Add critical section notes

Author: 0x326

CSE 381 (Systems II - OS, Currency, Virtualization, and Security)/Multithreading/Critical Section/MutualExclusionAlgorithm.java

@@ -0,0 +1,25 @@
+public abstract class MutualExclusionAlgorithm {
+    private static final int MAX_WAIT_TIME = 3000;
+
+    public abstract void requestCriticalSection(int t);
+
+    public abstract void notifyDone(int t);
+
+    public static void doCriticalWork() {
+        try {
+            // Simulate work
+            Thread.sleep((int) (Math.random() * MAX_WAIT_TIME));
+        } catch (InterruptedException e) {
+            // Do nothing
+        }
+    }
+
+    public static void doWork() {
+        try {
+            // Simulate work
+            Thread.sleep((int) (Math.random() * MAX_WAIT_TIME));
+        } catch (InterruptedException e) {
+            // Do nothing
+        }
+    }
+}

CSE 381 (Systems II - OS, Currency, Virtualization, and Security)/Multithreading/Critical Section/PetersonsAlgorithm.java

@@ -0,0 +1,18 @@
+public class PetersonsAlgorithm extends MutualExclusionAlgorithm {
+    private volatile int turn = 0;
+    private volatile boolean[] threadWantsCriticalSection = {false, false};
+
+    public void requestCriticalSection(int threadId) {
+        int otherThreadId = 1 - threadId;
+
+        threadWantsCriticalSection[threadId] = true;
+        turn = otherThreadId;
+        while (threadWantsCriticalSection[otherThreadId] && turn == otherThreadId) {
+            Thread.yield();
+        }
+    }
+
+    public void notifyDone(int threadId) {
+        threadWantsCriticalSection[threadId] = false;
+    }
+}

CSE 381 (Systems II - OS, Currency, Virtualization, and Security)/Multithreading/Critical Section/PetersonsAlgorithmTest.java

@@ -0,0 +1,43 @@
+public class PetersonsAlgorithmTest {
+    public static void main(String[] args) {
+        MutualExclusionAlgorithm mutexAlg = new PetersonsAlgorithm();
+
+        Worker thread1 = new Worker("Thread 1", 0, mutexAlg);
+        Worker thread2 = new Worker("Thread 2", 1, mutexAlg);
+
+        thread1.start();
+        thread2.start();
+    }
+}
+
+class Worker extends Thread {
+    private String name;
+    private int threadId;
+    private MutualExclusionAlgorithm mutexAlg;
+
+    public Worker(String name, int threadId, MutualExclusionAlgorithm mutexAlg) {
+        this.name = name;
+        this.threadId = threadId;
+        this.mutexAlg = mutexAlg;
+    }
+
+    public void run() {
+        while (true) {
+            System.out.printf("%s: requesting critical section\n", name);
+            mutexAlg.requestCriticalSection(threadId);
+            System.out.printf("%s: entered critical section\n", name);
+            long startTime = System.nanoTime();
+
+            System.out.printf("%s: doing critical work\n", name);
+            MutualExclusionAlgorithm.doCriticalWork();
+
+            mutexAlg.notifyDone(threadId);
+            long endTime = System.nanoTime();
+            double duration = (endTime - startTime) / 1000000.0;
+            System.out.printf("%s: left critical section (critical work lasted for %f ms)\n", name, duration);
+
+            System.out.printf("%s: doing normal work\n", name);
+            MutualExclusionAlgorithm.doWork();
+        }
+    }
+}