Question

(c) Assume that . accessing and searching the TLB takes 5 ns, . transferring the data from the TLB into a register takes 10 ns, . updating a record in the TLB takes 15 ns, .accessing and searching the PT takes 50 ns, .transferring the data from the PT into a register takes 70 ns, . the TLB hit ratio is 0.6. How many nanoseconds are needed on average to perform all the TLB and PT operations needed to compute the physical address for a given virtual address (assuming that all referenced pages are in main memory) (c) A hard disk spins at 6000 rprn (revolution per minute), and it takes 100 μs (on average) for the head to traverse one track. Consider the following sequence of disk track requests: 27, 129, 110, 186, 147, 41, 10, 64, 120. Assume that initially the head is at track 30 and is moving in the direction of decreasing track number. Compute the time it takes to serve the requests using FIFO, SSF (shortest seek first) and SCAN (elevator) algorithms.

Answer 1

Solution

c)

FCFS:

Disk head movement is

30->27->129->110->186->147->41->10->64->120

=(30‐27) + (129‐27) + (129‐110) + (186‐110) + (186‐147) + (147‐41) + (41‐10) + (64‐10) + (120‐64)

=486

Time taken= 486*100ms

=48.6 ms

SSF

30->27->41->64->110->120->129->147->186->10

(30‐27) + (41‐27) + (64‐41) + (110‐64) + (120‐110) + (129‐120) + (147‐129) + (186‐147) + (186‐10)

=338     

Time taken= 338*100ms

=33.8 ms

SCAN

30->27->10->0->41->64->110->120->129->147->186

=(30‐27) + (27‐10) + (10‐0) + (41‐0) + (64‐41) + (110‐64) + (120‐110) + (129‐120) + (147‐129) + (186‐147)

=216

Time taken= 216*100ms

=21.6 ms

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