatz, Galvin and Gagne  2002 13.4 Operating System Concepts Interrupts CPU Interrupt request line triggered by I/O device Interrupt handler receives interrupts Maskable to ignore or delay some interrupts Interrupt vector to dispatch interrupt to correct handler  Based on priority  Some unmaskable Interrupt mechanism also used for exceptions

5. Silberschatz, Galvin and Gagne  2002 13.5 Operating System Concepts Interrupt-Driven I/O Cycle

6. Silberschatz, Galvin and Gagne  2002 13.6 Operating System Concepts Intel Pentium Processor Event-Vector Table

7. Silberschatz, Galvin and Gagne  2002 13.7 Operating System Concepts Direct Memory Access Used to avoid programmed I/O for large data movement Requires DMA controller Bypasses CPU to transfer data directly between I/O device and memory

8. Silberschatz, Galvin and Gagne  2002 13.8 Operating System Concepts Six Step Process to Perform DMA Transfer

9. Silberschatz, Galvin and Gagne  2002 13.9 Operating System Concepts Application I/O Interface I/O system calls encapsulate device behaviors in generic classes Device-driver layer hides differences among I/O controllers from kernel Devices vary in many dimensions  Character-stream or block  Sequential or random-access  Sharable or dedicated  Speed of operation  read-write, read only, or write only

10. Silberschatz, Galvin and Gagne  2002 13.10 Operating System Concepts A Kernel I/O Structure

11. Silberschatz, Galvin and Gagne  2002 13.11 Operating System Concepts Characteristics of I/O Devices

12. Silberschatz, Galvin and Gagne  2002 13.12 Operating System Concepts Block and Character Devices Block devices include disk drives  Commands include read, write, seek  Raw I/O or file-system access  Memory-mapped file access possible Character devices include keyboards, mice, serial ports  Commands include get, put  Libraries layered on top allow line editing

13. Silberschatz, Galvin and Gagne  2002 13.13 Operating System Concepts Blocking and Nonblocking I/O Blocking - process suspended until I/O completed  Easy to use and understand  Insufficient for some needs Nonblocking - I/O call returns as much as available  User interface, data copy (buffered I/O)  Implemented via multi-threading  Returns quickly with count of bytes read or written Asynchronous - process runs while I/O executes  Difficult to use  I/O subsystem signals process when I/O completed

14. Silberschatz, Galvin and Gagne  2002 13.14 Operating System Concepts Kernel I/O Subsystem Scheduling  Some I/O request ordering via per-device queue  Some OSs try fairness Buffering - store data in memory while transferring between devices  To cope with device speed mismatch  To cope with device transfer size mismatch  To maintain “copy semantics”

15. Silberschatz, Galvin and Gagne  2002 13.15 Operating System Concepts Kernel I/O Subsystem Caching - fast memory holding copy of data  Always just a copy  Key to performance Spooling - hold output for a device  If device can serve only one request at a time  i.e., Printing Device reservation - provides exclusive access to a device  System calls for allocation and deallocation  Watch out for deadlock

16. Silberschatz, Galvin and Gagne  2002 13.16 Operating System Concepts Life Cycle of An I/O Request

17. Silberschatz, Galvin and Gagne  2002 13.17 Operating System Concepts Disk Structure Disk drives are addressed as large 1-dimensional arrays of logical blocks, where the logical block is the smallest unit of transfer. The 1-dimensional array of logical blocks is mapped into the sectors of the disk sequentially.  Sector 0 is the first sector of the first track on the outermost cylinder.  Mapping proceeds in order through that track, then the rest of the tracks in that cylinder, and then through the rest of the cylinders from outermost to innermost.

18. Silberschatz, Galvin and Gagne  2002 13.18 Operating System Concepts Disk Scheduling The operating system is responsible for using hardware efficiently — for the disk drives, this means having a fast access time and disk bandwidth. Access time has two major components  Seek time is the time for the disk are to move the heads to the cylinder containing the desired sector.  Rotational latency is the additional time waiting for the disk to rotate the desired sector to the disk head. Minimize seek time Seek time  seek distance Disk bandwidth is the total number of bytes transferred, divided by the total time between the first request for service and the completion of the last transfer.

19. Silberschatz, Galvin and Gagne  2002 13.19 Operating System Concepts Disk Scheduling (Cont.) Several algorithms exist to schedule the servicing of disk I/O requests. We illustrate them with a request queue (0-199). 98, 183, 37, 122, 14, 124, 65, 67 Head pointer 53

20. Silberschatz, Galvin and Gagne  2002 13.20 Operating System Concepts FCFS Illustration shows total head movement of 640 cylinders.

21. Silberschatz, Galvin and Gagne  2002 13.21 Operating System Concepts SSTF Selects the request with the minimum seek time from the current head position. SSTF scheduling is a form of SJF scheduling; may cause starvation of some requests. Illustration shows total head movement of 236 cylinders.

22. Silberschatz, Galvin and Gagne  2002 13.22 Operating System Concepts SSTF (Cont.)

23. Silberschatz, Galvin and Gagne  2002 13.23 Operating System Concepts SCAN The disk arm starts at one end of the disk, and moves toward the other end, servicing requests until it gets to the other end of the disk, where the head movement is reversed and servicing continues. Sometimes called the elevator algorithm. Illustration shows total head movement of 208 cylinders.

24. Silberschatz, Galvin and Gagne  2002 13.24 Operating System Concepts SCAN (Cont.)

25. Silberschatz, Galvin and Gagne  2002 13.25 Operating System Concepts C-SCAN Provides a more uniform wait time than SCAN. The head moves from one end of the disk to the other. servicing requests as it goes. When it reaches the other end, however, it immediately returns to the beginning of the disk, without servicing any requests on the return trip. Treats the cylinders as a circular list that wraps around from the last cylinder to the first one.

26. Silberschatz, Galvin and Gagne  2002 13.26 Operating System Concepts C-SCAN (Cont.)

27. Silberschatz, Galvin and Gagne  2002 13.27 Operating System Concepts C-LOOK Version of C-SCAN Arm only goes as far as the last request in each direction, then reverses direction immediately, without first going all the way to the end of the disk.

28. Silberschatz, Galvin and Gagne  2002 13.28 Operating System Concepts C-LOOK (Cont.)

29. Silberschatz, Galvin and Gagne  2002 13.29 Operating System Concepts Selecting a Disk-Scheduling Algorithm SSTF is common and has a natural appeal SCAN and C-SCAN perform better for systems that place a heavy load on the disk. Performance depends on the number and types of requests. Requests for disk service can be influenced by the file- allocation method. The disk-scheduling algorithm should be written as a separate module of the operating system, allowing it to be replaced with a different algorithm if necessary. Either SSTF or LOOK is a reasonable choice for the default algorithm.

30. Silberschatz, Galvin and Gagne  2002 13.30 Operating System Concepts Disk Management Low-level formatting, or physical formatting — Dividing a disk into sectors that the disk controller can read and write. To use a disk to hold files, the operating system still needs to record its own data structures on the disk.  Partition the disk into one or more groups of cylinders.  Logical formatting or “making a file system”. Boot block initializes system.  The bootstrap is stored in ROM.  Bootstrap loader program. Methods such as sector sparing used to handle bad blocks.

31. Silberschatz, Galvin and Gagne  2002 13.31 Operating System Concepts MS-DOS Disk Layout

32. Silberschatz, Galvin and Gagne  2002 13.32 Operating System Concepts Swap-Space Management Swap-space — Virtual memory uses disk space as an extension of main memory. Swap-space can be carved out of the normal file system,or, more commonly, it can be in a separate disk partition. Swap-space management  4.3BSD allocates swap space when process starts; holds text segment (the program) and data segment.  Kernel uses swap maps to track swap-space use.  Solaris 2 allocates swap space only when a page is forced out of physical memory, not when the virtual memory page is first created.