Operating Systems

1. Operating Systems reportspost topic on WebCT Sassn 5PDL = 10 Novproject = 1 Dec

2. Operating Systems wassn7 assigned due next time

3. Operating Systems

4. Device Management II. Device Management D. DASD example 4. OS seek stategies

5. Device Management a. FCFS tracks are serviced in the order they arrived

6. Device Management data organization assume 50 tracks on disc (0..49) assume 1ms to move R/W head 1 track

7. Device Management track 0 at “outside” track max at “inside”

8. Device Management R/W head at track 20 list of pending track requests: {2, 26, 19, 43}

9. Device Management at track 20

10. Device Management move to track 2

11. Device Management move to track 26

12. Device Management move to track 19

13. Device Management move to track 43

14. Device Management how much time? 20 2 26 19 43 [20-2] + [26-2] + [26-19] + [43-19] 18 + 24 + 7 + 24 = 73 msec

15. Device Management animation link

16. Device Management b. SSTF Shortest Seek Time First track closest to current request is serviced next

17. Device Management adv: short seek times dis: favors “closer” tracks may indefinitely postpone “far” tracks

18. Device Management SSTF will reorder pending requests eg, {2, 26, 19, 43} becomes {19, 26, 43, 2}

19. Device Management at track 20

20. Device Management move to track 19

21. Device Management move to track 26

22. Device Management move to track 43

23. Device Management move to track 2

24. Device Management how much time? 20 19 26 43 2 [20-19] + [26-19] + [43-26] + [43-2] 1 + 7 + 17 + 41 = 66 msec

25. Device Management c. SCAN continue R/W head travel in one direction satisfying requests “direction bit”

26. Device Management SCAN moves all the way across disc surface eg, from lower (outer) tracks to higher (inner)(track 0 is outermost)

27. Device Management SCAN analogy wiper blade

28. Device Management adv: no starvation dis: may miss a “close” track if moving in other direction

29. Device Management SCAN will reorder pending requests eg, {2, 26, 19, 43} “in” becomes {26, 43, [49], 19, 2}

30. Device Management at track 20

31. Device Management move to track 26

32. Device Management move to track 43

33. Device Management move to track 49

34. Device Management move to track 19

35. Device Management move to track 2

36. Device Management how much time? 2026 43 49 19 2 [26-20] + [43-26] + [49-43] + [49-19] + [19-2] 6 + 17 + 6 + 30 + 17 = 76 msec

37. Device Management d. LOOK variation of SCAN R/W head doesn’t move to extreme edges of disc if no requests

38. Device Management at track 20

39. Device Management move to track 26

40. Device Management move to track 43

41. Device Management move to track 19

42. Device Management move to track 2

43. Device Management how much time? 20 26 43 19 2 [26-20] + [43-26] + [43-19] + [19-2] 6 + 17 + 24 + 17 = 64 msec

44. Device Management e. C-SCAN circular SCAN R/W head moves from outer to inner track

45. Device Management when inner track is reached quickly repositions head (no reading) back to outer track begins a new “sweep”

46. Device Management CSCAN analogy squeegee

47. Device Management not even close … … to hernia movers

48. Device Management C-SCAN will reorder pending requests eg, {2, 26, 19, 43} “in” becomes {26, 43, 2, 19}

49. Device Management at track 20

50. Device Management move to track 26