Operating System ( Disk Scheduling ).pptx
- 2. Disk Scheduling The operating system is responsible for using hardware efficiently — for the disk drives, this means having a fast access time and disk bandwidth. 2
- 3. Disk Scheduling (Cont.) Several algorithms exist to schedule the servicing of disk I/O requests. We illustrate them with a request queue (0-199). 98, 183, 37, 122, 14, 124, 65, 67 Head pointer 53 3
- 4. FCFS 4 Illustration shows total head movement of 640 cylinders.
- 5. SSTF Selects the request with the minimum seek time from the current head position. SSTF scheduling is a form of SJF scheduling; may cause starvation of some requests. Illustration shows total head movement of 236 cylinders. 5
- 6. SSTF (Cont.) 6 Total head movement - 12+2+30+23+84+24+2+59=236 (65-53)
- 7. SCAN The disk arm starts at one end of the disk, and moves toward the other end, servicing requests until it gets to the other end of the disk, where the head movement is reversed and servicing continues. Sometimes called the elevator algorithm. Illustration shows total head movement of 208 cylinders. 7
- 8. SCAN (Cont.) 8 Total head movement - 16+23+51+2+31+24+2+59=208 (14-53=51)
- 9. C-SCAN C(Circular)-Scan Provides a more uniform wait time than SCAN. The head moves from one end of the disk to the other. servicing requests as it goes. When it reaches the other end, however, it immediately returns to the beginning of the disk, without servicing any requests on the return trip. Treats the cylinders as a circular list that wraps around from the last cylinder to the first one. 9
- 11. LOOK Version of C-SCAN Arm only goes as far as the last request in each direction, then reverses direction immediately, without first going all the way to the end of the disk. 11
- 12. LOOK (Cont.) 12
- 13. Selecting a Disk- Scheduling Algorithm SSTF is common and has a natural appeal SCAN and C-SCAN perform better for systems that place a heavy load on the disk. Performance depends on the number and types of requests. Either SSTF or LOOK is a reasonable choice for the default algorithm. 13
- 14. Device Management Devices are typically physical/hardware devices such as computers, laptops, servers. device management refers to the control of input/output devices such as keyboards, printers, magnetic tape, USB ports etc. One of the important jobs of an Operating System is to manage various I/O devices including mouse, keyboards, touch pad, disk drives.
- 15. Features of Device Management The operating system interacts with the device controllers through device drivers while allocating the device to the various processes running on the system. Device drivers are system software programs that connect processes and device controllers. The device management in the operating system is responsible for implementing the APIs. The device controller, which is used in device management operations, consists primarily of three registers: command, status, and data.
- 16. Device Driver A device driver is a computer program that operates or controls a particular device attached to a computer A driver provides a software interface to hardware devices, enabling operating systems and other computer programs to access hardware functions A driver communicates with the device through the computer bus or communications subsystem to which the hardware connects. Once the device sends data back to the driver, the driver may invoke routines in the original calling program. When a calling program invokes a driver's routine, the driver issues the commands to the device.
- 17. Working of Device driver A device driver performs the following jobs, such as: To accept request from the device-independent software above to it. Making sure that the request is executed successfully. Interact with the device controller to take and give I/O and perform required error handling.