CSMC 412Operating SystemsProf. Ashok K Agrawala© 2006 Ashok AgrawalaSet 13November 2006 1CMSC 412 Set 13I/O Systems• I/O Hardware• Application I/O Interface• Kernel I/O Subsystem• Transforming I/O Requests to Hardware Operations• Streams• PerformanceNovember 2006 2CMSC 412 Set 13Objectives• Explore the structure of an operating system ’s I/O subsystem• Discuss the principles of I/O hardware and its complexity• Provide details of the performance aspects of I/O hardware and softwareNovember 2006 3CMSC 412 Set 13I/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/ONovember 2006 4CMSC 412 Set 13A Typical PC Bus StructureNovember 2006 5CMSC 412 Set 13Device I/O Port Locations on PCs (partial)November 2006 6CMSC 412 Set 13Polling• Determines state of device – command-ready– busy– Error• Busy-wait cycle to wait for I/O from deviceNovember 2006 7CMSC 412 Set 13Interrupts• 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 exceptionsNovember 2006 8CMSC 412 Set 13Interrupt-Driven I/O CycleNovember 2006 9CMSC 412 Set 13Intel Pentium Processor Event-Vector TableNovember 2006 10CMSC 412 Set 13Direct 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 November 2006 11CMSC 412 Set 13Six Step Process to Perform DMA TransferNovember 2006 12CMSC 412 Set 13Application 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 onlyNovember 2006 13CMSC 412 Set 13A Kernel I/O StructureNovember 2006 14CMSC 412 Set 13Characteristics of I/O DevicesNovember 2006 15CMSC 412 Set 13Block 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 editingNovember 2006 16CMSC 412 Set 13Network 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)November 2006 17CMSC 412 Set 13Clocks 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 timersNovember 2006 18CMSC 412 Set 13Blocking 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 completedNovember 2006 19CMSC 412 Set 13Two I/O MethodsSynchronousAsynchronousNovember 2006 20CMSC 412 Set 13Kernel 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 m aintain “copy sem antics”November 2006 21CMSC 412 Set 13Device-status TableNovember 2006 22CMSC 412 Set 13Sun Enterprise 6000 Device-Transfer RatesNovember 2006 23CMSC 412 Set 13Kernel 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 deadlockNovember 2006 24CMSC 412 Set 13Error 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 reportsNovember 2006 25CMSC 412 Set 13I/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 tooNovember 2006 26CMSC 412 Set 13Use of a System Call to Perform I/ONovember 2006 27CMSC 412 Set 13Kernel 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, m em ory allocation, “dirty” blocks• Some use object-oriented methods and message passing to implement I/ONovember 2006 28CMSC 412 Set 13UNIX I/O Kernel StructureNovember 2006 29CMSC 412 Set 13I/O Requests to Hardware Operations• Consider reading a file from disk for a process:– 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 processNovember 2006 30CMSC 412 Set 13Life Cycle of An I/O RequestNovember 2006 31CMSC 412 Set 13STREAMS• STREAM – a full-duplex communication channel between a user-level process and a device in Unix System V and beyond• 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 queuesNovember 2006 32CMSC 412 Set 13The STREAMS
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