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Scheduling criteria (turnaround time, waiting time, throughput) in Operating Systems - Cheat Sheet & Quick Revision

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beginner
What is turnaround time in process scheduling?
Turnaround time is the total time taken from when a process enters the system until it completes. It includes both waiting time and execution time.
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beginner
Define waiting time in the context of CPU scheduling.
Waiting time is the total time a process spends waiting in the ready queue before it gets CPU time for execution.
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beginner
What does throughput measure in scheduling?
Throughput measures how many processes are completed per unit time. Higher throughput means more work is done in less time.
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intermediate
How are turnaround time and waiting time related?
Turnaround time includes waiting time plus the actual execution time of the process. So, turnaround time = waiting time + burst time.
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intermediate
Why is minimizing waiting time important in scheduling?
Minimizing waiting time improves system responsiveness and user experience because processes spend less time idle before execution.
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What does turnaround time include?
ATime spent in I/O operations only
BOnly execution time
COnly waiting time
DWaiting time plus execution time
Which scheduling criterion measures the number of processes completed per unit time?
AWaiting time
BTurnaround time
CThroughput
DResponse time
If a process has a burst time of 5 ms and waiting time of 3 ms, what is its turnaround time?
A8 ms
B5 ms
C3 ms
D2 ms
Why is high throughput desirable in scheduling?
AIt means fewer processes are completed
BIt means more processes are completed quickly
CIt increases waiting time
DIt reduces CPU utilization
Which of the following is NOT a scheduling criterion?
AMemory size
BWaiting time
CThroughput
DTurnaround time
Explain the difference between turnaround time and waiting time in process scheduling.
Think about the total time a process spends in the system versus just the waiting part.
You got /3 concepts.
    Why is throughput an important measure in CPU scheduling? Give a real-life example.
    Consider how fast work gets done in a busy place.
    You got /3 concepts.

      Practice

      (1/5)
      1. Which scheduling criterion measures the total time taken from the arrival of a process to its completion?
      easy
      A. Turnaround time
      B. Waiting time
      C. Throughput
      D. Response time

      Solution

      1. Step 1: Understand the definition of turnaround time

        Turnaround time is the total time from when a process arrives until it finishes execution.

      2. Step 2: Compare with other criteria

        Waiting time is only the time a process waits before starting, throughput is number of processes completed per time, and response time is time until first response.

      3. Final Answer:

        Turnaround time -> Option A

      4. Quick Check:

        Turnaround time = total process duration [OK]
      Hint: Turnaround = arrival to finish total time [OK]
      Common Mistakes:
      • Confusing waiting time with turnaround time
      • Mixing throughput with time durations
      • Thinking response time equals turnaround time
      2. Which of the following correctly defines waiting time in process scheduling?
      easy
      A. Number of processes completed per unit time
      B. Time from process arrival to completion
      C. Time a process spends in the ready queue before execution
      D. Time taken by CPU to execute the process

      Solution

      1. Step 1: Define waiting time

        Waiting time is the time a process spends waiting in the ready queue before it starts running on the CPU.

      2. Step 2: Eliminate other options

        Time from process arrival to completion describes turnaround time, C describes throughput, and D is CPU burst time, not waiting time.

      3. Final Answer:

        Time a process spends in the ready queue before execution -> Option C

      4. Quick Check:

        Waiting time = time before execution [OK]
      Hint: Waiting time = time before process runs [OK]
      Common Mistakes:
      • Confusing waiting time with turnaround time
      • Thinking waiting time includes execution time
      • Mixing throughput with waiting time
      3. Consider three processes with the following completion times (in seconds): P1=10, P2=15, P3=20. If all arrived at time 0, what is the throughput if the total time observed is 20 seconds?
      medium
      A. 0.20 processes per second
      B. 0.15 processes per second
      C. 0.10 processes per second
      D. 0.25 processes per second

      Solution

      1. Step 1: Calculate total processes completed

        All three processes (P1, P2, P3) completed within 20 seconds, so total completed = 3.

      2. Step 2: Calculate throughput

        Throughput = number of processes completed / total time = 3 / 20 = 0.15 processes per second.

      3. Final Answer:

        0.15 processes per second -> Option B

      4. Quick Check:

        Throughput = 3/20 = 0.15 [OK]
      Hint: Throughput = completed tasks ÷ total time [OK]
      Common Mistakes:
      • Dividing total time by number of processes instead of reverse
      • Counting incomplete processes
      • Using average completion time instead of total time
      4. A scheduler reports the following for a process: Arrival time = 0, Start time = 5, Completion time = 12. The waiting time is incorrectly calculated as 7 seconds. What is the correct waiting time?
      medium
      A. 5 seconds
      B. 7 seconds
      C. 12 seconds
      D. 0 seconds

      Solution

      1. Step 1: Understand waiting time formula

        Waiting time = Start time - Arrival time = 5 - 0 = 5 seconds.

      2. Step 2: Identify error in reported waiting time

        The reported waiting time of 7 seconds is incorrect because it does not match the formula.

      3. Final Answer:

        5 seconds -> Option A

      4. Quick Check:

        Waiting time = start - arrival = 5 [OK]
      Hint: Waiting time = start time minus arrival time [OK]
      Common Mistakes:
      • Using completion time instead of start time
      • Adding instead of subtracting times
      • Confusing waiting time with turnaround time
      5. A system runs 4 processes with arrival times and burst times as follows:
      P1: arrival=0, burst=4
      P2: arrival=1, burst=3
      P3: arrival=2, burst=1
      P4: arrival=3, burst=3
      If the scheduler uses First-Come-First-Serve (FCFS), what is the average turnaround time?
      hard
      A. 3.5 seconds
      B. 4.5 seconds
      C. 5.0 seconds
      D. 6.0 seconds

      Solution

      1. Step 1: Calculate completion times using FCFS

        Process order by arrival: P1(0), P2(1), P3(2), P4(3).
        P1 completes at 0+4=4,
        P2 starts at 4, completes at 4+3=7,
        P3 starts at 7, completes at 7+1=8,
        P4 starts at 8, completes at 8+3=11.

      2. Step 2: Calculate turnaround times

        Turnaround = completion - arrival:
        P1: 4-0=4,
        P2: 7-1=6,
        P3: 8-2=6,
        P4: 11-3=8.
        Average turnaround = (4+6+6+8)/4 = 24/4 = 6.0 seconds.

      3. Final Answer:

        6.0 seconds -> Option D

      4. Quick Check:

        Average turnaround = 24/4 = 6.0 [OK]
      Hint: Turnaround = completion - arrival; average = sum/number [OK]
      Common Mistakes:
      • Calculating turnaround as burst time only
      • Ignoring arrival times in scheduling order
      • Mixing waiting time with turnaround time