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# Program for SSTF Disk Scheduling Algorithm

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.

The basic idea is the tracks that are closer to the current disk head position should be serviced first in order to minimize the seek operations is basically known as Shortest Seek Time First (SSTF).

## Advantages of Shortest Seek Time First (SSTF)

• Better performance than the FCFS scheduling algorithm.
• It provides better throughput.
• This algorithm is used in Batch Processing systems where throughput is more important.
• It has a 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 requests and ignores the far-away processes.
• There is a lack of predictability because of the high variance of response time.
• Switching direction slows things down.

## Algorithm

Step 1: Let the Request array represents an array storing indexes of tracks that have been requested. ‘head’ is the position of the disk head.

Step 2: Find the positive distance of all tracks in the request array from the head.

Step 3: Find a track from the requested array which has not been accessed/serviced yet and has a minimum distance from the head.

Step 4: Increment the total seek count with this distance.

Step 5: Currently serviced track position now becomes the new head position.

Step 6: Go to step 2 until all tracks in the request array have not been serviced.

Example:

Request sequence = {176, 79, 34, 60, 92, 11, 41, 114}

The following chart shows the sequence in which requested tracks are serviced using SSTF.

Therefore, the total seek count is calculated as: SSTF (Shortest Seek Time First)

`= (50-41)+(41-34)+(34-11)+(60-11)+(79-60)+(92-79)+(114-92)+(176-114)= 204which can also be directly calculated as: (50-11) + (176-11)`

## Implementation

The 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 the head and the track position. ‘accessed’ is a boolean variable that tells whether the track has been accessed/serviced before by the disk head or not.

## C++

 `// C++ program for implementation of``// SSTF disk scheduling``#include ``using` `namespace` `std;` `// Calculates difference of each ``// track number with the head position``void` `calculatedifference(``int` `request[], ``int` `head,``                         ``int` `diff[], ``int` `n)``{``    ``for``(``int` `i = 0; i < n; i++)``    ``{``        ``diff[i] = ``abs``(head - request[i]);``    ``}``}` `// Find unaccessed track which is``// at minimum distance from head``int` `findMIN(``int` `diff[], ``int` `n)``{``    ``int` `index = -1;``    ``int` `minimum = 1e9;``  ` `    ``for``(``int` `i = 0; i < n; i++)``    ``{``        ``if` `(!diff[i] && minimum > diff[i])``        ``{``            ``minimum = diff[i];``            ``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] = { { 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;``        ` `        ``// Increase the total count``        ``seekcount += diff[index];``        ` `        ``// 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] = 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] && minimum > diff[i]) {``            ``minimum = diff[i];``            ``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 = head;` `    ``for` `(let i = 0; i < n; i++) {``        ``calculatedifference(request, head, diff, n);``        ``let index = findMIN(diff, n);``        ``diff[index] = 1;` `        ``// Increase the total count``        ``seekcount += diff[index];` `        ``// 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 = 204Seek Sequence: 50, 41, 34, 11, 60, 79, 92, 114, 176 `

Time Complexity: O(N^2)

Auxiliary Space: O(N)

## FAQs on SSTF Disk Scheduling Algorithm

### 1. Will Starvation occur for some requests in SSTF?

Yes, when a continuos stream of requests are coming then it will lead to starvation.

### 2. Whether SSTF is suitable for minimizing disk arm movement?

Yes, SSTF is helpful in minimizing disk arm movement and it also helps in reducing seek time.

### 3. In case of two requests having the same seek time, which request SSTF will follow?

If multiple requests have the same seek time, SSTF can choose any of them, depending on the implementation.