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This project explores various device management strategies implemented in operating systems, focusing on disk scheduling algorithms such as FCFS, SSTF, SCAN, LOOK, and C-SCAN. It examines request handling processes, analyzes time complexity for specific track requests, and provides visual animations to illustrate their operational mechanics. The assignment aims to elucidate how different strategies can impact system performance, particularly in managing read/write head movements on disks.
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Operating Systems reportspost topic on WebCT Sassn 5PDL = 10 Novproject = 1 Dec
Operating Systems wassn7 assigned due next time
Device Management II. Device Management D. DASD example 4. OS seek stategies
Device Management a. FCFS tracks are serviced in the order they arrived
Device Management data organization assume 50 tracks on disc (0..49) assume 1ms to move R/W head 1 track
Device Management track 0 at “outside” track max at “inside”
Device Management R/W head at track 20 list of pending track requests: {2, 26, 19, 43}
Device Management at track 20
Device Management move to track 2
Device Management move to track 26
Device Management move to track 19
Device Management move to track 43
Device Management how much time? 20 2 26 19 43 [20-2] + [26-2] + [26-19] + [43-19] 18 + 24 + 7 + 24 = 73 msec
Device Management animation link
Device Management b. SSTF Shortest Seek Time First track closest to current request is serviced next
Device Management adv: short seek times dis: favors “closer” tracks may indefinitely postpone “far” tracks
Device Management SSTF will reorder pending requests eg, {2, 26, 19, 43} becomes {19, 26, 43, 2}
Device Management at track 20
Device Management move to track 19
Device Management move to track 26
Device Management move to track 43
Device Management move to track 2
Device Management how much time? 20 19 26 43 2 [20-19] + [26-19] + [43-26] + [43-2] 1 + 7 + 17 + 41 = 66 msec
Device Management c. SCAN continue R/W head travel in one direction satisfying requests “direction bit”
Device Management SCAN moves all the way across disc surface eg, from lower (outer) tracks to higher (inner)(track 0 is outermost)
Device Management SCAN analogy wiper blade
Device Management adv: no starvation dis: may miss a “close” track if moving in other direction
Device Management SCAN will reorder pending requests eg, {2, 26, 19, 43} “in” becomes {26, 43, [49], 19, 2}
Device Management at track 20
Device Management move to track 26
Device Management move to track 43
Device Management move to track 49
Device Management move to track 19
Device Management move to track 2
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
Device Management d. LOOK variation of SCAN R/W head doesn’t move to extreme edges of disc if no requests
Device Management at track 20
Device Management move to track 26
Device Management move to track 43
Device Management move to track 19
Device Management move to track 2
Device Management how much time? 20 26 43 19 2 [26-20] + [43-26] + [43-19] + [19-2] 6 + 17 + 24 + 17 = 64 msec
Device Management e. C-SCAN circular SCAN R/W head moves from outer to inner track
Device Management when inner track is reached quickly repositions head (no reading) back to outer track begins a new “sweep”
Device Management CSCAN analogy squeegee
Device Management not even close … … to hernia movers
Device Management C-SCAN will reorder pending requests eg, {2, 26, 19, 43} “in” becomes {26, 43, 2, 19}
Device Management at track 20
Device Management move to track 26
