Presentation-4 Group-A1 Professor Mohamed Khalil Anita Kanuganti Hemanth Rao

DISK SCHEDULING IN AN OPERATING SYSTEM

Agenda Disk Structure Why Disk Scheduling? Example Scheduling Algorithms 1. FCFS 2. SSTF 3. SCAN 4. C-SCAN 5. LOOK 6. C-LOOK Comparison of different Algorithms Selection of right scheduling Algorithm References

Disk Structure A hard disk drive(usually called “drive”) records data on the surface of metal plates called platters that are coated with a substance containing ground-up iron, or other substances that allow zeros and ones to be recorded as tiny spots of magnetism. Modern disk drives are addressed as large one- dimensional arrays of logical blocks (smallest unit of transfer) which are mapped onto the sectors of the disk sequentially. Mapping converts a logical block number into an old-style disk address that consists of a cylinder number, a track,number within that cylinder and a sector number within that track.

Disk structure contd, Advantages of Mapping: Advantages of Mapping: 1. Mapping hides defective sectors by substituting them with spare sectors from elsewhere in the disk. 2. Number of sectors per track is not constant. This is solved in modern disks by organizing into “zones”.

Why Disk Scheduling ? To use hardware efficiently. To increase disk bandwidth: 1. Disk bandwidth—The total number of bytes transferred, divided by the total time between the first request for service and the completion of the last transfer. To reduce the Access time which is caused by: 1. Seek time -- The time for the disk arm to move the heads to the cylinder containing the desired sector. 2. Rotational Latency – The additional time waiting for the disk to rotate the desired sector to the disk head.

Example Consider, a disk queue with requests for I/O to blocks on cylinders 98, 183, 37, 122, 14, 124, 65, 67 in that order.

Scheduling Algorithms FCFS—First Come First Served 1. It is Fair– no starvation 2. Disadvantage– Low performance due to long swings from one part of the disk to another

FCFS Scheduling Head is at 53

Scheduling Algorithms cont, SSTF Scheduling– Shortest-seek-time first 1. Selects minimum seek time from the current position 2. Better Performance 3. Disadvantage: Causes starvation of some requests i.e., A request for a remote part of the disk may never get serviced

SSTF

Scheduling Algorithms cont, SCAN Scheduling—Scans from the start of the disk to end servicing all requests and reverses back servicing the remainder of requests just like an Elevator(also called Elevator Algorithm) Disadvantage: If 2 requests is in front of it and it has to scan the whole disk and return back serve the rest of it.

SCAN

Scheduling Algorithms cont, C-SCAN– Similar to SCAN but with a difference that when head reaches at the end of the disk, it immediately returns to the beginning of the disk,without servicing any requests on the return trip. More Uniform wait time

C-SCAN

Scheduling Algorithms cont, LOOK & C-LOOK—Similar to SCAN but reverses direction when hits the last request in the current direction 1. C-LOOK only moves similar to C-SCAN 1. Much better performance 2. Advantage since it does not has to go all the way to the end of the disk 3. Most widely used and optimized

C-LOOK

Comparison FCFS is not mostly used because of its low speed and delay SSTF is common and has a natural appeal SCAN & C-SCAN perform better for systems that place a heavy load on the disk, because they are less likely to have the starvation problem LOOK & C-LOOK perform much better

Selection of right Algorithm The criteria of selecting algorithm is based on: 1. Number of requests 2. Type of requests 3. File Allocation Method

References “Applied Operating System Concepts” by Abraham Silberschatz, Peter Galvin, Greg Gagne

Questions?