Unit 1 Device Management. Understanding Operating Systems

Transcription

1 Unit 1 Device Management 1

2 Device Management Paper Storage Media Magnetic Tape Storage Magnetic Disk Storage Optical Disc Storage 2

3 Device Management Functions Track status of each device (such as tape drives, disk drives, printers, plotters, and terminals). Use preset policies to determine which process will get a device and for how long. Allocate the devices. Deallocate the devices at 2 levels: At process level when I/O command has been executed & device is temporarily released At job level when job is finished & device is permanently released. 3

4 System Devices Differences among system s peripheral devices are a function of characteristics of devices, and how well they re managed by the Device Manager. Most important differences among devices Speeds Degree of sharability. By minimizing variances among devices, a system s overall efficiency can be dramatically improved. 4

5 Components of the I/O Subsystem Channel 1 Control Unit 1 Control Unit 2 Disk 1 Disk 2 Disk 3 CPU Tape 1 Tape 2 Channel 2 Control Unit 3 Control Unit 4 Tape 3 Tape 4 Disk 4 Disk 5 5

6 I/O Subsystem : I/O Channel I/O Channel -- keeps up with I/O requests from CPU and pass them down the line to appropriate control unit. Programmable units placed between CPU and control unit. Synchronize fast speed of CPU with slow speed of the I/O device. Make it possible to overlap I/O operations with processor operations so the CPU and I/O can process concurrently. Use channel programs that specifies action to be performed by devices & controls transmission of data between main memory & control units. Entire path must be available when an I/O command is initiated. 6

7 I/O Subsystem : I/O Control Unit I/O control unit interprets signal sent by channel. One signal for each function. At start of I/O command, info passed from CPU to channel: I/O command (READ, WRITE, REWIND, etc.) Channel number Address of physical record to be transferred (from or to secondary storage) Starting address of a memory buffer from which or into which record is to be transferred 7

8 Communication Among Devices Each unit in I/O subsystem can finish its operation independently from others. CPU is free to process data while I/O is being performed, which allows for concurrent processing and I/O. Success of operation depends on system s ability to know when device has completed operation. Uses a hardware flag that must be tested by CPU. 8

9 I/O Scheduler I/O scheduler performs same job as Process Scheduler-- it allocates the devices, control units, and channels. Under heavy loads, when # requests > # available paths, I/O scheduler must decide which request satisfied first. I/O requests are not preempted: once channel program has started, it s allowed to continue to completion even though I/O requests with higher priorities may have entered queue. Feasible because programs are relatively short (50 to 100 ms). 9

10 I/O Scheduler - 2 Some systems allow I/O scheduler to give preferential treatment to I/O requests from high-priority programs. If a process has high priority then its I/O requests also has high priority and is satisfied before other I/O requests with lower priorities. I/O scheduler must synchronize its work with traffic controller to make sure that a path is available to satisfy selected I/O requests. 10

11 First Come First Served (FCFS) Device Scheduling Algorithm Simplest device-scheduling algorithm: Easy to program and essentially fair to users. On average, it doesn t meet any of the three goals of a seek strategy. Remember, seek time is most time-consuming of functions performed here, so any algorithm that can minimize it is preferable to FCFS. 11

12 Shortest Seek Time First (SSTF) Device Scheduling Algorithm Uses same underlying philosophy as shortest job next where shortest jobs are processed first & longer jobs wait. Request with track closest to one being served (that is, one with shortest distance to travel) is next to be satisfied. Minimizes overall seek time. Favors easy-to-reach requests and postpones traveling to those that are out of way. 12

13 SCAN Device Scheduling Algorithm SCAN uses a directional bit to indicate whether the arm is moving toward the center of the disk or away from it. Algorithm moves arm methodically from outer to inner track servicing every request in its path. When it reaches innermost track it reverses direction and moves toward outer tracks, again servicing every request in its path. 13

14 LOOK (Elevator Algorithm) : A Variation of SCAN Arm doesn t necessarily go all the way to either edge unless there are requests there. Looks ahead for a request before going to service it. Eliminates possibility of indefinite postponement of requests in out-of-the-way places at either edge of disk. As requests arrive each is incorporated in its proper place in queue and serviced when the arm reaches that track. 14

15 Other Variations of SCAN N-Step SCAN -- holds all requests until arm starts on way back. New requests are grouped together for next sweep. C-SCAN (Circular SCAN) -- arm picks up requests on its path during inward sweep. When innermost track has been reached returns to outermost track and starts servicing requests that arrived during last inward sweep. Provides a more uniform wait time. C-LOOK (optimization of C-SCAN) --sweep inward stops at last high-numbered track request, so arm doesn t move all the way to last track unless it s required to do so. Arm doesn t necessarily return to the lowest-numbered track; it returns only to the lowest-numbered track that s requested. 15