Exercise #$1[12$ pts$]$: Warm$-$up

Consider a system that uses Round$-$Robin scheduling with a context$-$switching$/$scheduling overhead of duration $0\mathrm{.}1$ms$.$ We represent the execution timeline of a job mix on this system by indicating $1$ms of computation for a process via an uppercase letter. For example, the following string:

AAAoBoCoAAoDDD

means that process A executes for $3$ms$,$ then process B for $1$ms$,$ then process C for $1$ms$,$ then process A again for $2$ms$,$ then process D for $3$ms$,$ with appropriate context$-$switches in between. This example execution lasts $10$ms $+4*0\mathrm{.}1$ms $=10\mathrm{.}4$ms $(10$ms of actual process execution, $\mathrm{.}4$ms of context$-$switch overhead$).$

Question #$1[4$ pts$]$:

Say that the system is running $3$ jobs, A$,$ B$,$ and C$,$ and that all of them are CPU$-$intensive $($i$.$e$.,$ each one does one infinitely long CPU burst$).$ The system begins with A on the CPU at the beginning its time quantum while B and C are in the Ready Queue, in that order.

a$)$ Show the execution pattern $($as a string of A$$s$,$ B$$s$,$ C$,$ and o$$s$)$ assuming that the scheduler time quantum is equal to $4$ ms$.$ Show the execution for more than $20$ms $($but less than $30$ms$).$

b$)$ In the long run $($i$.$e$,$ assuming jobs don$$t ever terminate$),$ what percentage of the CPU time is wasted doing context$-$switching$/$scheduling$?$

Question #$2[4$ pts$]$

Consider the following execution on our system:

AAAAAoBBBBoCoDDDDDoAAAAAoCoDDDDDoAAAoCoDDDDDoAAAAAoBBBBB

a$)$ Out of the four jobs, which one do you think is a more interactive process $($e$.$g$.,$ a text editor$)$ and why?

b$)$ In general $($not specific to the execution above$),$ is it possible to have a job run for longer than the time quantum with a Round Robin scheduling algorithm? Explain your answer.

Question #$3[4$ pts$]$:

Consider a job mix in which there is one CPU$-$intensive job, A$,$ and $3$ I$/$O$-$intensive jobs, B$,$ C$,$ and D$.$ The scheduler uses Round Robin scheduling with a time quantum of $3$ ms$.$ Here is an observed execution, where initially A is running and B$,$ C$,$ and D are in the ready queue in this order:

AAAoBBoCoDoAAAoBBoAAAoDoAAAoBBoCoDoAAAoBBo

Answer the following questions based on the above. Explain and give a specify a part of the execution.

a$)$ Could any of A$$s I$/$O$-$burst time be $6$ ms$?$ Explain.

b$)$ Could any of B$$s I$/$O$-$burst time be $4$ ms$?$ Explain.