133 lines
5.9 KiB
Markdown
133 lines
5.9 KiB
Markdown
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#### ****1 -** What are atomic variables?**
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#### Explain their purpose and how they differ from ordinary (non-atomic) variables.
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#### **Answer:**
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Atomic variables are variables on which reading and writing operations are atomic.</br>
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It means that when a Thread is making changes to a variable, no other Thread can interfere with it.</br>
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They Use hardware-level instructions to ensure that only one thread can modify the value at a time without locks.</br>
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But for ordinary variables, operations like balance += amount are not atomic for example if another thread interrupts between these steps, it causes Race Conditions and data corruption.</br>
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---
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#### **2 -** Name at least four classes from the `java.util.concurrent.atomic` package that provide atomic operations for different data types.
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#### For one of them, briefly describe a typical use case.
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#### **Answer:**
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- AtomicInteger
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- AtomicLong
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- AtomicBoolean
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- AtomicReference
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Example :
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```
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private AtomicLong balance = new AtomicLong(0);
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public void deposit(long amount)
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{
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balance.addAndGet(amount);
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}
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```
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Atomically adds amount to current balance.</br>
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It is fast & it does not need `synchronized`
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.<br>
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But it is only good for simple operations on one variable.
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---
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#### **3 -** Compare locks with atomic variables.
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#### In which scenarios is using a lock a better choice than an atomic variable, and vice versa?
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#### **Answer:**
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| Feature | Lock (ReentrantLock / synchronized) | Atomic Variables |
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|:-------------------------|:----------------------------------------------------------------------------:|:------------------------------------------------------------------:|
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| Complexity of operations | Great for complex, multi-step operations | Great for simple operations on a variable |
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| Scope of Locking | You can lock blocks of code | It only locks the variable itself |
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| Conditional Logic | Supports conditional checks;You can check if (balance > 0) and then withdraw | Difficult; requires loops with compareAndSet which can be complex. |
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| Resource Management | Requires manual unlocking in finally | Automatic |
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| Deadlock | Dangerous | None |
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**When to Use Which?**
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**Use Locks When:**
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- You need to perform compound operations involving multiple state variables atomically (e.g., in our banking project: balance -= amount AND target.balance += amount must happen together).
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- You need to wait for a condition (using Condition.await()/signal()).
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- The critical section contains complex logic that cannot be reduced to a single atomic instruction.
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**Use Atomic Variables When:**
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- You are modifying a single variable (e.g., a global counter, a status flag).
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- Performance is critical and contention is expected to be low.
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- You want to avoid the complexity of managing lock lifecycles (acquire/release).
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---
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#### **4 -** A program is completely free of race conditions but still performs poorly under high contention.
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#### Explain how this situation can occur.
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#### Discuss at least three concurrency-related factors that may limit scalability even when correctness is guaranteed.
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#### **Answer:**
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Even without race conditions, high contention causes bottlenecks due to:
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- **Thread Contention:** Threads block waiting for locks, turning parallel execution into sequential processing. CPU time is wasted managing queues rather than computing.
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- **Context Switching Overhead:** Frequent blocking/unblocking forces the OS to save/restore thread states. High switch rates consume CPU cycles needed for actual work.
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- **False Sharing:** Unrelated variables in the same cache line cause unnecessary cache invalidations across cores, forcing slow main memory accesses despite logical independence.
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---
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####
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#### **5 -** Many concurrent systems experience performance degradation as the number of threads increases.
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#### Explain why adding more threads does not always improve performance.
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#### Your answer should discuss concepts such as:
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#### Context switching
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#### Contention
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#### coherence
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#### Synchronization overhead
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#### **Answer:**
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Performance degrades with excessive threads due to:
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- **Synchronization Overhead:** Lock acquisition/release costs exceed computation time for small tasks.
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- **Contention:** Increased probability of lock conflicts leads to long wait times, shifting from parallel to serialized execution.
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- **Context Switching:** Beyond core limits, CPUs spend more time switching contexts than executing instructions (“thread explosion”).
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- **Cache Coherence:** Frequent writes to shared data trigger inter-core synchronization (MESI protocol), saturating the communication bus and increasing latency
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---
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#### **6 -** Deadlocks often only appear in production, not during testing.
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#### Explain why this might happen from a thread-scheduling perspective.
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#### Describe two strategies a developer can use to increase the likelihood of exposing deadlocks during testing.
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#### **Answer:**
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**Why they hide in testing:**
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- Timing: Tests are fast and deterministic; production has I/O/network delays that alter thread interleaving.
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- Concurrency: Tests use few threads; deadlocks often require high concurrency to trigger naturally.
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**Strategies to Expose Deadlocks:**
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- Stress Testing: Spawn hundreds of threads performing random operations to exponentially increase the chance of hitting cyclic dependency windows.
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- Inject Delays: Add Thread.sleep() or use tools like JMH to disrupt natural flow, increasing the likelihood that a thread holds one lock while waiting for another.
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