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.