# SCAN（电梯）磁盘调度算法解析和实现

2021年3月9日15:53:33 发表评论 542 次浏览

## 本文概述

SCAN(电梯)算法

。结果, 中端的请求得到了更多的服务, 那些到达磁盘臂后面的请求将不得不等待。

SCAN(电梯)算法的优点

1. 该算法简单易懂。
2. SCAN算法没有饥饿感。
3. 该算法优于FCFS调度算法。

SCAN(电梯)算法的缺点

1. 实施更复杂的算法。
2. 该算法是不公平的, 因为它会导致磁头刚访问的气缸的等待时间较长。
3. 这样会导致磁头移动到磁盘末端, 这样到达手臂位置之前的请求将立即得到服务, 但是到达手臂位置后面的其他一些请求将不得不等待请求完成。

1. 让Request数组表示一个数组, 该数组存储已按其到达时间的升序请求的音轨的索引。 "磁头"是磁盘磁头的位置。
2. 让方向代表头部向左还是向右移动。
3. 在头部移动的方向上, 所有人都一一追踪。
4. 计算磁头到磁头的绝对距离。
5. 以此距离增加总寻道数。
6. 当前服务的轨道位置现在变为新的头位置。
7. 转到步骤3, 直到到达磁盘的一端。
8. 如果我们到达磁盘末端, 请反转方向, 然后继续执行步骤2, 直到请求阵列中的所有磁道都未送达为止。

``````Input:
Request sequence = {176, 79, 34, 60, 92, 11, 41, 114}
Direction = left (We are moving from right to left)

Output:
Total number of seek operations = 226
Seek Sequence is
41
34
11
0
60
79
92
114
176``````

``````= (50-41)+(41-34)+(34-11)
+(11-0)+(60-0)+(79-60)
+(92-79)+(114-92)+(176-114)
= 226``````

## C ++

``````// C++ program to demonstrate
// SCAN Disk Scheduling algorithm

#include <bits/stdc++.h>
using namespace std;

int size = 8;
int disk_size = 200;

void SCAN( int arr[], int head, string direction)
{
int seek_count = 0;
int distance, cur_track;
vector< int > left, right;
vector< int > seek_sequence;

// appending end values
// which has to be visited
// before reversing the direction
if (direction == "left" )
left.push_back(0);
else if (direction == "right" )
right.push_back(disk_size - 1);

for ( int i = 0; i < size; i++) {
left.push_back(arr[i]);
right.push_back(arr[i]);
}

// sorting left and right vectors
std::sort(left.begin(), left.end());
std::sort(right.begin(), right.end());

// run the while loop two times.
// one by one scanning right
// and left of the head
int run = 2;
while (run--) {
if (direction == "left" ) {
for ( int i = left.size() - 1; i >= 0; i--) {
cur_track = left[i];

// appending current track to seek sequence
seek_sequence.push_back(cur_track);

// calculate absolute distance
distance = abs (cur_track - head);

// increase the total count
seek_count += distance;

// accessed track is now the new head
}
direction = "right" ;
}
else if (direction == "right" ) {
for ( int i = 0; i < right.size(); i++) {
cur_track = right[i];
// appending current track to seek sequence
seek_sequence.push_back(cur_track);

// calculate absolute distance
distance = abs (cur_track - head);

// increase the total count
seek_count += distance;

// accessed track is now new head
}
direction = "left" ;
}
}

cout << "Total number of seek operations = "
<< seek_count << endl;

cout << "Seek Sequence is" << endl;

for ( int i = 0; i < seek_sequence.size(); i++) {
cout << seek_sequence[i] << endl;
}
}

// Driver code
int main()
{

// request array
int arr[size] = { 176, 79, 34, 60, 92, 11, 41, 114 };
string direction = "left" ;

return 0;
}``````

## Java

``````// Java program to demonstrate
// SCAN Disk Scheduling algorithm
import java.util.*;

class GFG
{

static int size = 8 ;
static int disk_size = 200 ;

static void SCAN( int arr[], int head, String direction)
{
int seek_count = 0 ;
int distance, cur_track;
Vector<Integer> left = new Vector<Integer>(), right = new Vector<Integer>();
Vector<Integer> seek_sequence = new Vector<Integer>();

// appending end values
// which has to be visited
// before reversing the direction
if (direction == "left" )
else if (direction == "right" )

for ( int i = 0 ; i < size; i++)
{
}

// sorting left and right vectors
Collections.sort(left);
Collections.sort(right);

// run the while loop two times.
// one by one scanning right
// and left of the head
int run = 2 ;
while (run-- > 0 )
{
if (direction == "left" )
{
for ( int i = left.size() - 1 ; i >= 0 ; i--)
{
cur_track = left.get(i);

// appending current track to seek sequence

// calculate absolute distance

// increase the total count
seek_count += distance;

// accessed track is now the new head
}
direction = "right" ;
}
else if (direction == "right" )
{
for ( int i = 0 ; i < right.size(); i++)
{
cur_track = right.get(i);

// appending current track to seek sequence

// calculate absolute distance

// increase the total count
seek_count += distance;

// accessed track is now new head
}
direction = "left" ;
}
}

System.out.print( "Total number of seek operations = "
+ seek_count + "\n" );

System.out.print( "Seek Sequence is" + "\n" );

for ( int i = 0 ; i < seek_sequence.size(); i++)
{
System.out.print(seek_sequence.get(i) + "\n" );
}
}

// Driver code
public static void main(String[] args)
{

// request array
int arr[] = { 176 , 79 , 34 , 60 , 92 , 11 , 41 , 114 };
String direction = "left" ;

}
}

// This code is contributed by 29AjayKumar``````

## C#

``````// C# program to demonstrate
// SCAN Disk Scheduling algorithm
using System;
using System.Collections.Generic;

class GFG
{

static int size = 8;
static int disk_size = 200;

static void SCAN( int []arr, int head, String direction)
{
int seek_count = 0;
int distance, cur_track;
List< int > left = new List< int >(), right = new List< int >();
List< int > seek_sequence = new List< int >();

// appending end values
// which has to be visited
// before reversing the direction
if (direction == "left" )
else if (direction == "right" )

for ( int i = 0; i < size; i++)
{
}

// sorting left and right vectors
left.Sort();
right.Sort();

// run the while loop two times.
// one by one scanning right
// and left of the head
int run = 2;
while (run-- >0)
{
if (direction == "left" )
{
for ( int i = left.Count - 1; i >= 0; i--)
{
cur_track = left[i];

// appending current track to seek sequence

// calculate absolute distance

// increase the total count
seek_count += distance;

// accessed track is now the new head
}
direction = "right" ;
}
else if (direction == "right" )
{
for ( int i = 0; i < right.Count; i++)
{
cur_track = right[i];

// appending current track to seek sequence

// calculate absolute distance

// increase the total count
seek_count += distance;

// accessed track is now new head
}
direction = "left" ;
}
}

Console.Write( "Total number of seek operations = "
+ seek_count + "\n" );
Console.Write( "Seek Sequence is" + "\n" );
for ( int i = 0; i < seek_sequence.Count; i++)
{
Console.Write(seek_sequence[i] + "\n" );
}
}

// Driver code
public static void Main(String[] args)
{

// request array
int []arr = { 176, 79, 34, 60, 92, 11, 41, 114 };
String direction = "left" ;

}
}

// This code is contributed by 29AjayKumar``````

``````Total number of seek operations = 226
Seek Sequence is
41
34
11
0
60
79
92
114
176``````

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