P2L2: Threads and Concurrency

Question 1

What are benefits of multithreading?

Answer 1

• parallelization => speed up - specialization => hot cache - efficiency => lower ram requirement 2 cheaper IPC Single CPU - If the the t_idle > 2*t_ctx_switch then it is useful to use more than 1 thread to avoid wasting too much time in the t_idle If the app is multithreaded and the OS is multithreaded that is helpful because threads can working for each app and OS services

Question 2

Process vs. thread, describe the distinctions. What happens on a process vs. thread context switch.

Answer 2

• Context switch time in thread is less than in process - Threads usually result in hotter caches when multiple exist compared to thread - Threads can share a virtual address space

Question 3

When is it useful to add more threads, when does adding threads lead to pure overhead?

Answer 3

..

Question 4

What are the possible sources of overhead associated with multithreading?

Answer 4

..

Question 5

What are mutexes?

Answer 5

• It is a construct with infor: locked, owner and blocked threads Functions - lock - unlock - Problem: Two thread manipulate and modify the same data resources at the same time and the result is unpredictable and sometimes wrong - Solution: Mutex uses a lock so that one thread does the work and modified the data resource exclusively. Only then the thread is done the other can continue

Question 6

What are condition variables?

Answer 6

wait(mutex, cond) signal(cond) Problem: - Having to loop continuously into lock, check the condition and if not unlock. Waste lock time

Question 7

What are spurious wake-ups, how do you avoid them, and can you always avoid them?

Answer 7

• Thread are awaken from waiting, but they can’t proceed because mutex is still locked. Therefore they are put in the mutex waiting queue - Doesn’t necessarily affect correctness, but affects performance - You can rewrite, put signal and broadcast out of lock if they do not depend on a condition related to the protected data resource and correctness no affected. Else, signal and broadcast need to be in lock

Question 8

Do you understand the need for using a while() look for the predicate check in the critical section entry code examples in the lessons?

Answer 8

If not used: - It is possible that when the signal is emitted the another thread affected the predicate condition. If not checked then the predicate condition logic could we not followed If used: - The predicate condition logic is always followed because we checked it again to verify it after the signal

Question 9

What’s a simple way to prevent deadlocks? Why?

Answer 9

• Two or more competing threads are waiting on each other to complete, but none of them ever do - prevent cycle in wait graph - Detection and recovery -Do nothing

Question 10

Describe the boss-worked multithreading pattern.

Answer 10

• Boss assigns work to workers. Place work in a queue - Worker: performs entire tasks

Question 11

If you need to improve a performance metric like throughput or response time, what could you do in a boss-worker model? What are the limiting factors in improving performance with this pattern

Answer 11

• Throughput = 1/ boss time per order - Reduce boss time per order - Queue synchronization

Question 12

Describe the pipelined multithreading pattern.

Answer 12

+ specialization

• balancing and synchronization overheads

Question 13

If you need to improve a performance metric like throughput or response time, what could you do in a pipelined model? What are the limiting factors in improving performance with this pattern?

Answer 13

• Improve the weakest point in the pipeline by adding more threads
• The limiting factors are maintaining pipeline balance, and additional synchronization operations, weakest point

Question 14

Answer 14