Merge pull request #1866 from khwaishchawla/main

two level scheduling

Author: pankaj-bind

Miscellaneous Algorithms/two level scheduling/program.c

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+#include <stdio.h>
+#include <stdlib.h>
+
+struct Process {
+    int pid;
+    int arrival_time;
+    int burst_time;
+    int waiting_time;
+    int turnaround_time;
+};
+
+void calculateTimes(struct Process *processes, int n) {
+    int total_waiting = 0, total_turnaround = 0;
+    processes[0].waiting_time = 0;
+    processes[0].turnaround_time = processes[0].burst_time;
+
+    for (int i = 1; i < n; i++) {
+        processes[i].waiting_time = processes[i - 1].waiting_time + processes[i - 1].burst_time;
+        processes[i].turnaround_time = processes[i].waiting_time + processes[i].burst_time;
+
+        total_waiting += processes[i].waiting_time;
+        total_turnaround += processes[i].turnaround_time;
+    }
+
+    printf("\nProcess ID\tArrival Time\tBurst Time\tWaiting Time\tTurnaround Time\n");
+    for (int i = 0; i < n; i++) {
+        printf("%d\t\t%d\t\t%d\t\t%d\t\t%d\n", processes[i].pid, processes[i].arrival_time,
+               processes[i].burst_time, processes[i].waiting_time, processes[i].turnaround_time);
+    }
+    printf("\nAverage Waiting Time: %.2f", (float)total_waiting / n);
+    printf("\nAverage Turnaround Time: %.2f\n", (float)total_turnaround / n);
+}
+
+void twoLevelScheduling(struct Process *processes, int n) {
+    // Step 1: Sort processes by arrival time (Long-term scheduling)
+    for (int i = 0; i < n - 1; i++) {
+        for (int j = i + 1; j < n; j++) {
+            if (processes[i].arrival_time > processes[j].arrival_time) {
+                struct Process temp = processes[i];
+                processes[i] = processes[j];
+                processes[j] = temp;
+            }
+        }
+    }
+
+    // Step 2: FCFS on sorted list (Short-term scheduling)
+    calculateTimes(processes, n);
+}
+
+int main() {
+    int n;
+    printf("Enter the number of processes: ");
+    scanf("%d", &n);
+
+    struct Process processes[n];
+    for (int i = 0; i < n; i++) {
+        printf("Enter arrival time and burst time for process %d: ", i + 1);
+        processes[i].pid = i + 1;
+        scanf("%d %d", &processes[i].arrival_time, &processes[i].burst_time);
+    }
+
+    twoLevelScheduling(processes, n);
+    return 0;
+}

Miscellaneous Algorithms/two level scheduling/readme.md

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+Two-level scheduling is a strategy used in operating systems to manage processes efficiently. It divides the scheduling process into two distinct phases to balance resource usage and system responsiveness, particularly in systems with a mix of interactive and batch processes. 
+
+### Description of Two-Level Scheduling
+
+1. **First Level (Long-Term Scheduling):**
+   - The long-term scheduler, also called the admission scheduler, determines which processes are admitted to the system for execution.
+   - It decides the overall mix of active processes in the system by selecting a subset from a larger pool of tasks. Only a portion of processes are allowed into memory based on system resource availability.
+   - Long-term scheduling helps control the degree of multiprogramming (i.e., the number of concurrent processes in memory).
+
+2. **Second Level (Short-Term Scheduling):**
+   - The short-term scheduler, or CPU scheduler, operates on the processes that are already in memory.
+   - It frequently selects one of these processes to execute on the CPU, switching between processes as needed to optimize CPU utilization and system responsiveness.
+   - Short-term scheduling uses various algorithms (e.g., round-robin, priority scheduling) to decide which process gets CPU time next.
+
+### Pros of Two-Level Scheduling
+
+1. **Improved Resource Utilization:**
+   - Long-term scheduling controls memory usage by limiting the number of concurrent processes, helping to reduce memory thrashing (frequent page swaps).
+
+2. **Enhanced System Performance:**
+   - By focusing on short-term scheduling among active processes, it achieves higher CPU utilization and responsiveness, especially beneficial for interactive applications.
+
+3. **Reduced Overhead in Process Management:**
+   - By admitting only a manageable number of processes to memory, the system reduces overhead in handling context switching and managing process queues.
+
+4. **Flexibility with Different Process Types:**
+   - Works well in systems with mixed workloads by allowing batch jobs to run efficiently in the background and giving priority to interactive tasks.
+
+### Cons of Two-Level Scheduling
+
+1. **Increased Complexity:**
+   - Requires careful design and tuning to manage the two levels of scheduling effectively, adding to the complexity of the operating system.
+
+2. **Potential Latency for Batch Processes:**
+   - Long-term scheduling may delay certain processes (especially lower-priority or batch jobs), affecting the turnaround time for such tasks.
+
+3. **Memory Management Challenges:**
+   - Determining the right mix of processes admitted to memory requires sophisticated memory management, and poor decisions may lead to inefficient memory usage.
+
+4. **Higher Initial Overhead:**
+   - The process of admitting and scheduling processes at two levels can introduce some initial overhead, potentially slowing down system responsiveness under heavy load conditions. 
+
+Two-level scheduling is particularly beneficial for multi-user systems and real-time applications where maintaining responsiveness for interactive users is critical. However, the additional complexity requires careful management and may not be suitable for simpler or single-user systems.