Synchronization;Barriers;Semaphores

Question 1

What do we do if we have a memory location which is neither thread-local nor immutable?

Answer 1

We need to make sure we synchronize the resource properly.

Question 2

What is thread local?

Answer 2

Do not use the memory location in more than 1 thread, by giving each thread its own copy of a resource, provided that the threads dont need to communicate through this resource

Question 3

What is immutable? And how do we make a resource immutable?

Answer 3

Do not write to the memory location. You can enforcethis by declaring such variable final

Question 4

Remember there are 3 ways to make concurreny happen, at least one of them needs to be met. What are these 3?

Answer 4

Synchronized
Immutable
Thread local

Question 5

What does a thread do when encountering a synchronized keyword?

Answer 5

First it attempts to acquire the lock to the specified object. Until it obtains the lock it will block

Question 6

Are Java lock reentrant and what does this mean?

Answer 6

Java locks are reentrent. A lock is reentren if it can be acquired multiple times by the same thread

Question 7

How do changes made inside a synchronized block appear to other thread?

Answer 7

They appear to take place instanttaneously

Question 8

What problem do long critical sections protected by locks do to the program? Thinking of Amdahls Law

Answer 8

We reduce the possible speedup of the program, because critical sections protected by locks arent parallizable. Remember sequential work is a bottle neck.

Question 9

Where can conditions be implemented?

Answer 9

Critical sections protected by locks and the thread must hold the lock to react to the condition.

Question 10

How do we implement conditions on locks in Java?

List the methods

Answer 10

• wait()
• notify()
• notifyAll()

Question 11

What does wait() do?

Answer 11

Thread is moved into an inactive state and is inserted into the waiting queue. The thread releases the lock on which wait() was called.

Question 12

what does notify() do?

Answer 12

Some thread is removed from the waiting queue and moved into an ctive state. The thread that was just woken up will then proceed to attempt to re-acquire the lock normally.

We do not have control over which thread is woken up

Question 13

What does notifyAll() do?

Answer 13

• All threads are removed from the waiting queue and moved into an active state.
• All these threads will then attempt to reacquire the lock.

Question 14

Why do we use while loops insted of if statements when using conditions in locked sections?

Answer 14

• the operating system might randomly wake up a waiting thread
• another thread might interrupt the waiting thread this leads to an InterruptException

Question 15

What are the methods in java.util.concurrent.locks library called? and what equals wait(), notify() and notifyAll()

Answer 15

The methods are
await() is equal to wait()
signal() is equal to notify()
signalAll() is equal to notifyAll()

Question 16

what is important to know about the wait() method?

Tipp: spurious wakeups

Answer 16

Always enclose a wait() statement in a while loop. Remember the operating system could wake up a thread at any time without calling notify()

Question 17

Only call wait() and notify() under what condition?

Answer 17

the corresponding object must hold the lock

Question 18

Code for implementing synchronized keyword.

What variables need to be created and how is the lock acquired?

Answer 18

Following is the code for a lock that can be acquired:
private static Object lock = new Object(); 

To acquire the lock just write:
synchronized(lock) {…..}

Example see picture

Question 19

How to implement a condition lock? What is the code? What variables are needed?

Answer 19

We need to create a lock like this:
private static Lock l = new ReentantLock();

To create a condition we do this as follows using the lock just created:
private static Condition menQueue = 
l.newCondition();

Acquiring and releasing the lock happens as follows:

l. lock() (acquiring the lock l)
l. unlcok() (releaseing the lock l)

see picture for example

Question 20

How can we let more than one thread enter a critical section?

Answer 20

Using Semaphores. A Semaphore S has a capacity of N

Question 21

A Semaphore S with capacity of N has two methods name them.

Answer 21

acquire(S): Block until N > 0, then decrements N.

release(S): Increments N

Question 22

What does a Barrier do?

Answer 22

It blocks all threads up until a certain threshold of N threads. Once the threshhold has been reached all threads waiting in the barrier are allowed to continue.

Question 23

What are the 2 different types of barriers?

Answer 23

reusable and non-reusable

Question 24

What is a reusable barrier?

Answer 24

the barrier can reset to its initial state and be reused.

Question 25

what is a non-reusable barrier?

Answer 25

it cant be reused

Question 26

Give an implementation of a non-reusable barrier.

How are Semaphores used in the implementation?

Answer 26

A new barrier is initiated wit a Semaphore of capacity 0

What is important is that the count for the threshhold of the barrier implements mutual exclusion

Question 27

What is a semaphore using a nightclub as an example?

Answer 27

Think of semaphores as bouncers at a nightclub. There are a dedicated number of people that are allowed in the club at once. If the club is full no one is allowed to enter, but as soon as one person leaves another person might enter.

A way to limit the number of consumers for a specific resource.

Question 28

What does the picture say about semaphores.

Answer 28

A semaphore makes resources available for a specific number of threads. This number is repsented by an integer. In the picture Thread 1 acquires the semaphore first which has 2 places. After acquiring it the integer is decremented to 1. Now Thread 2 acquires the resource the int value is now 0, so therefore Thread 3 can’t acquire the resource. After realeasing the number increments again and Thread 3 could enter.

Question 29

Give an implementation of a semaphore. What is important to remember?

Answer 29

Important all operations are atomic.

S := the value of the semaphore ( the number of bikes available to take)

Question 30

Between what barriers do we distinguish?

Answer 30

reusable and non-reusable

Question 31

Give an implementation of a non-reusable barrier. In code form, explain the steps. What arguments does it take?

Answer 31

We need a threshhold (threads that need to build up until we continue execution.

We need a counter (counts how many threads have reached the barrier and are waiting)

We also need barrier reference to a new Semaphore

Each thread that arrive increments the counter and attempts to acquire the barrier semaphore, thereby going into a blocked state. Once all threads have arrived count == threshhold, the barrier semaphore is set to 1, allowing one thread to pass. Whenever a thread passes the barrier it immediately releases it again so that the next thread may proceed with its execution.

This method is called turnstile