We use the elevator algorithm to service requests on a disk with the following characteristics:
1. There are 10,001 tracks, numbered 0 to 10,000.
2. It takes time 1+(t/1000) milliseconds to move the headttracks.
3. Once arriving at a track, it takes 1 ms. to satisfy a request for I/O at that track.
4. The decision regarding what track to visit next is made based on all requests made at or before the time at which the previous read completes, subject to the "elevator" rule that the head continues to sweep in the same direction, if possible.
5. At time 0, the head is at track 0, at rest.
Here is the sequence of I/O requests and the times at which they are made:
|
0
|
7000
|
|
5
|
2000
|
|
10
|
9000
|
|
15
|
6000
|
|
20
|
3000
|
Simulate the behavior of the elevator algorithm for these requests. Which of the following correctly describes the time (in milliseconds) at which the I/O operation at a certain track begins?