I/O Hardware

1. I/O Hardware • Incredible variety of I/O devices Operating System Concepts

2. I/O Hardware • Devices vary in many dimensions • Direction • Read, Write, Read-Write • Character, Block • Speed • Latency, Transfer rate, Delay between operations • Access • Sequential, Random • Sharing • Sharable, Dedicated • IO subsystem reduces perceived differences for apps, and optimizes performances for apps Operating System Concepts

3. I/O Hardware • Common concepts (provide abstraction) • Port (serial, parallel, ethernet) • Bus (daisy chain or shared direct access) • Controller (host adapter operates ports/bus/device) • See my picture Operating System Concepts

4. A Kernel I/O Structure Operating System Concepts

5. 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 • Makes OS independent of the IO hardware • Provided by the device/controller manufacturers • DDs are part of the OS (not processes) usually • OS defines interface to DDs • Non-standard across OSs => device manufacturers have to provide a DD for each OS (bugger) • Applications normally reach the DDs via the OS • Escape entry (e.g., ioctl) allows more direct access Operating System Concepts

6. Kernel I/O Subsystem • Scheduling • To maximize performance • I/O request ordering via per-device queue • Some OSs try fairness • Device reservation - provides exclusive access to a device (e.g., in VMS) • System calls for allocation and deallocation • Wait for device on call, e.g., NT • Watch out for deadlock Operating System Concepts

7. Kernel I/O Subsystem • Buffering - store data in memory while transferring between devices • To cope with device speed mismatch • E.g., modem to disk (x1000) • Double buffering • To cope with device transfer size mismatch • E.g., keyboard to disk • Collating network packets • To maintain “copy semantics” • Caching - fast memory holding copy of data • Always just a copy (as opposed to a buffer) • Often implemented in buffer system • Key to performance Operating System Concepts

8. Error Handling • OS can recover from transient failures E.g., disk read, device unavailable, network failures • Permanent failures • OS can make devices unavailable • Need operator intervention • System calls return an error code when I/O fails • System error logs hold problem reports • More detailed information than return values • HW diagnostic information, e.g., from SCSI controllers Operating System Concepts

9. Blocking and Non-blocking I/O • Blocking - process suspended until I/O completed • Easy to use and understand • Insufficient for some needs • Non-blocking - I/O call returns as much as available • E.g., user interface, data copy (buffered I/O) • Can be 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 Operating System Concepts

10. Life Cycle of An I/O Request • Request may be satisfied immediately, e.g., in cache • The request for the DD may have to be queued • CPU runs async with device • DD waits in sync with device • It’s blocking I/O. • Non-blocking I/O always “can satisfy request” • Asynchronous I/O does not block the process • Direct I/O instructions • Placed in registers • Status, control, data-in, data-out • Memory-mapped I/O • Maps registers onto RAM • Can be faster than I/O instructions Operating System Concepts

11. An Alternative - Polling • Synchronous communication • Controller waits for command-ready bit in controller status register bit to be set • Host waits for busy bit in controller status register to be clear (initially it is clear) • Host places command in command register, and any required data in data register • Host sets command-ready bit, and waits for it to clear • Controller notices command-ready bit, sets busy bit, clears command-ready bit. • Host loops waiting for busy bit to clear, while controller does IO • Controller clears busy and command-ready bits, and loops • Vantages • Done once for each byte • Wasteful of CPU time if IO takes long • Can use offboard CPU, e.g., in SCSI controller • Useful for fast data streams Operating System Concepts

12. Direct Memory Access • Used to avoid programmed I/O for large data movement • If device transmits close to memory speeds, little time is left for processing • Requires DMA controller • Bypasses CPU to transfer data directly between I/O device and memory • DMA controller steals RAM cycles from CPU Operating System Concepts

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

14. Spooling • Spooling - hold output for a device • If device can serve only one request at a time • Provides asynchronous I/O • i.e., Printing and the lpd Operating System Concepts

15. Network Devices • Approaches vary widely • Pipes • FIFOs • Streams • Queues • Mailboxes • Socket interface • Separates network protocol from network operation • Includes select functionality Operating System Concepts

16. Kernel Data Structures • Kernel keeps state info for I/O components, including open file tables, network connections, character device state • Many, many complex data structures to track buffers, memory allocation, “dirty” blocks • Some use object-oriented methods and message passing to implement I/O, e.g., NT, Nachos, nu Operating System Concepts

17. 4.3 BSD Kernel I/O Structure • Cooked interfaces are buffered • Block buffers • C-lists • Raw interfaces are unbuffered • Devices have major and minor device numbers. • Major number used as index into array of DD entry points • Direct access via ioctl() and /dev files Operating System Concepts

18. Block Buffer Cache • Consist of buffer headers, each of which can point to a piece of physical memory, and contain a device number and a block number on the device. • The buffer headers for blocks not currently in use are kept in several linked lists: • Buffers not recently used, or with invalid contents (AGE list). • Buffers recently used, linked in LRU order (LRU list). • Buffers with no associated physical memory (EMPTY list). • On read the cache is searched. • If the block is found it is used - no I/O transfer is necessary. • If it is not found, a buffer is chosen from the AGE list, or the LRU list if AGE is empty. • On write, if the block is in the cache, then write and set dirty bit • Dirty blocks are output by regular sync() • Blocks may be fragmented, and headers are taken from EMPTY Operating System Concepts

19. Raw Device Interfaces • The raw deviceinterface — unlike the block interface, it bypasses the block buffer cache. • Each disk driver maintains a queue of pending transfers: • whether it is a read or a write • a main memory address for the transfer • a device address for the transfer • a transfer size • Can use user memory for a transfer record Operating System Concepts

20. C-Lists • Terminal drivers use a character buffering system which involves keeping small blocks of characters in linked lists. • A write system call to a terminal enqueues characters on a list for the device. An initial transfer is started, and interrupts cause dequeueing of characters and further transfers, i.e., it’s asynchronous • Input is similarly interrupt driven. • It is also possible to have the device driver bypass the canonical queue and return characters directly from the raw queue — raw mode (used by full-screen editors and other programs that need to react to every keystroke). Operating System Concepts