A system runs a good mix of I/O-bound tasks and CPU-bound task. On one instance,

Question

A system runs a good mix of I/O-bound tasks and CPU-bound task. On one instance, there were 5 I/O-bound tasks and 6 CPU-bound tasks. Assume that the I/O-bound tasks issue an I/O operation once for every two millisecond of CPU computing and that each I/O operation takes 10 milliseconds to complete. Also assume that the context-switching overhead is 0.5 millisecond and that all processes are long-running tasks. Describe the CPU utilization for a round-robin scheduler when:

a. The time quantum is 1 millisecond

b. The time quantum is 10 milliseconds

Explanation / Answer

Answer---

a). Given ,The time quantum is 1 millisecond.

Irrespective of which process is scheduled, the scheduler incurs a 0.1 millisecond context-switching cost for every context-switch.

This results in a CPU utilization of ====== ( 1 / 1.1 ) * 100

====== ( 1000 / 11 ) = 0.91

====== 91 %

b). Given ,The time quantum is 10 millisecond.

The I/O-bound tasks incur a context-switch after using up only 1 millisecond of the time quantum.

The time required to cycle through all the processes is therefore 10 * 1.1 + 10.1 ( as each I/O-bound task executes for 1 millisecond and then incur the context-switch task, whereas the CPU- bound task executes for 10 millisecond before incurring a context switch )

Therefore, This results in CPU Utilization of ====== ( 20 / 21.1 ) * 100

====== 20000 / 211 = 0.94

====== 94 %

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