1. SSTF磁盘调度算法:
SSTF 代表最短寻道时间优先。顾名思义,该算法服务于最接近当前头部或指针位置的任务请求。在这里,头部的方向在确定整个头部运动中起着至关重要的作用。如果请求之间出现联系,则头部将在其正在进行的方向上为遇到它的请求提供服务。
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 的当前磁头位置head 是 53 并且会朝正确的方向移动。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
2.看磁盘调度算法:
Look Algorithm 实际上是 SCAN Algorithm 的改进版本。在这个算法中,磁头从磁盘的一侧开始第一个请求,并通过服务其间的所有请求向另一端移动。与 SCAN 不同,在这种情况下,头部不是走到最后一个轨道,而是走到最后一个请求,然后改变方向。与 SSTF 不同,它不处理最接近当前头部或指针位置的任务请求。
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 的当前磁头位置head 是 53 并且会向右移动。使用LOOK算法计算读/写磁头的磁道移动总数。
总的头部运动,
= (65-53)+(98-65)+(122-98)
+(124-122)+(183-124)+(183-40)+(40-10)
= 303
SSTF和LOOK磁盘调度算法的区别:
LOOK SCHEDULING ALGORITHM | SSTF SCHEDULING ALGORITHM | |
---|---|---|
1. | The performance of LOOK is better than SSTF. | SSTF lags in performance. |
2. | LOOK results in increased total seek time. | It reduces total seek time as compared to LOOK. |
3. | It provides low variance in average waiting time and response time. | This algorithm provides high variance average response time and waiting time. |
4. | As shown in above example, direction of head gets reversed when it serves the last request in one direction. | But here, direction of head plays an important role, in order to break tie between requests. |
5. | In this algorithm, there is an overhead for finding end request. | Here, there is an overhead for finding out closest request. |
6. | LOOK does not cause starvation to any request. | Here, the request which are far from head will suffer starvation. |
7. | LOOK algorithm can handle requests more effectively than SSTF. | Here handling of request is not so good as compared to LOOK algorithm. |