Disk Scheduling
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This document discusses disk scheduling algorithms. It provides background on disk drives and describes several disk scheduling algorithms - First Come First Serve (FCFS), Shortest Seek Time First (SSTF), SCAN, CSCAN, LOOK, and CLOOK. It defines key terms like seek time, rotational latency, transfer time, and response time. For each algorithm, it provides an example to illustrate how the algorithm works and calculates the total seek time. The goal of disk scheduling is to minimize seek times and optimize disk bandwidth utilization.
Disk Scheduling
- 1. B.Tech – CS 2nd Year Operating System (KCS- 401) Dr. Pankaj Kumar Operating System KCS – 401 Disk Scheduling Dr. Pankaj Kumar Associate Professor – CSE SRMGPC Lucknow
- 2. Background B.Tech – CS 2nd Year Operating System (KCS- 401) Dr. Pankaj Kumar Magnetic disks provide bulk of secondary storage of modern computers Drives rotate at 60 to 250 times per second Transfer rate is rate at which data flow between drive and computer Positioning time (random-access time) is time to move disk arm to desired cylinder (seek time) and time for desired sector to rotate under the disk head (rotational latency) Head crash results from disk head making contact with the disk surface -- That’s bad Disks can be removable Drive attached to computer via I/O bus Busses vary, including EIDE, ATA, SATA, USB, Fibre Channel, SCSI, SAS, Firewire Host controller in computer uses bus to talk to disk controller built into drive or storage array
- 3. Hard Disk Mechanism B.Tech – CS 2nd Year Operating System (KCS- 401) Dr. Pankaj Kumar Platters range from .85” to 14” (historically) Commonly 3.5”, 2.5”, and 1.8” Range from 30GB to 3TB per drive Performance Transfer Rate – theoretical – 6 Gb/sec Effective Transfer Rate – real – 1Gb/sec Seek time from 3ms to 12ms – 9ms common for desktop drives Average seek time measured or calculated based on 1/3 of tracks Latency based on spindle speed 1 / (RPM / 60) = 60 / RPM Average latency = ½ latency
- 4. Hard Disk Performance B.Tech – CS 2nd Year Operating System (KCS- 401) Dr. Pankaj Kumar Access Latency = Average access time = average seek time + average latency For fastest disk 3ms + 2ms = 5ms For slow disk 9ms + 5.56ms = 14.56ms Average I/O time = average access time + (amount to transfer / transfer rate) + controller overhead For example to transfer a 4KB block on a 7200 RPM disk with a 5ms average seek time, 1Gb/sec transfer rate with a .1ms controller overhead = 5ms + 4.17ms + 0.1ms + transfer time = Transfer time = 4KB / 1Gb/s * 8Gb / GB * 1GB / 10242KB = 32 / (10242) = 0.031 ms Average I/O time for 4KB block = 9.27ms + .031ms = 9.301ms
- 5. Hard Disk Performance B.Tech – CS 2nd Year Operating System (KCS- 401) Dr. Pankaj Kumar Disk drives are addressed as large 1-dimensional arrays of logical blocks, where the logical block is the smallest unit of transfer Low-level formatting creates logical blocks on physical media The 1-dimensional array of logical blocks is mapped into the sectors of the disk sequentially Sector 0 is the first sector of the first track on the outermost cylinder Mapping proceeds in order through that track, then the rest of the tracks in that cylinder, and then through the rest of the cylinders from outermost to innermost Logical to physical address should be easy Except for bad sectors Non-constant # of sectors per track via constant angular velocity
- 6. Disk Scheduling B.Tech – CS 2nd Year Operating System (KCS- 401) Dr. Pankaj Kumar The operating system is responsible for using hardware efficiently — for the disk drives, this means having a fast access time and disk bandwidth Minimize seek time Seek time seek distance Disk bandwidth is the total number of bytes transferred, divided by the total time between the first request for service and the completion of the last transfer There are many sources of disk I/O request OS System processes Users processes I/O request includes input or output mode, disk address, memory address, number of sectors to transfer OS maintains queue of requests, per disk or device Idle disk can immediately work on I/O request, busy disk means work must queue Optimization algorithms only make sense when a queue exists
- 7. Disk Scheduling B.Tech – CS 2nd Year Operating System (KCS- 401) Dr. Pankaj Kumar • Seek Time: Seek time is the time taken to locate the disk arm to a specified track where the data is to be read or write. So the disk scheduling algorithm that gives minimum average seek time is better. • Rotational Latency: Rotational Latency is the time taken by the desired sector of disk to rotate into a position so that it can access the read/write heads. So the disk scheduling algorithm that gives minimum rotational latency is better. • Transfer Time: Transfer time is the time to transfer the data. It depends on the rotating speed of the disk and number of bytes to be transferred. • Disk Response Time: Response Time is the average of time spent by a request waiting to perform its I/O operation. Average Response time is the response time of the all requests. Variance Response Time is measure of how individual request are serviced with respect to average response time. So the disk scheduling algorithm that gives minimum variance response time is better. • Disk Access Time: Disk Access Time is: Disk Access Time = Seek Time + Rotational Latency + Transfer Time
- 8. First Come First Serve (FCFS) B.Tech – CS 2nd Year Operating System (KCS- 401) Dr. Pankaj Kumar FCFS is the simplest of all the Disk Scheduling Algorithms. In FCFS, the requests are addressed in the order they arrive in the disk queue. Let us understand this with the help of an example. Example: Suppose the order of request is- (82,170,43,140,24,16,190) And current position of Read/Write head is : 50 Advantages: •Every request gets a fair chance •No indefinite postponement Disadvantages: •Does not try to optimize seek time •May not provide the best possible service So, total seek time: =(82-50)+(170-82)+(170-43)+(140-43)+(140-24)+(24-16)+(190-16) =642
- 9. Shortest Seek Time First (SSTF) B.Tech – CS 2nd Year Operating System (KCS- 401) Dr. Pankaj Kumar Advantages: • Average Response Time decreases • Throughput increases Disadvantages: • Overhead to calculate seek time in advance • Can cause Starvation for a request if it has higher seek time as compared to incoming requests • High variance of response time as SSTF favors only some requests So, total seek time: = (50-43)+(43-24)+(24-16)+(82-16)+(140-82)+(170-40)+(190-170) =208 In SSTF (Shortest Seek Time First), requests having shortest seek time are executed first. So, the seek time of every request is calculated in advance in the queue and then they are scheduled according to their calculated seek time. As a result, the request near the disk arm will get executed first. Example: Suppose the order of request is- (82,170,43,140,24,16,190) And current position of Read/Write head is : 50
- 10. (SCAN) B.Tech – CS 2nd Year Operating System (KCS- 401) Dr. Pankaj Kumar Advantages: • High throughput • Low variance of response time • Average response time Disadvantages: • Long waiting time for requests for locations just visited by disk arm So, total seek time: =(199-50)+(199-16) =332 In SCAN algorithm the disk arm moves into a particular direction and services the requests coming in its path and after reaching the end of disk, it reverses its direction and again services the request arriving in its path. So, this algorithm works as an elevator and hence also known as elevator algorithm. As a result, the requests at the midrange are serviced more and those arriving behind the disk arm will have to wait. Example: Suppose the requests to be addressed are-82,170,43,140,24,16,190. And the Read/Write arm is at 50, and it is also given that the disk arm should move “towards the larger value”.
- 11. (CSCAN) B.Tech – CS 2nd Year Operating System (KCS- 401) Dr. Pankaj Kumar So, total seek time: =(199-50)+(199-0)+(43-0)=391 In SCAN algorithm, the disk arm again scans the path that has been scanned, after reversing its direction. So, it may be possible that too many requests are waiting at the other end or there may be zero or few requests pending at the scanned area. These situations are avoided in CSCAN algorithm in which the disk arm instead of reversing its direction goes to the other end of the disk and starts servicing the requests from there. So, the disk arm moves in a circular fashion and this algorithm is also similar to SCAN algorithm and hence it is known as C-SCAN (Circular SCAN). Example: Suppose the requests to be addressed are-82,170,43,140,24,16,190. And the Read/Write arm is at 50, and it is also given that the disk arm should move “towards the larger value”.
- 12. (LOOK) B.Tech – CS 2nd Year Operating System (KCS- 401) Dr. Pankaj Kumar So, total seek time: =(190-50)+(190-16) =314 It is similar to the SCAN disk scheduling algorithm except for the difference that the disk arm in spite of going to the end of the disk goes only to the last request to be serviced in front of the head and then reverses its direction from there only. Thus it prevents the extra delay which occurred due to unnecessary traversal to the end of the disk. Example: Suppose the requests to be addressed are-82,170,43,140,24,16,190. And the Read/Write arm is at 50, and it is also given that the disk arm should move “towards the larger value”.
- 13. (CLOOK) B.Tech – CS 2nd Year Operating System (KCS- 401) Dr. Pankaj Kumar So, total seek time: =(190-50)+(190-16)+(43-16) =341 It is similar to the SCAN disk scheduling algorithm except for the difference that the disk arm in spite of going to the end of the disk goes only to the last request to be serviced in front of the head and then reverses its direction from there only. Thus it prevents the extra delay which occurred due to unnecessary traversal to the end of the disk. Example: Suppose the requests to be addressed are-82,170,43,140,24,16,190. And the Read/Write arm is at 50, and it is also given that the disk arm should move “towards the larger value”.
- 14. (CLOOK) B.Tech – CS 2nd Year Operating System (KCS- 401) Dr. Pankaj Kumar So, total seek time: =(190-50)+(190-16)+(43-16) =341 It is similar to the SCAN disk scheduling algorithm except for the difference that the disk arm in spite of going to the end of the disk goes only to the last request to be serviced in front of the head and then reverses its direction from there only. Thus it prevents the extra delay which occurred due to unnecessary traversal to the end of the disk. Example: Suppose the requests to be addressed are-82,170,43,140,24,16,190. And the Read/Write arm is at 50, and it is also given that the disk arm should move “towards the larger value”.