# REPORT
### Hesam Ghazi
### 403222015
## 1. Atomic Variables
Atomic variables provide thread-safe single-variable operations performed atomically using CPU-supported compare-and-swap (CAS) instructions. Unlike ordinary variables, they prevent race conditions without explicit synchronization for simple operations.

## 2. Atomic Classes
- `AtomicInteger`
- `AtomicLong`
- `AtomicBoolean`
- `AtomicReference<T>`

**Use case:** `AtomicInteger` is commonly used as a thread-safe counter shared among multiple threads.

## 3. Locks vs Atomic Variables
**Atomic variables**
- Best for simple read-modify-write operations.
- Non-blocking and usually faster under low contention.
- Limited to simple operations.

**Locks**
- Suitable for protecting multiple variables or complex critical sections.
- Easier to implement compound operations atomically.
- Introduce blocking and context-switch overhead.

## Bonus Task 

```java
import java.util.concurrent.atomic.AtomicInteger;

public class AtomicDemo {
    static int normal = 0;
    static AtomicInteger atomic = new AtomicInteger(0);

    public static void main(String[] args) throws Exception {
        Thread[] threads = new Thread[10];
        for (int i = 0; i < threads.length; i++) {
            threads[i] = new Thread(() -> {
                for (int j = 0; j < 100000; j++) {
                    normal++;
                    atomic.incrementAndGet();
                }
            });
        }
        for (Thread t : threads) t.start();
        for (Thread t : threads) t.join();

        System.out.println("Normal: " + normal);
        System.out.println("Atomic: " + atomic.get());
    }
}
```

Expected: `AtomicInteger` always prints 1000000, while `normal` is usually smaller because of race conditions.

## 4. Correct but Poor Performance
A program may be race-free but still scale poorly because:
1. High lock contention forces threads to wait.
2. Excessive synchronization increases overhead.
3. False sharing and cache coherence traffic reduce CPU efficiency.
4. Frequent blocking decreases parallelism.

## 5. Why More Threads Can Hurt
- **Context switching:** CPU spends time switching threads.
- **Contention:** Threads compete for shared resources.
- **Cache coherence:** Shared data invalidates CPU caches.
- **Synchronization overhead:** Locks and coordination consume execution time.
- Too many threads may exceed available CPU cores, reducing throughput.

## 6. Why Deadlocks Often Appear Only in Production
Deadlocks depend on thread scheduling, which is nondeterministic. Testing usually explores only a small subset of possible execution orders, whereas production workloads create many timing combinations.

Two strategies to expose deadlocks:
1. Perform stress tests with many threads and randomized execution timing.
2. Insert artificial delays (sleep/yield) around lock acquisition to increase unfavorable interleavings.