P3L4 Synchronization - Intro

Question 1

Visual Metaphor

Synchonization is like __________________________________

1. Repeatedly checking if can continue working
   
   • Sync using _____________
2. Wait for a signal to continue working
   
   • Sync using __________ and ____________ variables
3. Waiting hurts performance
   
   • Wasted _____ cycles
   • _______ effects

Answer 1

Synchonization is like … waiting for a co-worker to finish so you can continue working

1. Repeatedly checking if can continue working
   
   • Sync using spinlocks
2. Wait for a signal to continue working
   
   • Sync using mutexes and condition variables
3. Waiting hurts performance
   
   • Wasted CPU cycles
   • Cache effects

Question 2

How is a spinlock different from a mutex?

Answer 2

When a spinlock is locked, and a thread is attempting to lock it, that thread is not blocked. Instead, the thread is spinning. It is running on the CPU and repeatedly checking to see if the lock has become free.

The thread will burn CPU cycles until the lock becomes free or until the thread is preempted for some reason. (e.g. timeslice up)

Question 3

True or False?

If a semaphore is initialized with a counter of 1, it will behave like a mutex, only allowing one thread at a time to pass.

Answer 3

True - called a binary semaphore

sem_t m

sem_init(&m, 0, 1)

Question 4

What are reader/writer locks good for?

Answer 4

to distinguish among different types of resource access.

For instance, we commonly want to distinguish “read” accesses - those that do not modify a shared resource - from “write” accesses - those that do modify a shared resource.

Question 5

Why is it useful to have more powerful synchronization constructs, like reader-writer locks or monitors?

Answer 5

These constructs are more powerful because of their expressiveness.

Dealing with higher-level semantics that abstract away some of the lower-level components helps developers reduce the area for mistakes.

Question 6

Name a bunch of synchronization constructs (11)

Answer 6

1. mutex
2. spinlock
3. semaphore
4. barrier
5. condition variable
6. read-write lock
7. monitor
8. serializers
9. path-expressions
10. rendez-vous points
11. Optimistic wait point sync (RCU)

Question 7

What do all synchronization constructs have in common?

Answer 7

all need hardware support!