I/O Management and Disk SchedulingCategories of I/O DevicesSlide 3Slide 4Differences in I/O DevicesPowerPoint PresentationSlide 7Slide 8Slide 9Techniques for Performing I/OEvolution of the I/O FunctionSlide 12Slide 13Direct Memory AccessDMASlide 16Slide 17Slide 18Slide 19Slide 20Operating System Design IssuesSlide 22Slide 23I/O BufferingSlide 25Single BufferSlide 27Slide 28Slide 29Double BufferCircular BufferSlide 32Disk Performance ParametersTiming of a Disk I/O TransferSlide 35Disk Scheduling PoliciesSlide 37Slide 38Slide 39Slide 40Slide 41Slide 42Slide 43Disk Scheduling AlgorithmsRAID 0 (non-redundant)Slide 46RAID 1 (mirrored)RAID 2 (redundancy through Hamming code)RAID 3 (bit-interleaved parity)RAID 4 (block-level parity)RAID 5 (block-level distributed parity)RAID 6 (dual redundancy)Disk CacheLeast Recently UsedSlide 55Least Frequently UsedUNIX SVR4 I/OWindows 2000 I/OI/O Management and Disk SchedulingChapter 11Categories of I/O Devices•Human readable–Used to communicate with the user–Printers–Video display terminals•Display•Keyboard•MouseCategories of I/O Devices•Machine readable–Used to communicate with electronic equipment–Disk and tap drives–Sensors–Controllers–ActuatorsCategories of I/O Devices•Communication–Used to communicate with remote devices–Digital line drivers–ModemsDifferences in I/O Devices•Data rate–May be differences of several orders of magnitude between the data transfer ratesDifferences in I/O Devices•Application–Disk used to store files requires file-management software–Disk used to store virtual memory pages needs special hardware and software to support it–Terminal used by system administrator may have a higher priorityDifferences in I/O Devices•Complexity of control•Unit of transfer–Data may be transferred as a stream of bytes for a terminal or in larger blocks for a disk•Data representation–Encoding schemes•Error conditions–Devices respond to errors differentlyDifferences in I/O Devices•Programmed I/O–Process is busy-waiting for the operation to complete•Interrupt-driven I/O–I/O command is issued–Processor continues executing instructions–I/O module sends an interrupt when doneTechniques for Performing I/O•Direct Memory Access (DMA)–DMA module controls exchange of data between main memory and the I/O device–Processor interrupted only after entire block has been transferredEvolution of the I/O Function•Processor directly controls a peripheral device•Controller or I/O module is added–Processor uses programmed I/O without interrupts–Processor does not need to handle details of external devicesEvolution of the I/O Function•Controller or I/O module with interrupts–Processor does not spend time waiting for an I/O operation to be performed•Direct Memory Access–Blocks of data are moved into memory without involving the processor–Processor involved at beginning and end onlyEvolution of the I/O Function•I/O module is a separate processor•I/O processor–I/O module has its own local memory–Its a computer in its own rightDirect Memory Access•Takes control of the system form the CPU to transfer data to and from memory over the system bus•Cycle stealing is used to transfer data on the system bus•The instruction cycle is suspended so data can be transferred•The CPU pauses one bus cycle•No interrupts occur–Do not save contextDMADMA•Cycle stealing causes the CPU to execute more slowly•Number of required busy cycles can be cut by integrating the DMA and I/O functions•Path between DMA module and I/O module that does not include the system busDMADMADMAOperating System Design Issues•Efficiency–Most I/O devices extremely slow compared to main memory–Use of multiprogramming allows for some processes to be waiting on I/O while another process executes–I/O cannot keep up with processor speed–Swapping is used to bring in additional Ready processes which is an I/O operationOperating System Design Issues•Generality–Desirable to handle all I/O devices in a uniform manner–Hide most of the details of device I/O in lower-level routines so that processes and upper levels see devices in general terms such as read, write, open, close, lock, unlockI/O Buffering•Reasons for buffering–Processes must wait for I/O to complete before proceeding–Certain pages must remain in main memory during I/OI/O Buffering•Block-oriented–Information is stored in fixed sized blocks–Transfers are made a block at a time–Used for disks and tapes•Stream-oriented–Transfer information as a stream of bytes–Used for terminals, printers, communication ports, mouse, and most other devices that are not secondary storageSingle Buffer•Operating system assigns a buffer in main memory for an I/O request•Block-oriented–Input transfers made to buffer–Block moved to user space when needed–Another block is moved into the buffer•Read aheadI/O BufferingSingle Buffer•Block-oriented–User process can process one block of data while next block is read in–Swapping can occur since input is taking place in system memory, not user memory–Operating system keeps track of assignment of system buffers to user processesSingle Buffer•Stream-oriented–Used a line at time–User input from a terminal is one line at a time with carriage return signaling the end of the line–Output to the terminal is one line at a timeDouble Buffer•Use two system buffers instead of one•A process can transfer data to or from one buffer while the operating system empties or fills the other bufferCircular Buffer•More than two buffers are used•Each individual buffer is one unit in a circular buffer•Used when I/O operation must keep up with processI/O BufferingDisk Performance Parameters•To read or write, the disk head must be positioned at the desired track and at the beginning of the desired sector•Seek time–time it takes to position the head at the desired track•Rotational delay or rotational latency–time its takes for the beginning of the sector to reach the headTiming of a Disk I/O TransferDisk Performance Parameters•Access time–Sum of seek time and rotational delay–The time it takes to get in position to read or write•Data transfer occurs as the sector moves under the headDisk Scheduling Policies•Seek time is the reason for differences in performance•For a single disk there will be a number of I/O requests•If requests are selected randomly, we
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