MODIFIED FIRST COME FIRST SERVE (MFCFS)

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1 MODIFIED FIRST COME FIRST SERVE (MFCFS) 1 HAMID HUSAIN, 2 KARNAV GUPTA, 3 SHRADHA TANEJA 1,2,3 Department of Computer Science, Jamia Hamdard, New Delhi, India hamidhusain@outlook.com, karnavgupta@gmail.com, shradha.taneja@gmail.com Abstract- Disk scheduling requires a careful scrutinization of the requests which are pending in order to determine the most efficient way to serve the requests. There are two most common type of disk scheduling i.e. seek optimization and latency (or rotational) optimization. Each and every scheduling algorithm focuses on reducing the seek time for the pending requests, since seek time in order of magnitude is greater than latency time. Seek time calculates the time required for read/write heads to move between the tracks provided by the disk. The various form of scheduling algorithms are FCFS (First Come First Serve), SJF (Shortest Job First), C-SCAN (Circular Scan) and LOOK. FCFS being the simplest in its implementation, it doesn t provide fastest implementation (request can be accessed). This paper aims to improve the existing FCFS algorithm such that the seek time and rotational latency can be reduced by reducing average seek distance. Keywords- Operating system, disk scheduling, FCFS (First Come First Serve), Seek time, Average seek time. I. INTRODUCTION In many existing algorithms, inefficiency is often caused by inappropriate use of the rotational storage devices such as disks. Since many processes generate request for reading and writing disk track, the systems which use such kind of algorithm result in formation of waiting queue for each device. The question arises: How is the next record traversed by the disk scheduler. Now the main aim of disk scheduling algorithm is to provide a paramount way of traversing each track such that it requires minimum seek time and minimum heads. Another question arises: How to improve disk performance. Disk performance can be enhanced by establishing magnetic disks that provide high transfer rates. A magnetic disk is basically a collection of platters in the disk. Every platter has its own read-write head located at the top of platter. The surface of latter is divided into circular tracks which are further divided into sectors. Many researches are going on to improve the efficiency of disks [8]. This study focuses on improving the FCFS algorithm. II. RELATED WORK Significant contribution have been made by several computer scientists to improve the typical FCFS (First Come First Serve) algorithm. Various approaches have been put forward in the past few years to overcome the limitations of the classical FCFS algorithm. The Existing algorithm proposes a method wherein the head of the disk moves with the intention of serving the first I/O request first. On its way to serve the first request, it serves any waiting request in between. As per the other authors, have suggested different advents to beat the shortcomings of the conventional FCFS algorithm that is presently being used. The disk I/O operations are dependent on the computer system, disk controller hardware, the operating system and the nature of the I/O channel. III. PROPOSED ALGORITHM Let us assume that the operating system has to service the request from a to b. Let initially the header be at CH, CH=Current Head Tn=Total number of tracks ST=Seek Time a. AST= Average Seek Time 1. Sort the requests in ascending order 2. Calculate the difference between current head and other sorted requests. 3. Find the minimum difference from step 2, Say it is P. 4. Service the request with the minimum difference and change the current head position to the serviced request and save this position in another variable say A. 5. Calculate the number of requests on both sides of the current head found in step If the left side has less number of requests, serve the request CH - 1 and proceed in the same direction till end of queue. 7. Proceed till the end of queue and then move to position A+1 and serve the other requests till other end and add P to it. 8. Else satisfy request CH + 1 and progress in that direction till the end of queue. 9. Solve till the end of queue and then move to position A-1 and serve the other requests till other end and add P to it. 10. Calculate the Average seek time (AST) from the suggested algorithm. 11. AST = Σ ST(of each track)/ Σ Tn. 11

2 IV. FLOW CHART OF PROPOSED APPROACH: ABBREVIATIONS: 1. DQ= DISK QUEUE 2. IST = INITIAL SEEK TIME 3. CH = CURRENT DISK HEAD POSITION 4. ST= SEEK TIME 5. TR = TRACK REQUEST 6. N = TOTAL NO. OF REQUESTS VI. EXPERIMENTS PERFORMED To evaluate performance for our proposed algorithm, we have taken three different instances. CASE 1: We have taken the disk queue with request for I/O to blocks on cylinders 80, 50, 30, 40, 5 and 10. Initial disk head is at position 20. First we will sort the entire requests present in the request queue. After sorting, disk head will traverse to the nearest request while adding it s seek time. There after total number of request present to both sides of disk head is calculated, the disk head will traverse to the side which has least number of requests. Then the disk head after reaching one end of disk queue will then move to the request which is next to the first request and will then move till the other end adding the seek time. V. PERFORMANCE EVALUATION To evaluate the performance, we assumed that all I/O are independent of each other and have equal priority. All the requests are stored in request queue. The minimum track number is zero with least difference between them. Seek time is taken as the performance parameter. Also assuming all the requests to be of different values. 12

3 CASE 2: We have taken the disk queue with request for I/O to blocks on cylinders 25, 10, 151, 170, 62, 46, 74 and 111. Initial disk head is at position 45. First we will sort the entire requests present in the request queue. After sorting, disk head will traverse to the nearest request while adding it s seek time. There after total number of request present to both sides of disk head is calculated, the disk head will traverse to the side which has least number of requests. Then the disk head after reaching one end of disk queue will then move to the request which is next to the first request and will then move till the other end adding the seek time. 13

4 CASE 3: We have taken the disk queue with request for I/O to blocks on cylinders 23, 89, 132, 42 and 187. Initial disk head is at position 100. First we will sort the entire requests present in the request queue. After sorting, disk head will traverse to the nearest request while adding it s seek time. There after total number of request present to both sides of disk head is calculated, the disk head will traverse to the side which has least number of requests. Then the disk head after reaching one end of disk queue will then move to the request which is next to the first request and will then move till the other end adding the seek time. CONCLUSION Stimulation results shows that the proposed disk scheduling algorithm is always giving better performance than FCFS. After improvement in FCFS it has been found that service is fast and seek time has been reduced drastically. This algorithm can be implemented to improve the performance in the system in which FCFS is a preferable choice. 14

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