113.1Operating System ConceptsCSMC 412Operating SystemsProf. Ashok K Agrawala© 2004 Ashok AgrawalaSet 1313.2Operating System ConceptsI/O Systems I/O Hardware Application I/O Interface Kernel I/O Subsystem Transforming I/O Requests to Hardware Operations Streams Performance213.3Operating System ConceptsI/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/O13.4Operating System ConceptsA Typical PC Bus Structure313.5Operating System ConceptsDevice I/O Port Locations on PCs (partial)13.6Operating System ConceptsPolling Determines state of device ) command-ready) busy) Error Busy-wait cycle to wait for I/O from device413.7Operating System ConceptsInterrupts 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 unmaskableInterrupt mechanism also used for exceptions13.8Operating System ConceptsInterrupt-Driven I/O Cycle513.9Operating System ConceptsIntel Pentium Processor Event-Vector Table13.10Operating System ConceptsDirect 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 memory613.11Operating System ConceptsSix Step Process to Perform DMA Transfer13.12Operating System ConceptsApplication 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 only713.13Operating System ConceptsA Kernel I/O Structure13.14Operating System ConceptsCharacteristics of I/O Devices813.15Operating System ConceptsBlock and Character Devices Block devices include disk drives) Commands include read, write, seek ) Raw I/O or file-system access) Memory-mapped file access possibleCharacter devices include keyboards, mice, serial ports) Commands include get, put) Libraries layered on top allow line editing13.16Operating System ConceptsNetwork Devices Varying enough from block and character to have own interface Unix and Windows NT/9i/2000 include socket interface) Separates network protocol from network operation) Includes select functionalityApproaches vary widely (pipes, FIFOs, streams, queues, mailboxes)913.17Operating System ConceptsClocks 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 timers13.18Operating System ConceptsBlocking and Nonblocking I/O Blocking - process suspended until I/O completed) Easy to use and understand) Insufficient for some needsNonblocking - 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 writtenAsynchronous - process runs while I/O executes) Difficult to use) I/O subsystem signals process when I/O completed1013.19Operating System ConceptsKernel I/O Subsystem Scheduling) Some I/O request ordering via per-device queue) Some OSs try fairnessBuffering - 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”13.20Operating System ConceptsSun Enterprise 6000 Device-Transfer Rates1113.21Operating System ConceptsKernel I/O Subsystem Caching - fast memory holding copy of data) Always just a copy) Key to performanceSpooling - hold output for a device) If device can serve only one request at a time ) i.e., PrintingDevice reservation - provides exclusive access to a device) System calls for allocation and deallocation) Watch out for deadlock13.22Operating System ConceptsError 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 reports1213.23Operating System ConceptsKernel 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/O13.24Operating System ConceptsUNIX I/O Kernel Structure1313.25Operating System ConceptsI/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 process13.26Operating System ConceptsLife Cycle of An I/O Request1413.27Operating System ConceptsSTREAMS 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 queues13.28Operating System ConceptsThe STREAMS Structure1513.29Operating System ConceptsPerformance 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) Network traffic especially stressful13.30Operating System ConceptsIntercomputer Communications1613.31Operating System ConceptsImproving 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 throughput13.32Operating System ConceptsDevice-Functionality
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