Deadlock avoidance (Banker's algorithm) in Operating Systems - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare the maximum resource matrix in Banker's algorithm.

Operating Systems

max_resources = [[7, 5, 3], [3, 2, [1]], [9, 0, 2]]

Drag options to blanks, or click blank then click option'

A5

B4

C2

D3

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing allocated resources with maximum demand.

Using a value not consistent with the matrix pattern.

The maximum resource matrix defines the maximum demand of each process. Here, the missing value is 2 to match the example matrix.

2fill in blank

medium

Complete the code to calculate the Need matrix in Banker's algorithm.

Operating Systems

need = [[max_resources[i][j] - [1][i][j] for j in range(m)] for i in range(n)]

Drag options to blanks, or click blank then click option'

Arequest

Ballocation

Cavailable

Dmax_resources

Attempts:

3 left

💡 Hint

Common Mistakes

Subtracting available resources instead of allocated.

Using the wrong matrix for calculation.

The Need matrix is calculated by subtracting the allocated resources from the maximum resources for each process.

3fill in blank

hard

Fix the error in the safety check condition to verify if a process can be safely allocated resources.

Operating Systems

if all(need[i][j] <= [1][j] for j in range(m)):

Drag options to blanks, or click blank then click option'

Aallocation

Brequest

Cmax_resources

Davailable

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing Need with Allocation or Max resources.

Using the request matrix incorrectly.

The safety check compares the Need matrix with the Available resources to decide if the process can proceed safely.

4fill in blank

hard

Fill both blanks to update the Available resources and Allocation matrix after granting a request.

Operating Systems

available = [available[j] - [1][j] for j in range(m)]
allocation[process_id] = [allocation[process_id][j] + [2][j] for j in range(m)]

Drag options to blanks, or click blank then click option'

Arequest

Ballocation

Cavailable

Dneed

Attempts:

3 left

💡 Hint

Common Mistakes

Using Allocation or Need instead of Request for updates.

Updating Available incorrectly.

When a request is granted, Available resources decrease by the request amount, and Allocation for the process increases by the same request.

5fill in blank

hard

Fill all three blanks to check if the system is in a safe state after resource allocation.

Operating Systems

finish = [False] * n
work = available.copy()
for i in range(n):
    if not finish[i] and all(need[i][j] <= [1][j] for j in range(m)):
        work = [work[j] + [2][i][j] for j in range(m)]
        finish[i] = [3]

Drag options to blanks, or click blank then click option'

Awork

Ballocation

CTrue

DFalse

Attempts:

3 left

💡 Hint

Common Mistakes

Using Available instead of Work in the condition.

Marking finish as False instead of True.

Updating Work incorrectly.

The safety algorithm checks if Need is less than or equal to Work (initially Available). If yes, it adds Allocation of that process to Work and marks the process as finished (True).

Practice

1. What is the main purpose of the Banker's algorithm in operating systems?

easy

A. To increase CPU speed by optimizing instructions

B. To manage file storage on disk drives

C. To avoid deadlocks by checking if resource allocation is safe

D. To encrypt data for security purposes

Solution

1. Step 1: Understand the Banker's algorithm role

   The Banker's algorithm is designed to prevent deadlocks by simulating resource allocation and ensuring the system remains in a safe state.

2. Step 2: Compare options with the algorithm's purpose

   Only To avoid deadlocks by checking if resource allocation is safe correctly describes this purpose. Other options relate to unrelated system functions.

3. Final Answer:

   To avoid deadlocks by checking if resource allocation is safe -> Option C

4. Quick Check:

   Deadlock avoidance = Checking safe resource allocation [OK]

Hint: Banker's algorithm checks safety before granting resources [OK]

Common Mistakes:

• Confusing deadlock avoidance with performance optimization
• Thinking it manages storage or security
• Assuming it works after deadlock occurs

2. Which of the following is a required input for the Banker's algorithm to work correctly?

easy

A. The total CPU time each process will use

B. The maximum resource needs of each process

C. The size of the hard disk

D. The network bandwidth available

Solution

1. Step 1: Identify inputs needed by Banker's algorithm

   The algorithm requires the maximum resource needs of each process to predict safe allocation.

2. Step 2: Eliminate unrelated options

   CPU time, disk size, and network bandwidth are not inputs for this algorithm.

3. Final Answer:

   The maximum resource needs of each process -> Option B

4. Quick Check:

   Max resource needs = Required input [OK]

Hint: Know max needs to predict safe resource allocation [OK]

Common Mistakes:

• Confusing CPU time with resource needs
• Including unrelated system metrics
• Ignoring maximum resource requirements

3. Given the following data for three processes (P1, P2, P3) and one resource type with 10 units total:
Allocation: P1=3, P2=2, P3=2
Maximum: P1=7, P2=4, P3=3
Available: 1 unit
Which process can safely get its requested resources without causing deadlock?

medium

A. P1, because it needs 4 more units which are available

B. P2, because it needs 2 more units which are available

C. None, because available units are insufficient

D. P3, because it needs 1 more unit which is available

Solution

1. Step 1: Calculate each process's remaining need

   Need = Maximum - Allocation
   P1: 7-3=4 units
   P2: 4-2=2 units
   P3: 3-2=1 unit

2. Step 2: Compare needs with available units

   Available units = 1
   P1 needs 4 (not available)
   P2 needs 2 (not available)
   P3 needs 1 (available)
   Only P3 can be satisfied immediately, finishing first and releasing resources safely.

3. Final Answer:

   P3, because it needs 1 more unit which is available -> Option D

4. Quick Check:

   Need ≤ Available means safe allocation [OK]

Hint: Check need = max - allocation; compare with available [OK]

Common Mistakes:

• Miscalculating which process's need ≤ available
• Choosing process with higher need incorrectly
• Confusing allocation with need

4. A system using the Banker's algorithm has the following state:
Allocation: P1=1, P2=2
Maximum: P1=3, P2=3
Available: 1 unit
If process P1 requests 2 more units, what is the correct action?

medium

A. Deny the request to avoid unsafe state

B. Grant the request immediately

C. Ignore the request and continue

D. Increase available units to grant request

Solution

1. Step 1: Calculate P1's need and check availability

   P1's need = Maximum - Allocation = 3 - 1 = 2 units
   Available units = 1
   P1 requests 2 units, but only 1 is available.

2. Step 2: Determine if granting request is safe

   Granting 2 units is impossible now; it would cause an unsafe state or deadlock.
   Therefore, the request must be denied.

3. Final Answer:

   Deny the request to avoid unsafe state -> Option A

4. Quick Check:

   Request > Available means deny [OK]

Hint: Deny requests exceeding available resources [OK]

Common Mistakes:

• Granting requests without checking availability
• Ignoring unsafe states
• Assuming system can create resources

5. Consider a system with 3 processes (P1, P2, P3) and 2 resource types (R1 and R2).
Maximum needs:
P1: (7, 5), P2: (3, 2), P3: (9, 0)
Current allocation:
P1: (3, 2), P2: (2, 1), P3: (3, 0)
Available resources: (3, 2)
If P2 requests (1, 1), should the Banker's algorithm grant it? Why or why not?

hard

A. Yes, because granting keeps system in a safe state

B. No, because P2's request exceeds its maximum need

C. No, because granting leads to an unsafe state

D. Yes, because available resources are sufficient

Solution

1. Step 1: Check if request exceeds maximum need

   P2's maximum need = (3, 2)
   Current allocation = (2, 1)
   Request = (1, 1)
   New allocation would be (3, 2), which equals maximum need, so request is valid.

2. Step 2: Check if available resources can satisfy request and system remains safe

   Available = (3, 2)
   Request = (1, 1) ≤ Available, so resources are sufficient.
   After granting, Available becomes (2, 1).
   Check if system can finish all processes:
   - P1 needs (7-3, 5-2) = (4, 3)
   - P2 needs (3-3, 2-2) = (0, 0)
   - P3 needs (9-3, 0-0) = (6, 0)
   P2 can finish immediately, releasing (3, 2) back.
   Then Available becomes (5, 3), enough for P1 and P3 to finish.
   System is safe.

3. Final Answer:

   Yes, because granting keeps system in a safe state -> Option A

4. Quick Check:

   Request ≤ Max and safe state = Grant [OK]

Hint: Grant if request ≤ max and system stays safe after allocation [OK]

Common Mistakes:

• Denying valid requests due to misunderstanding max need
• Ignoring safety check after allocation
• Assuming available resources alone decide grant