1 / 19

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

chars
Download Presentation

Disk Scheduling

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  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

More Related