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Disk Scheduling

Disk Scheduling. 디스크에서 트랙 , 섹터 , 실린더 (1). 디스크에서 트랙 , 섹터 , 실린더 (2). Disk Scheduling (1). Access to a particular record of data : 0.1~0.001 sec. Seek Time Rotational Delay (Latency Time) : 3600 회전 / sec Data Transmission Time Why Scheduling FCFC

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Disk Scheduling

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  1. Disk Scheduling SunMoon University

  2. 디스크에서 트랙,섹터,실린더(1) SunMoon University

  3. 디스크에서 트랙,섹터,실린더(2) SunMoon University

  4. Disk Scheduling (1) • Access to a particular record of data : 0.1~0.001 sec. • Seek Time • Rotational Delay (Latency Time) : 3600회전 / sec • Data Transmission Time • Why Scheduling • FCFC • but can result in very long waiting times when the request rate becomes heavy SunMoon University

  5. Disk Scheduling (2) • Criteria for Scheduling Policies 1) Maximize thruput 2) Minimize mean response time (mean waiting time) 3) Minimize the variance in response time (predictability) SunMoon University

  6. Disk Scheduling (3) • Two Scheduling Method 1) Seek Optimization 2) Latency Optimization, when heavy load • Seek Optimization • Disk Queue : 78, 183, 37, 122, 14, 124, 65, 67 • Initial Head : 53 SunMoon University

  7. 디스크 접근요소 SunMoon University

  8. Disk Scheduling - FCFS • FCFS • fair • acceptable when light load • no attempt at optimizing the seek patterns • low thruput • fig 9.7 SunMoon University

  9. FCFS Illustration shows total head movement of 640 cylinders. SunMoon University

  10. Disk Scheduling - SSTF • SSTF (Shortest Seek Time First) • selects the request with the minimum seek time from the current head position • better thruput response time for moderate load • higher variance • no predictability • long waiting time for innermost / outermost track requests • good for batch processing • possible “starvation” • fig 9.8 SunMoon University

  11. SSTF (Cont.) SunMoon University

  12. Disk Scheduling - SCAN • SCAN • called the elevator algorithm • 헤드의 진행 방향에서 가장 가까운 것부터 • 진행 방향의 변화는 끝에 도달하거나, 더 이상 진행 방향 쪽으로 작업이 없을때 • 중산 job에는 두 번의 기회, 외곽 job은 한번의 기회 • thruput, mean response time, lower variance • 문제 : • 헤드 진행 방향으로는 작업 요청 밀도가 낮고, 헤드의 뒤쪽에 요청 밀도가 높은 경우 • fig 9.9 SunMoon University

  13. SCAN (Cont.) SunMoon University

  14. Disk Scheduling – C-SCAN • C-SCAN (Circular SCAN) • 헤드를 한 방향으로만 움직임 • outer ----> inner or • outer <---- inner • 안쪽과 바깥쪽의 실린더에 대한 차별 대우를 없앰 • lower variance in response time • N-Step SCAN • 헤드는 양 방향으로 움직임 • 단, 헤드가 움직이기 시작한 뒤에 도착한 작업들은 뒤돌아와서 처리 • 어떤 방향으로 진행이 시작될 당시 대기 중이던 요청들만 서비스 SunMoon University

  15. C-SCAN (Cont.) SunMoon University

  16. N-단계 SCAN 스케줄링 SunMoon University

  17. Disk Scheduling - LOOK • LOOK Scheduling • the head is only moved as far as the last request in each direction • cf.) SCAN, C-SCAN : move the head from one end of the disk to the other. SunMoon University

  18. C-LOOK (Cont.) SunMoon University

  19. Selecting a Disk-Scheduling Algorithm • Simulation results • low load --> SCAN • medium to heavy load --> C-SCAN • influenced by the file-allocation method • contiguously allocated file • a linked or indexed file • the location of directories and index blocks • placing the directories halfway between the inner and outer edge of the disk • placing the directories at either end SunMoon University

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