Chapter 13: I/O Systems - 6 th ed

1. Chapter 13: I/O Systems- 6th ed • I/O Hardware • Application I/O Interface • Kernel I/O Subsystem • Transforming I/O Requests to Hardware Operations • Streams • Performance Review Chapters 2 and 3, and instructors notes on: “Interrupt schemes and DMA” This chapter gives more focus to these chapters and topics. Instructor’s annotations in blue Updated 12/5/03 Operating System Concepts

2. I/O Hardware • Incredible variety of I/O devices • Common concepts • Port - basic interface to CPU - status, control, data • Bus (daisy chain or shared direct access) - main and specialized local (ex: PCI for main and SCSI for disks) • Controller (host adapter) - HW interface between Device and Bus - an adapter card or mother board moduleController has special purposes registers (commands, etc.) which when written to causes actions to take place - may be memory mapped • I/O instructions control devices - ex: in, out for Intel • Devices have addresses, used by • Direct I/O instructions - uses I/O instructions • Memory-mapped I/O - uses memory instructions Operating System Concepts

3. A Typical PC Bus Structure Operating System Concepts

4. Device I/O Port Locations on PCs (partial) Various ranges for a device includes both control and data ports Operating System Concepts

5. Polling • Handshaking • Determines state of device • command-ready • busy • Error • Busy-wait cycle to wait for I/O from deviceWhen not busy - set data in data port, set command in control port and let ‘er rip • Not desirable if excessive - since it is a busy wait which ties up CPU & interferes with productive work • Remember CS220 LABs Operating System Concepts

6. Interrupts • CPU Interrupt request line (IRQ) 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 • Application can go away after I/O request, but is til responsible for transferring data to memory when it becomes available from the device. • Can have “nested” interrupts (with Priorities) • See Instructors notes: “Use of Interrupts and DMA” • Soft interrupts or “traps” generated from OS in system calls. Operating System Concepts

7. Interrupt-Driven I/O Cycle Go away & do Something else ==> Operating System Concepts

8. Intel Pentium Processor Event-Vector Table Interrupts 0-31 are non-maskable - cannot be disabled Operating System Concepts

9. Direct Memory Access • With pure interrupt scheme, CPU was still responsible for transferring data from controller to memory (on interrupt) when device mad it available. • Now DMA will do this - all CPU has to do is set up DMA and user the data when the DMA-complete interrupt arrives. … Interrupts still used - but only to signal DMA Complete. • 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 • Cycle stealing: interference with CPU memory instructions during DMA transfer. - DMA takes priority - CPU pauses on memory part of word. Operating System Concepts

10. Six Step Process to Perform DMA Transfer Operating System Concepts

11. Application I/O Interface • The OS software interface to the I/O devices (an API to the programmer) • Attempts to abstract the characteristics of the many I/o devices into a few general classes. • 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 • units for data transfer bytes vs blocks • Sequential or random-access - access methods • Synchronous (predictable response times) vs asynchronous (unpredictable response times) • Sharable or dedicated - implications on deadlock • Speed of operation - device/software issue • read-write, read only, or write only - permissions Operating System Concepts

12. A Kernel I/O Structure System calls ==> … “user” API ==> Example: ioctl(…) generic call (roll your own) in UNIX (p. 468), and other more specific commands or calls open, read, ... Fig. 13.6 Operating System Concepts

13. Characteristics of I/O Devices Device driver must deal with these at a low level Use of I/O buffering Operating System Concepts

14. Block and Character Devices • Block devices include disk drives • example sectors or sector clusters on a disk • Commands/calls include read, write, seek • Access is typically through a file-system interface • Raw I/O or file-system access - “binary xfr” of file data - interpretation is in application (personality of file lost) • Memory-mapped (to VM) file access possible - use memory instructions rather than I/O instructions - very efficient (ex: swap space for disk). • Device driver xfr’s blocks at a time - as in paging • DMA transfer is block oriented • Character devices include keyboards, mice, serial ports • Device driver xfr’s byte at a time • Commands include get, put - character at a time • Libraries layered on top allow line editing- ex: keyboard input • could be beefed up to use a line at a time (buffering) • Block & character devices also determine the two general device driver catagories Operating System Concepts

15. Network Devices • Varying enough from block and character to have own interface - OS makes network device interface distinct from disk interface - due to significant differences between the two • Unix and Windows NT/9i/2000 include socket interface • Separates network protocol from network operation • Encapsulates details of various network devices for application … analogous to a file and the disk??? • Includes select functionality - used to manage and access sockets - returns info on packets waiting or ability to accept packets - avoids polling • Approaches vary widely (pipes, FIFOs, streams, queues, mailboxes) … you saw some of these! Operating System Concepts

16. Clocks and Timers • Provide current time, elapsed time, timer • If programmable, interval time used for timings, periodic interrupts • ioctl (on UNIX) covers odd aspects of I/O such as clocks and timers - a back door for device driver writers (roll your own). Can implement “secret” calls which may not be documented in a users or programming manual Operating System Concepts

17. Blocking and Nonblocking I/O • Blocking - process (making the request blocks - lets other process execute) suspended until I/O completed • Easy to use and understand • Insufficient for some needs • multi-threading - depends on role of OS in thread management • 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 - ex: read a “small” portion of a file very quickly, use it, and go back for more, ex: displaying video “continuously from a disk” • Asynchronous - process (making the asynch request)runs while I/O executes • Difficult to use - can it continue without the results of the I/O? • I/O subsystem signals process when I/O completed - via interrupt (soft), or setting of shared variable which is periodically tasted. Operating System Concepts

18. Kernel I/O Subsystem • See A Kernel I/O Structure slide - Fig 13.6 • 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 - de-couples application from device action • To cope with device transfer size mismatch • To maintain “copy semantics” - guarantee that the version of data written to device from a buffer is identical to that which was there at the time of the “write call” - even if on return of the system call, the user modifies buffer - OS copies data to kernel buffer before returning control to user. • Double or “ping-pong” buffers - write in one and read from another - decouples devices and applications… idea can be extended to multiple buffers accesses in a circular fashion Operating System Concepts

19. Sun Enterprise 6000 Device-Transfer Rates Operating System Concepts

20. Kernel I/O Subsystem - (continued) • Caching - fast memory holding copy of data • Always just a copy • Key to performance • How does this differ from a buffer? • Spooling - a buffer holding output/(input too) for a device • If device can serve only one request at a time • Avoids queuing applications making requests. • Data from an application is saved in a unique file associated with the application AND the particular request. Could be saved in files on a disk, or in memory. • Example: Printing • Device reservation - provides exclusive access to a device • System calls for allocation and deallocation • Watch out for deadlock - why? Operating System Concepts

21. 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 • CRC checks - especially over network transfers of a lot of data, for example video in real time. Operating System Concepts

22. Kernel Data Structures • Kernel keeps state info for I/O components, including open file tables, network connections, character device state • used by device drivers in manipulating devices and data transfer, and in for error recovery • data that has images on the disk must be kept in synch with disk copy. • Many, many complex data structures to track buffers, memory allocation, “dirty” blocks • Some use object-oriented methods and message passing to implement I/O • Make data structures object oriented classes to encapsulate the low level nature of the “device” - UNIX provides a seamless interface such as this. Operating System Concepts

23. UNIX I/O Kernel Data Structure Refer to chapter 11 and 12 on files Fig. 13.9 Operating System Concepts

24. Mapping I/O Requests to Hardware Operations • Consider reading a file from disk for a process:How is connection made from file-name to disk controller: • Determine device holding file • Translate name to device representation • Physically read data from disk into buffer • Make data available to requesting process • Return control to process • See the 10 step scenario on pp. 479-481 (Silberschatz, 6th ed.) for a clear description. Operating System Concepts

25. Life Cycle of An I/O Request Data already in buffer Ex read ahead Operating System Concepts

26. STREAMS (?) • STREAM – a full-duplex communication channel between a user-level process and a device • A STREAM consists of: - STREAM head interfaces with the user process - driver end interfaces with the device- zero or more STREAM modules between them. • Each module contains a read queue and a write queue • Message passing is used to communicate between queues Operating System Concepts

27. The STREAMS Structure Operating System Concepts

28. Performancesect 13.7 • I/O a major factor in system performance: • Places demands on CPU to execute device driver, kernel I/O code • resulting in context switching • interrupt overhead • Data copying - loads down memory bus • Network traffic especially stressful • See bulleted list on page 485(Silberschatz, 6th ed.) • Improving PerformanceSee bulleted list on page 485(Silberschatz, 6th ed.) • Reduce number of context switches • Reduce data copying • Reduce interrupts by using large transfers, smart controllers, polling • Use DMA • Move proccessing primitives to hardware • Balance CPU, memory, bus, and I/O performance for highest throughput Operating System Concepts

29. Intercomputer Communications- omit for now Operating System Concepts

30. Device-Functionality Progression Where should I/O functionality be implemented? Application level … device hardware Decision depends on trade-offs in the design layers: Operating System Concepts