FCFS 和 SSTF 磁盘调度算法的区别

先决条件 – 磁盘调度算法
1. FCFS 磁盘调度算法：
先到先得，顾名思义，该算法按照任务到达磁盘队列的顺序来处理任务。它是最简单易懂的磁盘调度算法。在这种情况下，头部或指针向任务到达的方向移动，并移动直到所有请求都得到服务。 FCFS 提供更多的平均等待时间和响应时间。然而，FCFS 算法在处理即将到来的请求方面有更公平的策略。

例子：
考虑一个有 200 个磁道 (0-199) 的磁盘，磁盘队列的 I/O 请求顺序如下：

98, 183, 40, 122, 10, 124, 65 

Read\Write 磁头当前磁头位置为 53。使用 FCFS 算法计算 Read/Write 磁头的磁道移动总数。

总的头部运动，

= (98-53)+(183-98)+(183-40)+(122-40)+(122-10)+(124-10)+(124-65)
= 640

2. SSTF 磁盘调度算法：
SSTF 代表 Shortest Seek Time First，顾名思义，它服务于最接近当前头部或指针位置的请求。在这个算法中，头指针的方向很重要。如果不知何故，我们遇到请求之间的联系，则头部将在其正在进行的方向上为请求提供服务。与 FCFS 相比，SSTF 算法在寻道时间上非常有效。

例子：
考虑一个有 200 个磁道 (0-199) 的磁盘，磁盘队列的 I/O 请求顺序如下：

98, 183, 40, 122, 10, 124, 65 

Read\Write 磁头的当前磁头位置为 53，将向右移动。使用SSTF算法计算读/写磁头的磁道移动总数。

总的头部运动，

= (65-53)+(65-40)+(40-10)+(98-10)+(122-98)+(124-122)+(183-124)
= 240

FCFS和SSTF磁盘调度算法的区别：

	FCFS SCHEDULING ALGORITHM	SSTF SCHEDULING ALGORITHM
1.	FCFS is not efficient in seek movements.	SSTF is very effective/efficient in seek movements.
2.	It results in increased total seek time.	It reduces the total seek time as compared to FCFS.
3.	It provides more average waiting time and response time.	This algorithm provides less average response time and waiting time.
4.	In this algorithm direction of head does not matters much, which we can clearly see in above example.	But here direction of head plays an important role, in order to break tie between requests and above example is a proof of it.
5.	This algorithm is the easy to understand and implement.	Here, there is an overhead of finding out the closest request.
6.	FCFS does not cause starvation to any request (but may suffer from Convoy effect.).	Here, the request which are far from head will suffer starvation.
7.	In FCFS algorithm there is decrement in Throughput.	Here in SSTF there is increment in Throughput.