Program for SSTF disk scheduling algorithm
Prerequisite – Disk scheduling algorithms
Given an array of disk track numbers and initial head position, our task is to find the total number of seek operations done to access all the requested tracks if Shortest Seek Time First (SSTF) is a disk scheduling algorithm is used.
Shortest Seek Time First (SSTF) –
Basic idea is the tracks which are closer to current disk head position should be serviced first in order to minimise the seek operations.
Advantages of Shortest Seek Time First (SSTF) –
- Better performance than FCFS scheduling algorithm.
- It provides better throughput.
- This algorithm is used in Batch Processing system where throughput is more important.
- It has less average response and waiting time.
Disadvantages of Shortest Seek Time First (SSTF) –
- Starvation is possible for some requests as it favours easy to reach request and ignores the far away processes.
- There is lack of predictability because of high variance of response time.
- Switching direction slows things down.
Algorithm –
- Let Request array represents an array storing indexes of tracks that have been requested. ‘head’ is the position of disk head.
- Find the positive distance of all tracks in the request array from head.
- Find a track from requested array which has not been accessed/serviced yet and has minimum distance from head.
- Increment the total seek count with this distance.
- Currently serviced track position now becomes the new head position.
- Go to step 2 until all tracks in request array have not been serviced.
Example –
Request sequence = {176, 79, 34, 60, 92, 11, 41, 114}
Initial head position = 50
The following chart shows the sequence in which requested tracks are serviced using SSTF.
Therefore, total seek count is calculated as:
= (50-41)+(41-34)+(34-11)+(60-11)+(79-60)+(92-79)+(114-92)+(176-114) = 204
Which can also be directly calculated as: (50-11)+(176-11)
Implementation –
Implementation of SSTF is given below. Note that we have made a node class having 2 members. ‘distance’ is used to store the distance between head and the track position. ‘accessed’ is a boolean variable which tells whether the track has been accessed/serviced before by disk head or not.
C++
// C++ program for implementation of // SSTF disk scheduling #include <bits/stdc++.h> using namespace std; // Calculates difference of each // track number with the head position void calculatedifference( int request[], int head, int diff[][2], int n) { for ( int i = 0; i < n; i++) { diff[i][0] = abs (head - request[i]); } } // Find unaccessed track which is // at minimum distance from head int findMIN( int diff[][2], int n) { int index = -1; int minimum = 1e9; for ( int i = 0; i < n; i++) { if (!diff[i][1] && minimum > diff[i][0]) { minimum = diff[i][0]; index = i; } } return index; } void shortestSeekTimeFirst( int request[], int head, int n) { if (n == 0) { return ; } // Create array of objects of class node int diff[n][2] = { { 0, 0 } }; // Count total number of seek operation int seekcount = 0; // Stores sequence in which disk access is done int seeksequence[n + 1] = {0}; for ( int i = 0; i < n; i++) { seeksequence[i] = head; calculatedifference(request, head, diff, n); int index = findMIN(diff, n); diff[index][1] = 1; // Increase the total count seekcount += diff[index][0]; // Accessed track is now new head head = request[index]; } seeksequence[n] = head; cout << "Total number of seek operations = " << seekcount << endl; cout << "Seek sequence is : " << "\n" ; // Print the sequence for ( int i = 0; i <= n; i++) { cout << seeksequence[i] << "\n" ; } } // Driver code int main() { int n = 8; int proc[n] = { 176, 79, 34, 60, 92, 11, 41, 114 }; shortestSeekTimeFirst(proc, 50, n); return 0; } // This code is contributed by manish19je0495 |
Java
// Java program for implementation of // SSTF disk scheduling class node { // represent difference between // head position and track number int distance = 0 ; // true if track has been accessed boolean accessed = false ; } public class SSTF { // Calculates difference of each // track number with the head position public static void calculateDifference( int queue[], int head, node diff[]) { for ( int i = 0 ; i < diff.length; i++) diff[i].distance = Math.abs(queue[i] - head); } // find unaccessed track // which is at minimum distance from head public static int findMin(node diff[]) { int index = - 1 , minimum = Integer.MAX_VALUE; for ( int i = 0 ; i < diff.length; i++) { if (!diff[i].accessed && minimum > diff[i].distance) { minimum = diff[i].distance; index = i; } } return index; } public static void shortestSeekTimeFirst( int request[], int head) { if (request.length == 0 ) return ; // create array of objects of class node node diff[] = new node[request.length]; // initialize array for ( int i = 0 ; i < diff.length; i++) diff[i] = new node(); // count total number of seek operation int seek_count = 0 ; // stores sequence in which disk access is done int [] seek_sequence = new int [request.length + 1 ]; for ( int i = 0 ; i < request.length; i++) { seek_sequence[i] = head; calculateDifference(request, head, diff); int index = findMin(diff); diff[index].accessed = true ; // increase the total count seek_count += diff[index].distance; // accessed track is now new head head = request[index]; } // for last accessed track seek_sequence[seek_sequence.length - 1 ] = head; System.out.println( "Total number of seek operations = " + seek_count); System.out.println( "Seek Sequence is" ); // print the sequence for ( int i = 0 ; i < seek_sequence.length; i++) System.out.println(seek_sequence[i]); } public static void main(String[] args) { // request array int arr[] = { 176 , 79 , 34 , 60 , 92 , 11 , 41 , 114 }; shortestSeekTimeFirst(arr, 50 ); } } |
Python3
# Python3 program for implementation of # SSTF disk scheduling # Calculates difference of each # track number with the head position def calculateDifference(queue, head, diff): for i in range ( len (diff)): diff[i][ 0 ] = abs (queue[i] - head) # find unaccessed track which is # at minimum distance from head def findMin(diff): index = - 1 minimum = 999999999 for i in range ( len (diff)): if ( not diff[i][ 1 ] and minimum > diff[i][ 0 ]): minimum = diff[i][ 0 ] index = i return index def shortestSeekTimeFirst(request, head): if ( len (request) = = 0 ): return l = len (request) diff = [ 0 ] * l # initialize array for i in range (l): diff[i] = [ 0 , 0 ] # count total number of seek operation seek_count = 0 # stores sequence in which disk # access is done seek_sequence = [ 0 ] * (l + 1 ) for i in range (l): seek_sequence[i] = head calculateDifference(request, head, diff) index = findMin(diff) diff[index][ 1 ] = True # increase the total count seek_count + = diff[index][ 0 ] # accessed track is now new head head = request[index] # for last accessed track seek_sequence[ len (seek_sequence) - 1 ] = head print ( "Total number of seek operations =" , seek_count) print ( "Seek Sequence is" ) # print the sequence for i in range (l + 1 ): print (seek_sequence[i]) # Driver code if __name__ = = "__main__" : # request array proc = [ 176 , 79 , 34 , 60 , 92 , 11 , 41 , 114 ] shortestSeekTimeFirst(proc, 50 ) # This code is contributed by # Shubham Singh(SHUBHAMSINGH10) |
C#
// C# program for implementation of // SSTF disk scheduling using System; public class node { // represent difference between // head position and track number public int distance = 0; // true if track has been accessed public Boolean accessed = false ; } public class SSTF { // Calculates difference of each // track number with the head position public static void calculateDifference( int []queue, int head, node []diff) { for ( int i = 0; i < diff.Length; i++) diff[i].distance = Math.Abs(queue[i] - head); } // find unaccessed track // which is at minimum distance from head public static int findMin(node []diff) { int index = -1, minimum = int .MaxValue; for ( int i = 0; i < diff.Length; i++) { if (!diff[i].accessed && minimum > diff[i].distance) { minimum = diff[i].distance; index = i; } } return index; } public static void shortestSeekTimeFirst( int []request, int head) { if (request.Length == 0) return ; // create array of objects of class node node []diff = new node[request.Length]; // initialize array for ( int i = 0; i < diff.Length; i++) diff[i] = new node(); // count total number of seek operation int seek_count = 0; // stores sequence in which disk access is done int [] seek_sequence = new int [request.Length + 1]; for ( int i = 0; i < request.Length; i++) { seek_sequence[i] = head; calculateDifference(request, head, diff); int index = findMin(diff); diff[index].accessed = true ; // increase the total count seek_count += diff[index].distance; // accessed track is now new head head = request[index]; } // for last accessed track seek_sequence[seek_sequence.Length - 1] = head; Console.WriteLine( "Total number of seek operations = " + seek_count); Console.WriteLine( "Seek Sequence is" ); // print the sequence for ( int i = 0; i < seek_sequence.Length; i++) Console.WriteLine(seek_sequence[i]); } // Driver code public static void Main(String[] args) { // request array int []arr = { 176, 79, 34, 60, 92, 11, 41, 114 }; shortestSeekTimeFirst(arr, 50); } } // This code contributed by Rajput-Ji |
Javascript
// Javascript Program for implementation of // SSTF disk scheduling function calculatedifference(request, head, diff, n) { for (let i = 0; i < n; i++) { diff[i][0] = Math.abs(head - request[i]); } } // Find unaccessed track which is // at minimum distance from head function findMIN(diff, n) { let index = -1; let minimum = 1e9; for (let i = 0; i < n; i++) { if (!diff[i][1] && minimum > diff[i][0]) { minimum = diff[i][0]; index = i; } } return index; } function shortestSeekTimeFirst(request, head, n) { if (n == 0) { return ; } // Create array of objects of class node let diff = new Array(n); for (let i = 0; i < n; i++) { diff[i] = new Array(2); } // Count total number of seek operation let seekcount = 0; // Stores sequence in which disk access is done let seeksequence = new Array(n + 1); seeksequence[0] = head; for (let i = 0; i < n; i++) { calculatedifference(request, head, diff, n); let index = findMIN(diff, n); diff[index][1] = 1; // Increase the total count seekcount += diff[index][0]; // Accessed track is now new head head = request[index]; seeksequence[i + 1] = head; } console.log( "Total number of seek operations = " + seekcount); console.log( "Seek sequence is : " ); // Print the sequence for (let i = 0; i <= n; i++) { console.log(seeksequence[i]); } } // Driver code let n = 8; let proc = [176, 79, 34, 60, 92, 11, 41, 114]; shortestSeekTimeFirst(proc, 50, n); // This code is contributed by ishankhandelwals. |
Output:
Total number of seek operations = 204 Seek Sequence is 50 41 34 11 60 79 92 114 176
Time Complexity: O(N^2)
Auxiliary Space: O(N)
Please Login to comment...