Past exam of the year 1 of the computer science course of the University of Oxford / 2017 / Digital systems, linear algebra, introduction to formal proof / 2 / d

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- Solution d

Solution

With the given ready-order rule and A first, the CPU timeline in milliseconds is

interval CPU 0 : 4 A 4 : 5 B 5 : 6 idle 6 : 10 A 10 : 11 B 11 : 12 idle 12 : 16 A 16 : 17 B

A performs input-output during

[4, 6)

and

[10, 12)

; B does so during

[5, 7)

and

[11, 13)

. In particular, B becoming ready at time 7 does not interrupt A's ongoing nonpreemptive burst. The completion times are

C_{A} = 16

and

C_{B} = 17

elapsed time = 17 ms, mean turnaround = 16.5 ms .

There are 15 ms of useful CPU execution and 2 ms of idle time on this particular timeline.

Figure 1.
CPU timelines and I/O waits for the two scheduling policies
. Both processes arrive at zero. Readiness determines queue order; A wins initial ties. The displayed completion times give mean turnarounds 16.5 ms without preemption and 14 ms with a two-millisecond quantum. Details.

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