SSTF最短寻道时间优先 Selects the request with the minimum seek time from the current head position.选择从当前磁头位置所需寻道时间最 短的请求。 SSTF scheduling is a form of SJF scheduling; may cause starvation of some requests.SSTF是SJF调度的一种形式; 有可能引起某些请求的饥饿。 Illustration shows total head movement of 236 cylinders. sn 图所示,磁头移动的总距离是236柱面 Applied Operating System Concepts 13.6 Silberschatz, Galvin, and Gagne @1999
13.6 Silberschatz, Galvin, and Gagne ©1999 Applied Operating System Concepts SSTF 最短寻道时间优先 • Selects the request with the minimum seek time from the current head position. 选择从当前磁头位置所需寻道时间最 短的请求。 • SSTF scheduling is a form of SJF scheduling; may cause starvation of some requests. SSTF是SJF调度的一种形式； 有可能引起某些请求的饥饿。 • Illustration shows total head movement of 236 cylinders. 如 图所示，磁头移动的总距离是236柱面
SSTF(Cont) queue=98,183,37,122,14,124,65,67 head starts at 53 014 37536567 98122124 183199 Applied Operating System Concepts 13.7 Silberschatz, Galvin, and Gagne @1999
13.7 Silberschatz, Galvin, and Gagne ©1999 Applied Operating System Concepts SSTF (Cont.)
scAN扫描算法 The disk arm starts at one end of the disk and moves toward the other end, servicing requests until it gets to the other end of the disk. where the head movement is reversed and servicing continues.磁头从磁盘的一端开始向另一端移动, 沿途响应访问请求,直到到达了磁盘的另一端,此时磁头反向 移动并继续响应服务请求。 Sometimes called the elevator algorithm.有时也称为电梯算 法。 Illustration shows total head movement of 208 cylinders. A 图所示,磁头移动的总距离是208柱面。 Applied Operating System Concepts 138 Silberschatz, Galvin, and Gagne @1999
13.8 Silberschatz, Galvin, and Gagne ©1999 Applied Operating System Concepts SCAN 扫描算法 • The disk arm starts at one end of the disk, and moves toward the other end, servicing requests until it gets to the other end of the disk, where the head movement is reversed and servicing continues. 磁头从磁盘的一端开始向另一端移动， 沿途响应访问请求，直到到达了磁盘的另一端，此时磁头反向 移动并继续响应服务请求。 • Sometimes called the elevator algorithm. 有时也称为电梯算 法。 • Illustration shows total head movement of 208 cylinders. 如 图所示，磁头移动的总距离是208柱面
SCAN (Cont 0J寸 3、e3e29 8 SSIS寸 」83J0 pesq asifa sf 23 dnen6=88:」833J5S'J」Sep'e⊥ Applied Operating System Concepts 13.9 Silberschatz, Galvin, and Gagne @1999
13.9 Silberschatz, Galvin, and Gagne ©1999 Applied Operating System Concepts SCAN (Cont.)
C-SCAN Provides a more uniform wait time than SCaN.提供比扫描 算法更均衡的等待时间。 The head moves from one end of the disk to the other servicing requests as it goes. When it reaches the other end however, it immediately returns to the beginning of the disk, without servicing any requests on the return trip.磁头从磁盘 的一段向另一端移动,沿途响应请求。当它到了另一端,就立 即回到磁盘的开始处,在返回的途中不响应任何请求。 Treats the cylinders as a circular list that wraps around from the last cylinder to the first o e.把所有柱面看成一个循环的 序列,最后一个柱面接续第一个柱面。 Applied Operating System Concepts 13.10 Silberschatz, Galvin, and Gagne @1999
13.10 Silberschatz, Galvin, and Gagne ©1999 Applied Operating System Concepts C-SCAN • Provides a more uniform wait time than SCAN. 提供比扫描 算法更均衡的等待时间。 • The head moves from one end of the disk to the other. servicing requests as it goes. When it reaches the other end, however, it immediately returns to the beginning of the disk, without servicing any requests on the return trip. 磁头从磁盘 的一段向另一端移动，沿途响应请求。当它到了另一端，就立 即回到磁盘的开始处，在返回的途中不响应任何请求。 • Treats the cylinders as a circular list that wraps around from the last cylinder to the first one. 把所有柱面看成一个循环的 序列，最后一个柱面接续第一个柱面