Operating Systems CMPSC 473

1. Operating SystemsCMPSC 473 I/O Management (1) November 30 2010 - Lecture 23 Instructor: Bhuvan Urgaonkar

2. Announcements • Project 3 due today • Please indicate time to meet TA on sheet • Project 4 out and due on Dec 10 • You need to study material on content addressable storage • Will release slides on this later today • Will release practice question set to help you prepare for the final exam (sometime this week) • Exam is on Dec 16

3. I/O Management: Topics • General issues in I/O management • Secondary storage management • Properties of media • Magnetic disks and Flash • Systems software for using these media • File Systems • Note: Would like you to read ahead for P4 • Material on content addressable storage: will post slides

4. I/O Hardware • Incredible variety of I/O devices • Two ways of communicating • Port • Bus • Plus … interrupts • Unidirectional: From device to CPU • Special CPU input lines/pins for these

5. Some familiar ports

6. I/O Hardware • Incredible variety of I/O devices • Two ways of communicating • Port • Bus • Controller: Electronics to operate a port/bus or a device • More complex controller: host adapter

7. Some examples PCI slots and card

8. Some examples SATA cable SATA ports on a motherboard

9. A Typical PC Bus Structure

10. I/O Hardware • I/O instructions control devices • Devices have addresses, used by • Direct I/O instructions • These are part of the ISA • E.g., IN and OUT on Intel • Memory-mapped I/O • Not to be confused with memory mapped file IO!

11. Direct V. Memory Mapped IO • Direct I/O • Uses special instructions for accessing the I/O devices • i.e., different from loads/stores • IO devices have separate address space from general memory • Either accomplished by an extra IO pin in CPU’s physical interface • Or, separate bus for IO devices • Also called isolated IO

12. Direct V. Memory Mapped IO • Memory Mapped I/O • Uses loads and stores

13. Device I/O Port Locations on PCs (partial)

14. Communication between host and device • Employs registers on the port/controller • Typically four important registers • Data-in: read by the host to get input • Data-out: written by the host to send output • Status: Bits readable by the host that indicate state of port • Has the current command been completed? • Has a device error occurred? • Is there data available to be read from Data-in register? • Control: Writable by the host to start a command, select certain properties of the port (e.g., speed) • Interaction between host and controller • Polling • Interrupts

15. Polling • Determines state of device • command-ready • busy • Error • Busy-wait cycle to wait for I/O from device

16. 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 non-maskable • Interrupt mechanism also used for exceptions

18. Intel Pentium Processor Event-Vector Table

19. 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

20. Six Step Process to Perform DMA Transfer

21. 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

22. A Kernel I/O Structure

23. Characteristics of I/O Devices

24. 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

25. Network Devices • Varying enough from block and character to have own interface • Unix and Windows NT/9x/2000 include socket interface • Separates network protocol from network operation • Includes select functionality • Approaches vary widely (pipes, FIFOs, streams, queues, mailboxes)

26. Clocks and Timers • Provide current time, elapsed time, timer • Programmable interval timer used for timings, periodic interrupts • ioctl (on UNIX) covers odd aspects of I/O such as clocks and timers

27. Synchronous I/O • Blocking - process suspended until I/O completed • Easy to use and understand • Likely to be inefficient • Quiz: When and why?

28. Asynchronous I/O • Process runs while I/O executes • Returns immediately • “Event-driven” programming • Difficult to use • I/O subsystem signals process when I/O completed

29. Two I/O Methods Synchronous Asynchronous

30. “Hybrid” IO • Nonblocking - I/O call returns as much as available • One approach: Implemented via multi-threading • Some threads do blocking I/O, while others continue executing • Other approach: OS provides non-blocking call • Does not halt execution of application for an extended time, returns quickly with count of bytes read or written • Good e.g., select () system call for network I/O

31. 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”

32. 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

33. Error Handling • OS can recover from disk read, device unavailable, transient write failures • Most return an error number or code when I/O request fails • System error logs hold problem reports

34. I/O Protection • User process may accidentally or purposefully attempt to disrupt normal operation via illegal I/O instructions • All I/O instructions defined to be privileged • I/O must be performed via system calls • Memory-mapped and I/O port memory locations must be protected too

35. Use of a System Call to Perform I/O Monitor = privileged/kernel mode

36. Life Cycle of An I/O Request

37. Performance • I/O a major factor in system performance: • Demands CPU to execute device driver, kernel I/O code • Context switches due to interrupts • Data copying

38. Improving Performance • Reduce number of context switches • Reduce data copying • Reduce interrupts by using large transfers, smart controllers, polling • Use DMA • Balance CPU, memory, bus, and I/O performance for highest throughput

39. I/O W/O a Unified Buffer Cache

40. Unified Buffer Cache • A unified buffer cache uses the same page cache to cache both memory-mapped pages and ordinary file system I/O

41. I/O Using a Unified Buffer Cache

42. Recovery • Consistency checking – compares data in directory structure with data blocks on disk, and tries to fix inconsistencies • Use system programs to back up data from disk to another storage device (floppy disk, magnetic tape, other magnetic disk, optical) • Recover lost file or disk by restoring data from backup