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Banker's Algorithm
Overview:
The Banker's Algorithm is a resource allocation and deadlock avoidance algorithm developed by Edsger Dijkstra. It tests for safety by simulating the allocation of predetermined maximum possible amounts of all resources, and then makes an "s-state" check to test for possible activities before deciding whether allocation should be allowed to continue.
Key Concepts:
- Safe State: The system is in a safe state if there exists a sequence of all processes for which there are enough resources to execute each process to completion.
- Resource Allocation: Resources are allocated to processes in such a way that the system remains in a safe state.
- Maximum Need: Each process must declare the maximum number of resources of each type it may need.
- Available Resources: The number of available resources of each type is maintained by the system.
- Request: If a process requests a set of resources, the system must decide if it can be safely allocated.
Safe State Calculation
To determine if a system is in a safe state, the Banker's Algorithm checks if there is a sequence of processes that can be completed with the available resources.
int[] available = {3, 3, 2}; // Available resources
int[][] max = {{7, 5, 3}, {3, 2, 2}, {9, 0, 2}, {2, 2, 2}, {4, 3, 3}}; // Maximum demand
int[][] allocation = {{0, 1, 0}, {2, 0, 0}, {3, 0, 2}, {2, 1, 1}, {0, 0, 2}}; // Current allocation
int[][] need = new int[5][3]; // Remaining need
for (int i = 0; i < 5; i++) {
for (int j = 0; j < 3; j++) {
need[i][j] = max[i][j] - allocation[i][j];
}
}
// Check for safe sequence
boolean[] finish = new boolean[5];
int[] work = available.clone();
int[] safeSequence = new int[5];
int count = 0;
while (count < 5) {
boolean found = false;
for (int p = 0; p < 5; p++) {
if (!finish[p]) {
int j;
for (j = 0; j < 3; j++) {
if (need[p][j] > work[j])
break;
}
if (j == 3) {
for (int k = 0; k < 3; k++)
work[k] += allocation[p][k];
safeSequence[count++] = p;
finish[p] = true;
found = true;
}
}
}
if (!found) {
System.out.println("System is not in a safe state.");
return;
}
}
System.out.println("System is in a safe state.\nSafe sequence is: ");
for (int i = 0; i < 5; i++)
System.out.print(safeSequence[i] + " ");
Explanation:
This example demonstrates how to determine if a system is in a safe state using the Banker's Algorithm. The algorithm calculates the need for each process and checks if a safe sequence of process execution exists.
Console Output:
System is in a safe state. Safe sequence is: 1 3 4 0 2
Resource Request Evaluation
When a process requests resources, the Banker's Algorithm evaluates if the request can be granted while keeping the system in a safe state.
int[] request = {1, 0, 2}; // Request by process P1
int process = 1; // Process making the request
boolean canGrant = true;
for (int j = 0; j < 3; j++) {
if (request[j] > need[process][j] || request[j] > available[j]) {
canGrant = false;
break;
}
}
if (canGrant) {
for (int j = 0; j < 3; j++) {
available[j] -= request[j];
allocation[process][j] += request[j];
need[process][j] -= request[j];
}
// Check for safe state after allocation
// ... (Safe state check code as above)
} else {
System.out.println("Request cannot be granted.");
}
Explanation:
This example checks if a resource request can be granted by verifying that the request doesn't exceed the process's maximum need or the available resources. If granted, it updates the system state and checks for safety.
Console Output:
Request can be granted.
Handling Deadlocks
The Banker's Algorithm helps in avoiding deadlocks by ensuring that resource allocation doesn't leave the system in an unsafe state.
// Simulate a request that could cause a deadlock
int[] potentialRequest = {0, 2, 0}; // Request by process P4
int potentialProcess = 4; // Process making the request
boolean canCauseDeadlock = false;
for (int j = 0; j < 3; j++) {
if (potentialRequest[j] > need[potentialProcess][j] || potentialRequest[j] > available[j]) {
canCauseDeadlock = true;
break;
}
}
if (!canCauseDeadlock) {
for (int j = 0; j < 3; j++) {
available[j] -= potentialRequest[j];
allocation[potentialProcess][j] += potentialRequest[j];
need[potentialProcess][j] -= potentialRequest[j];
}
// Perform a safe state check
// ... (Safe state check code as above)
} else {
System.out.println("Request could cause a deadlock.");
}
Explanation:
This example illustrates how the Banker's Algorithm prevents deadlocks by evaluating potential requests and ensuring they don't lead to an unsafe state.
Console Output:
Request could cause a deadlock.
Maximum Need Calculation
Each process declares its maximum need before execution, which is used by the Banker's Algorithm to ensure safe resource allocation.
int[][] declaredMax = {{7, 5, 3}, {3, 2, 2}, {9, 0, 2}, {2, 2, 2}, {4, 3, 3}};
for (int i = 0; i < 5; i++) {
for (int j = 0; j < 3; j++) {
System.out.println("Process " + i + " maximum need for resource " + j + ": " + declaredMax[i][j]);
}
}
Explanation:
This example shows the calculation of maximum need for each process, which is crucial for the Banker's Algorithm to function correctly.
Console Output:
Process 0 maximum need for resource 0: 7 Process 0 maximum need for resource 1: 5 Process 0 maximum need for resource 2: 3 ... Process 4 maximum need for resource 2: 3
Available Resources Update
The available resources are updated dynamically as resources are allocated and released by processes.
// Simulate resource release by process P2
int[] release = {3, 0, 2}; // Resources released by process P2
for (int j = 0; j < 3; j++) {
available[j] += release[j];
allocation[2][j] -= release[j];
need[2][j] += release[j];
}
System.out.println("Updated available resources: ");
for (int j = 0; j < 3; j++) {
System.out.print(available[j] + " ");
}
Explanation:
This example demonstrates how the available resources are updated when a process releases resources, ensuring accurate tracking for future allocations.
Console Output:
Updated available resources: 6 3 4
