Deadlock avoidance (Banker's algorithm) in Operating Systems - Step-by-Step Execution

Concept Flow - Deadlock avoidance (Banker's algorithm)

Start: System State

↓

Request from Process

↓

Check if Request <= Available Resources?

No→Deny Request

Yes↓

Pretend to Allocate Resources

↓

Check System Safety

↓

Is System Safe?

No→Deny Request

Yes↓

Grant Request

↓

Update System State

↓

End

The Banker's algorithm checks each resource request by simulating allocation and testing if the system remains in a safe state before granting it.

Execution Sample

Operating Systems

Available = [3, 3, 2]
Request = [1, 0, 2]
if Request <= Available:
  # Pretend allocate
  # Check safety
  if safe:
    # grant
  else:
    # deny

This code simulates checking a resource request against available resources and system safety before granting it.

Analysis Table

Step	Action	Request	Available Before	Available After	Safety Check Result	Decision
1	Receive request	[1,0,2]	[3,3,2]	[3,3,2]	N/A	Proceed
2	Check if request <= available	[1,0,2]	[3,3,2]	[3,3,2]	N/A	Yes, continue
3	Pretend allocate resources	[1,0,2]	[3,3,2]	[2,3,0]	N/A	Simulate
4	Perform safety check	[1,0,2]	[2,3,0]	[2,3,0]	Safe	Grant request
5	Update system state	[1,0,2]	[3,3,2]	[2,3,0]	Safe	Request granted

💡 Request granted because it is less than available and system remains safe after allocation.

State Tracker

Variable	Start	After Step 1	After Step 3	After Step 5
Available	[3,3,2]	[3,3,2]	[2,3,0]	[2,3,0]
Request	N/A	[1,0,2]	[1,0,2]	[1,0,2]
Decision	N/A	Proceed	Simulate	Granted

Key Insights - 3 Insights

Why do we pretend to allocate resources before actually granting the request?

Pretending to allocate helps check if the system will remain safe after allocation. This is shown in step 3 and 4 of the execution_table where allocation is simulated before the safety check.

What does it mean if the safety check fails after pretending to allocate?

If safety check fails, it means granting the request could lead to deadlock. So the request is denied to keep the system safe, as explained in the decision column of the execution_table.

Why must the request be less than or equal to available resources?

Because you cannot allocate more resources than what is currently free. Step 2 in the execution_table confirms this check before proceeding.

Visual Quiz - 3 Questions

Look at the execution_table, what is the Available resource vector after pretending to allocate resources?

A [3,3,2]

B [1,0,2]

C [2,3,0]

D [3,2,2]

💡 Hint

Check the 'Available After' column at Step 3 in the execution_table.

At which step does the algorithm decide if the system is safe after the pretend allocation?

A Step 2

B Step 4

C Step 1

D Step 5

💡 Hint

Look at the 'Safety Check Result' column in the execution_table.

If the request was [4,0,0], what would happen at Step 2?

A Request would be denied because it exceeds available resources

B Request would be granted immediately

C Pretend allocation would happen

D Safety check would pass

💡 Hint

Refer to Step 2 where request is compared to available resources.

Concept Snapshot

Banker's Algorithm checks resource requests by:
1. Comparing request to available resources.
2. Pretending to allocate resources.
3. Running a safety check to avoid deadlocks.
4. Granting request only if system stays safe.
This prevents unsafe states and deadlocks.

Full Transcript

The Banker's algorithm is a method to avoid deadlocks by carefully checking resource requests. When a process requests resources, the system first checks if the request is less than or equal to the currently available resources. If not, the request is denied immediately. If yes, the system pretends to allocate these resources and then performs a safety check to see if the system will remain in a safe state where all processes can finish eventually. If the safety check passes, the request is granted and the system state is updated. If not, the request is denied to avoid deadlock. This step-by-step checking ensures the system never enters an unsafe state that could cause deadlock.

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