I/O Management and Disk schedule

1. I/O Management and Disk schedule

2. Device Purpose Partner Data rate (KB/sec) I/O Devices Keyboard Input Human .01 Mouse Input Human .02 Voice input Input Human .02 Scanner Input Human 200 Voice output Output Human .06 Line printer Output Human 1 Laser printer Output Human 100 Graphics display Output Human 30,000 CPU to frame buffer Output Human 200 Network terminal Input or output Machine .05 Network-LAN Input or output Machine 200 Optical disk Storage Machine 500 Magnetic tape Storage Machine 2,000 Magnetic disk Storage Machine 2,000

3. No interrupts Interrupts I/O Techniques I/O to memory transfer through processor Programmed I/O Interrupt-driven I/O Direct I/O to memory transfer Direct memory access (DMA) • DMA unit uses the bus to transfer data to/from memory • when the CPU is not using it, or • forces CPU to temporarily suspend (cycle stealing) • I/O channels • selector • multiplexor

4. Device I/O Device I/O Device I/O Scheduling & Control Scheduling & Control Scheduling & Control Hardware Hardware Hardware User processes User processes User processes I/O organization File System Logical I/O Communic. architecture Physical organization Local File system Remote

5. I/O Buffering • Why buffer? • I/O too slow • not possible to swap out a whole process • risk of single-process deadlock • page in memory would have to be locked • I/O devices are: • block-oriented: disk and other storage devices • stream-oriented: terminals, printers, com ports, mouse • Types of buffers (blocks, line or byte at-a-time) • single buffer • double • circular

6. Disk concepts: review See the following page http://home.ubalt.edu/abento/751/6iomgmt/os0606.html

7. Seek time • time to move disk arm to a track • S = m x n + s (m is a constant per disk, number of tracks transversed, s startup time) • Rotational delay • waiting time for a given sector align with the head • disks: =~ 3,600 rpm, average = 8.3 msec • diskettes= 300-600 rpm, average = 100 and 200 msec • Access time = seek + rotational delay • Transfer time • time to spin record by the head (to read or write) • T = b/ rN (b is number of bytes to be transferred, r rotation speed rpm/sec, N number of bytes on a track) Disk performance parameters

8. OS maintains a queue of requests for each I/O device Name Description Characteristic Disk scheduling policies Selection according to requestor: RSS random scheduling for analysis only FIFO first-in, first-out fairest of them all PRI priority by process control not based on disk queue management LIFO last-in, first-out maximize locality and resource utilization Selection according to requested item: SSTF shortest service first high use, small queues SCAN back and forth over disk better service distribution C-SCAN one way with fast return lower service variability N-step-SCAN SCAN of N records at a time service guarantee FSCAN N-step-SCAN with N= queue size load sensitive

9. a buffer in main memory for disk sectors • locality and locality of reference is what makes it to score hits • reading ahead and write-back delay • data delivery from the cache: • move from cache to user memory • shared memory and pointers: point do not move • Replacement strategy (similar to pages) • LRU (least recently used) • LFU (least frequently used) • FBR (frequency-based replacement) • Hits are not moved to top of queue as they might, pointers are used • Empirical: misses and hits are a function of the cache size Disk Cache