磁盘调度算法详解：SCAN与C-SCAN的原理与C语言实现

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磁盘调度算法详解：SCAN与C-SCAN的原理与C语言实现

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https://juejin.cn/post/7457805689586417701

磁盘调度算法是操作系统中用于优化磁盘I/O性能的关键技术。本文将深入探讨两种常见的磁盘调度算法：SCAN和C-SCAN，并通过C语言代码实现来帮助读者更好地理解这些算法的工作原理。

1. 介绍

磁盘调度算法用于确定磁盘I/O请求的顺序。这些算法在优化磁盘性能和减少访问时间方面起着至关重要的作用。本博文将重点介绍两种流行的磁盘调度算法：SCAN和C-SCAN。

2. 理解磁盘调度

磁盘调度是将磁盘I/O请求安排得尽可能减少寻道时间和最大化吞吐量的过程。其目标是减少访问磁盘数据所需的时间，从而提高系统性能。

3. 磁盘调度算法的类型

3.1 SCAN

SCAN算法将磁盘臂向一个方向移动，处理其路径上的请求。当磁盘臂到达磁盘末端时，它会改变方向，继续处理请求。

电梯算法：SCAN算法使臂部沿单一方向移动，类似于电梯在楼层之间移动。
减少寻道时间：通过沿单一方向移动，SCAN算法减少了方向变更的次数，从而减少了寻道时间。

3.2 C-SCAN

C-SCAN 是 SCAN 算法的变体，它沿一个方向移动磁盘臂，处理请求，然后迅速返回到另一端，不处理相反方向的请求。

环形扫描：在到达一端后，机械臂立即返回另一端，不处理任何请求，形成环形路径。
更公平的分配：C-SCAN确保所有请求最终都会得到处理，防止饥饿现象。

4. 在C语言中实现磁盘调度

以下是一个简单的C程序，模拟SCAN和C-SCAN算法：

#include <stdio.h>
#include <stdlib.h>

void scan(int *requests, int n, int head, int direction) {
    int i, j, temp;
    int seek_sequence[n + 1];
    int seek_operations = 0;
    // Create a copy of requests
    int *request_copy = malloc(n * sizeof(int));
    for (i = 0; i < n; i++) {
        request_copy[i] = requests[i];
    }
    // Sort the requests
    for (i = 0; i < n; i++) {
        for (j = i + 1; j < n; j++) {
            if (request_copy[i] > request_copy[j]) {
                temp = request_copy[i];
                request_copy[i] = request_copy[j];
                request_copy[j] = temp;
            }
        }
    }
    // Find the index of the head in the sorted array
    int index;
    for (i = 0; i < n; i++) {
        if (request_copy[i] == head) {
            index = i;
            break;
        }
    }
    // Service in the current direction
    for (i = index; (direction == 1) ? (i < n) : (i >= 0); i += direction) {
        seek_sequence[seek_operations] = request_copy[i];
        seek_operations++;
    }
    // Reverse direction
    direction = -direction;
    // Move to the other end and service in the new direction
    if (direction == -1) {
        for (i = index - 1; i >= 0; i--) {
            seek_sequence[seek_operations] = request_copy[i];
            seek_operations++;
        }
    } else {
        for (i = index + 1; i < n; i++) {
            seek_sequence[seek_operations] = request_copy[i];
            seek_operations++;
        }
    }
    // Print the seek sequence
    printf("SCAN Seek Sequence:\n");
    for (i = 0; i < seek_operations; i++) {
        printf("%d ", seek_sequence[i]);
    }
    printf("\n");
    free(request_copy);
}

void c_scan(int *requests, int n, int head, int direction) {
    int i, j, temp;
    int seek_sequence[n + 1];
    int seek_operations = 0;
    // Create a copy of requests
    int *request_copy = malloc(n * sizeof(int));
    for (i = 0; i < n; i++) {
        request_copy[i] = requests[i];
    }
    // Sort the requests
    for (i = 0; i < n; i++) {
        for (j = i + 1; j < n; j++) {
            if (request_copy[i] > request_copy[j]) {
                temp = request_copy[i];
                request_copy[i] = request_copy[j];
                request_copy[j] = temp;
            }
        }
    }
    // Find the index of the head in the sorted array
    int index;
    for (i = 0; i < n; i++) {
        if (request_copy[i] == head) {
            index = i;
            break;
        }
    }
    // Service in the current direction
    for (i = index; (direction == 1) ? (i < n) : (i >= 0); i += direction) {
        seek_sequence[seek_operations] = request_copy[i];
        seek_operations++;
    }
    // Move to the other end without servicing
    if (direction == 1) {
        seek_sequence[seek_operations] = 199; // Assume disk size is 200
        seek_operations++;
    } else {
        seek_sequence[seek_operations] = 0;
        seek_operations++;
    }
    // Service in the new direction
    if (direction == 1) {
        for (i = 0; i < index; i++) {
            seek_sequence[seek_operations] = request_copy[i];
            seek_operations++;
        }
    } else {
        for (i = index + 1; i < n; i++) {
            seek_sequence[seek_operations] = request_copy[i];
            seek_operations++;
        }
    }
    // Print the seek sequence
    printf("C-SCAN Seek Sequence:\n");
    for (i = 0; i < seek_operations; i++) {
        printf("%d ", seek_sequence[i]);
    }
    printf("\n");
    free(request_copy);
}

int main() {
    int requests[] = {55, 58, 39, 18, 90, 160, 150, 38, 184};
    int n = sizeof(requests) / sizeof(requests[0]);
    int head = 50;
    int direction = 1; // 1 for moving towards higher cylinders, -1 for lower
    scan(requests, n, head, direction);
    c_scan(requests, n, head, direction);
    return 0;
}

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