Feeling the heat of an upcoming software engineering interview? Don’t sweat it! Concurrency questions can be tricky, but with the right preparation, you can ace them and impress the interviewer
This comprehensive guide delves into the top 5 concurrency interview questions, providing insightful tips and solutions to help you shine. Get ready to showcase your expertise and land your dream job!
Top 5 Concurrency Interview Questions: A Deep Dive
1, ReadWrite Lock Sharing the Stage
Imagine a busy library where many people can read books at the same time but only one writer can make changes to them. Design a “ReadWrite Lock” that allows this harmonious coexistence.
Tips:
- Define clear APIs for acquiring read/write locks.
- Prioritize reader access while ensuring writer exclusivity.
- Use a counter to track active readers and a boolean flag for active writers.
- Employ mutexes and condition variables to manage access and avoid deadlocks.
Common Pitfalls:
- Splitting lock acquisition/release across methods (deadlock risk).
- Starvation of readers due to continuous writer access.
2. Dining Philosophers: A Feast of Fairness
Problem Five philosophers gather around a table with five forks, each needing two forks to eat Design a solution where every philosopher eats without causing a deadlock
Tips:
- Impose ordering on forks (e.g., numbered 0-4) to prevent circular wait.
- Use semaphores with a permit value of 1 to represent forks.
- Philosophers acquire the lower-numbered fork first to avoid conflicts.
Common Pitfalls:
- Circular wait: Philosophers endlessly wait for forks, creating a deadlock.
- Starvation: One philosopher is continuously denied access to forks.
3. Uber Ride Sharing the Journey
Problem: Democrats and Republicans at a conference need Uber rides. Design a system where rides can have either all Democrats, all Republicans, or two of each (fistfights are a no-no!).
Tips:
- Model the problem as a class with methods for Democrats/Republicans to request rides.
- Use semaphores to differentiate between waiting Democrats and Republicans.
- Signal the appropriate semaphore(s) when a ride is ready to accommodate both parties.
Common Pitfalls:
- Starvation: One party continuously waits for rides, unable to join.
- Deadlock: Republicans and Democrats wait for each other indefinitely.
4. Asynchronous to Synchronous: Bridging the Gap
Problem: You have an AsyncExecutor class that performs tasks asynchronously. Make its execution synchronous without modifying the original code.
Tips:
- Extend a new class from
AsyncExecutorand override theexecute()method. - Invoke the original asynchronous implementation using
super()within the overridden method. - Use condition variables and mutexes to signal the main thread when asynchronous execution is complete.
Common Pitfalls:
- Deadlock: The main thread and asynchronous execution wait for each other indefinitely.
- Unusable shared resources: Both asynchronous and synchronous code cannot access the resource.
5. Barber Shop: Keeping the Queue Moving
Problem: A barber shop has a waiting room with N chairs and a barber chair. Design a program to coordinate customers and the barber.
Tips:
- Maintain a count of waiting customers and available chairs.
- Use a semaphore for customers to wait on before being called for a haircut.
- Use a signaling mechanism (e.g., condition variable) to wake the barber when a customer arrives.
Common Pitfalls:
- Deadlock: The barber and customer wait for each other, creating a standstill.
- Starvation: Customers are continuously denied haircuts due to the barber’s busy schedule.
Your Concurrency Interview Toolkit: A Few Extra Tips
- Practice, practice, practice! The more you solve concurrency problems, the more comfortable you’ll be during your interview.
- Focus on clarity and communication. Explain your thought process and reasoning clearly to the interviewer.
- Don’t be afraid to ask questions. If you’re unsure about something, don’t hesitate to seek clarification.
- Show your enthusiasm and passion for concurrency. Your genuine interest will shine through and impress the interviewer.
By understanding these top 5 concurrency interview questions, practicing diligently, and showcasing your problem-solving skills, you’ll be well-equipped to ace your interview and land your dream software engineering job. Remember, concurrency is a powerful tool, and mastering it will open doors to exciting opportunities in the world of software development. So, go forth and conquer!
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2 Answers 2 Sorted by:
There are many ways to set up a mutex lock, but most of the time, it starts with the idea that the CPU architecture supports some form of atomic add and subtract. That is, an addition operation can be done on a memory variable that holds an integer and return the result without being messed up by another thread trying to get to the same memory location. Or at the very least, “atomic increment” and “atomic decrement”.
On modern Intel chips, for example, theres an instruction called XADD. When combined with the LOCK prefix it executes atomically and invalidates cached values across other cores. gcc implements a wrapper for this instruction called __sync_add_and_fetch. Win32 implements a similar function called InterlockedIncrement. Both are just calling LOCK XADD under the hood. Other CPU architectures should offer something similar.
So the most basic mutex lock could be implemented something like this. This is often called a “spin” lock. And this cheap version offers no ability to recursively enter the lock.
The above suffers from poor performance of “spinning” and doesnt guarantee any fairness. A higher priority thread could continue to win the EnterLock battle over a lower priority thread. It is also possible for the programmer to make a mistake and call LeaveLock on a thread that did not call EnterLock before. You could make the above work with a data structure that stores not only the lock integer but also the owner thread ID and the number of times the loop has been run.
The second concept for implementing a mutex is that the operating system can offer a wait and notify service such that a thread doesnt have to spin until the owner thread has released it. The thread or process waiting on lock can register itself with the OS to be put to sleep until the owner thread has released it. In OS terms, this is called a semaphore. Additionally, the OS level semaphore can also be used to implement locks across different processes and for the cases where the CPU doesnt offer an atomic add. And can be used to guaranteed fairness between multiple threads trying to acquire the lock.
Most implementations will try spinning for multiple attempts before falling back to making a system call.
I wouldnt say that this is a stupid question. On any level of abstraction for the position. On the high level you just say, you use standard library, or any threading library. You need to know how the compiler actually works and what is needed to make it work if you want to be a compiler developer.
To make a mutex work, you need a way to lock a resource so that it can be marked as being used by all threads at the same time. This is not trivial. You need to remember that two cores share memory, but they have caches. This piece of information must be guaranteed to be actual. So you do need support for hardware to ensure atomicity.
If you take at implementation of clang, they offload (at least in once case) implementation to pthreads, typedefs in threading support:
And if you dig through pthreads repo, you can find asm implementations of the interlocking operations. They rely on the lock asm keyword which make the operations atomic, i.e. no other thread can execute them at the same time. This eliminates racing conditions, and guarantees coherency.
Based on this, you can build a lock, which you can use for a mutex implementation.
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FANG Interview Question | Process vs Thread
FAQ
What is concurrency in an interview?
What is thread in Java interview questions?
How to implement concurrency in Python?
What is a lock variable?
A lock variable provides the simplest synchronization mechanism for processes. Some noteworthy points regarding Lock Variables are- Its a software mechanism implemented in user mode, i.e. no support required from the Operating System. Its a busy waiting solution (keeps the CPU busy even when its technically waiting).
How many processes can a lock variable be used for?
It can be used for more than two processes. When Lock = 0 implies critical section is vacant (initial value ) and Lock = 1 implies critical section occupied. Lock = 1; A more formal approach to the Lock Variable method for process synchronization can be seen in the following code snippet :
Are lock variables a good synchronization mechanism?
So like all easy things the Lock Variable Synchronization method comes with its fair share of Demerits but its a good starting point for us to develop better Synchronization Algorithms to take care of the problems that we face here. FAQ Q: Are lock variables the best synchronization mechanism in all scenarios?
Are lock variables efficient?
A: No, while lock variables are a simple mechanism for synchronization, they may not be efficient in scenarios where processes are frequently contending for access to a critical section. In such cases, other synchronization mechanisms like semaphores, monitors, or message passing may be more appropriate. Q: Can lock variables cause starvation?
