ISI2018-PCB-CS7

Question

A context switch from a process $P_{old}$ to a process $P_{new}$ consists of the following steps:

1. Step I:saving the context of $P_{old}$;
2. Step II: running the scheduling algorithm to pick $P_{new}$;
3. Step III: restoring the saved context of $P_{new}$.

Suppose Steps I and III together take $T_0$ units of time. The scheduling algorithm takes $nT_1$ units of time, where $n$ is the number of ready-to-run processes. The scheduling policy is round-robin with a time slice of $10$ms. Compute the CPU utilization for the following scenario: $k$ processes become ready at almost the same instant in the order $P_1, P_2, . . . , P_k;$ each process requires exactly one CPU burst of $20$ms and no I/O burst.

Answer 1

Few points to keep in mind.

• When process is scheduled first Step 3 not required.
• When process is scheduled for last time step 1 not required.

Ref: https://cs.stackexchange.com/questions/74049/does-a-context-switch-happen-when-a-process-has-terminated

 

Total time used for process = 2 * k * 10 (Each process is gets 2 time slice of 10 ms.)

In 1st cycle it takes  $kT_{1}$ for selecting which process to schedule and $T_{0}$ for saving the context.

In 2nd cycle it takes  $kT_{1}$ for selecting which process to schedule and $T_{0}$ for restoring the context.

Total Time =  $2 * k * 10 +  2k (kT_{1} + T_{0} )$ 

$CPU$ $Utitlization =$ $\frac{ Useful Work}{Total Time}$

                                =  $\frac{ 2 * k * 10}{2 k(kT_{1} + T_{0} ) + 2 * k * 10 }$

                                =  $\frac{ 10k}{ k^2T_{1} + kT_{0}  + 10k }$

Comments

this ans is not correct.

Answer 2

CPU Utilization = 10*k / [10*k + k*(T0 + k*T1)]

Comments

Can u give detailed solution.. I think answer should be $\frac{2\times K\times10}{2\times 10\times k + (T+KT1)(2k-1))}$ because burst time is 20ms

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I don’t have enough points to downvote you, otherwise, I would have surely downvoted you buddy !! Answering without explanation is against the spirit of GO

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See if you really follow THE GO SPIRIT  then come here to solve the question by your own. If you can't then 1st read the concepts then again try here till you find the answers (at last you will).