Debugging Multithreaded u0026 Concurrent Programs Quiz Quiz

Explore key concepts and debugging strategies for multithreaded and concurrent programs with this focused quiz. Improve your understanding of common concurrency issues, thread safety, and effective problem-solving in parallel execution environments.

1. Identifying Race Conditions

   Which issue often arises when two threads access and modify a shared variable at the same time without proper synchronization, leading to unpredictable results?

   1. Race Condition
   2. Livelock
   3. Starvation
   4. Deadlock

   Explanation: A race condition occurs when the outcome of a program depends on the timing of threads that access shared resources without synchronization, often causing random or unpredictable behavior. Deadlock involves two or more threads waiting forever for each other to release resources, while starvation happens when a thread never gets access to resources. Livelock is when threads keep responding to each other but can't make progress. Only 'Race Condition' matches the scenario described.

2. Recognizing Deadlock Symptoms

   What is a common symptom of deadlock in a concurrent program where two threads each hold a resource and wait for the other’s resource indefinitely?

   1. CPU usage spikes to maximum but processes finish successfully.
   2. Data is consistently overwritten in shared memory.
   3. The program freezes and no threads make further progress.
   4. The threads execute faster than normal.

   Explanation: When deadlock occurs, threads are stuck waiting for resources, causing the program to freeze with no progress. Faster execution and high CPU usage do not indicate deadlock; they could signal other issues. Data being overwritten is more related to race conditions, not deadlock. Thus, freezing and lack of progress best describe this situation.

3. Detecting Heisenbugs

   Why do some bugs in multithreaded programs, sometimes called 'Heisenbugs,' disappear when certain debugging techniques such as adding print statements are used?

   1. The added code changes the timing and thread interleaving.
   2. Debug logs replace all shared resource locks.
   3. Threads stop running when print statements are used.
   4. Print statements permanently fix data alignment.

   Explanation: Heisenbugs can disappear when debugging due to changes in execution timing caused by added print statements or logging, which alter thread scheduling and interleaving. Print statements do not inherently fix data alignment, nor do they replace resource locks. It's not accurate to say threads stop running due to printing. The timing change is the key factor.

4. Understanding Data Races

   In a concurrent scenario, what is the main effect of a data race when multiple threads read and write to the same variable without synchronization?

   1. All threads move to idle state automatically.
   2. Only one thread is affected, not others.
   3. The variable may become unpredictable or corrupted.
   4. The program detects and fixes the error automatically.

   Explanation: A data race happens when unsynchronized threads access and modify a shared variable, potentially leading to unpredictable or corrupted values. Threads do not enter an idle state, nor does the program repair such errors automatically. The effects are not isolated to one thread; instead, shared data can impact all threads accessing it.

5. Debugging Priority Inversion

   In concurrent programming, what problem occurs when a lower-priority thread holds a lock needed by a higher-priority thread, causing the higher-priority thread to wait unexpectedly long?

   1. Thread Starvation
   2. Priority Inversion
   3. Busy Waiting
   4. Atomic Operation

   Explanation: Priority inversion describes the scenario where a high-priority thread is blocked because a lower-priority thread holds a required lock. Busy waiting is when a thread actively waits without yielding the processor, and thread starvation is when a thread does not get CPU time due to scheduling. Atomic operation refers to an indivisible unit of computation and is unrelated. 'Priority Inversion' correctly identifies this problem.