Question 5070 – Operating-Systems
November 17, 2023Question 5215 – Operating-Systems
November 17, 2023Question 5068 – Operating-Systems
style=”font-weight: 400;”>Consider the input/output (I/O) requests made at different instants of time directed at a hypothetical disk having 200 tracks as given in the following table :
style=”font-weight: 400;”>Assume that :
style=”font-weight: 400;”>Current head position is at track no. 65
style=”font-weight: 400;”>Direction of last movement is towards higher numbered tracks
style=”font-weight: 400;”>Current clock time is 160 milliseconds
style=”font-weight: 400;”>Head movement time per track is
style=”font-weight: 400;”>1 millisecond.
style=”font-weight: 400;”>“look” is a variant of “SCAN” disk arm scheduling algorithm. In this algorithm, if no more I/O requests are left in current direction, the disk head reverses its direction. The seek times in Shortest Seek First (SSF) and “look” disk-arm scheduling algorithms respectively are
Correct Answer: B
Current head position is at track no. 65
The direction of last movement is towards higher numbered tracks
Current clock time is 160 milliseconds
Head movement time per track is 1 millisecond.
According to clock time, the serial no 1,2,3 and 4 tracks are ready to process.
SSTF( Shortest Seek time First )
Current head position is 65, according to SSTF next track is 75 but it will arrive at the time of 175, then it will process 85 first, then track 75 later 100 from there it will process 40 and then 12
so the requests are 65 → 85→ 75→ 100→ 40→ 12
= (85-65)+(85-75)+(100-75)+(100-40)+(40-12)
= 20+10+25+60+28
= 143 ms
Look method:
The requests are processed in the following way
Direction of last movement is towards higher number of tracks.
65→ 85→ 100→ 75→ 40→ 12
=(85-65)+(100-85)+(100-75)+(75-40)+(40-12)
=20+15+25+35+28
=123ms
Given data
Current head position is at track no. 65
The direction of last movement is towards higher numbered tracks
Current clock time is 160 milliseconds
Head movement time per track is 1 millisecond.
According to clock time, the serial no 1,2,3 and 4 tracks are ready to process.
SSTF( Shortest Seek time First )
Current head position is 65, according to SSTF next track is 75 but it will arrive at the time of 175, then it will process 85 first, then track 75 later 100 from there it will process 40 and then 12
so the requests are 65 → 85→ 75→ 100→ 40→ 12
= (85-65)+(85-75)+(100-75)+(100-40)+(40-12)
= 20+10+25+60+28
= 143 ms
Look method:
The requests are processed in the following way
Direction of last movement is towards higher number of tracks.
65→ 85→ 100→ 75→ 40→ 12
=(85-65)+(100-85)+(100-75)+(75-40)+(40-12)
=20+15+25+35+28
=123ms