Multilevel Queue Scheduling

// Multilevel Queue — two queue levels
// Queue 0 (highest): Interactive/System processes → Round Robin, quantum=4
// Queue 1 (lowest): Batch processes → FCFS

// Queue assignment rule: interactive tasks go to Queue 0, batch to Queue 1
// Scheduler: always drain Queue 0 before touching Queue 1

Queue<String> queue0 = new LinkedList<>();  // interactive (foreground)
Queue<String> queue1 = new LinkedList<>();  // batch (background)

// Simulate adding processes
queue0.add("UI-Render");
queue0.add("API-Request");
queue1.add("Nightly-Backup");
queue1.add("Log-Compression");
queue0.add("DB-Query");   // interactive arrives — preempts batch

System.out.println("=== Scheduling Order ===");
// Scheduler logic: always pick from queue0 first
while (!queue0.isEmpty() || !queue1.isEmpty()) {
    if (!queue0.isEmpty()) {
        System.out.println("Running (foreground): " + queue0.poll());
    } else {
        System.out.println("Running (background): " + queue1.poll());
    }
}
// Nightly-Backup and Log-Compression only run after ALL interactive tasks finish

// MLFQ conceptual diagram:
//
// Q0 (highest, quantum=4ms):  [new arrivals, I/O-bound tasks stay here]
//         ↓ demote (used full quantum without blocking)
// Q1 (medium, quantum=8ms):   [moderately CPU-bound]
//         ↓ demote (used full quantum again)
// Q2 (lowest, FCFS):          [CPU-intensive batch jobs]
//         ↑ promote (after waiting too long — aging)

// Simulating MLFQ process behavior
record MlfqProcess(String name, int remainingBurst, int queueLevel) {}

List<MlfqProcess> processes = new ArrayList<>(List.of(
    new MlfqProcess("Browser-Render", 3, 0),   // short burst → stays in Q0
    new MlfqProcess("Video-Encode",   50, 0),  // long burst → gets demoted
    new MlfqProcess("Git-Clone",       20, 0)  // medium burst → Q1
));

int[] quanta = {4, 8, Integer.MAX_VALUE};  // Q0=4ms, Q1=8ms, Q2=FCFS

for (MlfqProcess p : processes) {
    int q = p.queueLevel();
    int used = Math.min(p.remainingBurst(), quanta[q]);
    boolean usedFullQuantum = (p.remainingBurst() >= quanta[q]);
    int newLevel = usedFullQuantum ? Math.min(q + 1, 2) : q;  // demote if CPU-bound
    System.out.printf("%s: burst=%d, Q%d → used %dms → moved to Q%d%n",
        p.name(), p.remainingBurst(), q, used, newLevel);
}