chapter 4

Question 1

What does the Happens Before Guarantee for Reads of volatile Variables state?

Answer 1

A read of a volatile variable will happen before any subsequent reads of volatile and non-volatile variables

Question 2

What can happen if two or more threads are sharing an object without proper synchronization or volatile declarations?

Answer 2

Updates to the shared object made by one thread may not be visible to other threads.

Question 3

What happens if a thread running on CPU one changes the shared object in its CPU cache before flushing it back to main memory?

Answer 3

The changed version of the shared object is not visible to threads running on other CPUs.

Question 4

Give an example of a race condition (train ticket booking)

Answer 4

If two passengers try to book the only available ticket at the same time without synchronization, both might end up booking tickets, causing a problem.

Question 5

What are special hardware instructions that allow us to modify the content of a word or swap the contents of two words atomically?

Answer 5

Test-and-Set instruction and Compare-and-Swap instruction are special hardware instructions.

Question 6

How can you solve the problem of race condition with volatile variables?

Answer 6

You can use a Java synchronized block to guarantee only one thread can enter a given critical section of the code at any given time.

Question 7

Imagine thread A reads the variable count into its CPU cache, while thread B does the same into a different CPU cache. What happens if both threads increment the count?

Answer 7

The variable count will be incremented twice, once in each CPU cache.

Question 8

Do volatile variables guarantee solving the synchronization problem for race condition?

Answer 8

No, volatile variables do not guarantee solving the synchronization problem for race condition.

Question 9

What is the purpose of the AtomicBoolean variable in the counter example?

Answer 9

The AtomicBoolean variable is used as a lock to provide synchronization.

Question 10

What is the purpose of a memory barrier instruction?

Answer 10

To ensure that all loads and stores are completed before any subsequent load or store operations are performed.

Question 11

What can a memory barrier instruction prevent?

Answer 11

Reordered instructions and ensure store operations are completed in memory and visible to other processors before future load or store operations are performed.

Question 12

What can happen in multithreading applications without proper synchronization?

Answer 12

Reorder instruction problems, where instructions can be reordered leading to unexpected behavior.

Question 13

What do computer architectures provide to ensure memory modifications are visible to threads running on other processors?

Answer 13

Memory barriers or memory fences.

Question 14

Why can’t kernel developers make assumptions about memory modifications on a shared-memory multiprocessor?

Answer 14

Memory models vary by processor type.

Question 15

What is a weakly ordered memory model?

Answer 15

Where a memory modification of one processor may not be immediately visible to all other processors.

Question 16

What happens if synchronization is violated?

Answer 16

Multiple threads may access shared resources simultaneously, leading to unpredictable results.

Question 17

How can static synchronization be achieved in Java?

Answer 17

By applying the synchronized keyword to static methods or using a synchronized block with a static object.

Question 18

When does a thread acquire the monitor and continue executing the monitor region?

Answer 18

If no other thread is waiting in the entry set and no other thread currently owns the monitor.

Question 19

What happens if a thread arrives at the beginning of a monitor region that is already owned by another thread?

Answer 19

The newly arrived thread must wait in the entry set.

Question 20

What happens when the current owner of the monitor exits the monitor?

Answer 20

The newly arrived thread must compete with any other threads waiting in the entry set.

Question 21

When is cooperation important in Java monitors?

Answer 21

When one thread needs some data to be in a particular state and another thread is responsible for getting the data into that state.

Question 22

What should you do if you need to guarantee the exact sequence of thread access to a synchronized block?

Answer 22

implement Fairness yourself.

Question 23

How does static synchronization work?

Answer 23

It places the synchronized keyword before a static method, ensuring that the lock access is on the class, not on the object or method.

Question 24

What is static synchronization?

Answer 24

It is a way to ensure that multiple threads do not simultaneously enter a synchronized block or method with separate locks.

Question 25

How does static synchronization impact instruction reordering?

Answer 25

It places restrictions on the reordering of instructions before, inside, and after synchronized blocks, similar to the restrictions imposed by the volatile keyword.

Question 26

What does the synchronized keyword do?

Answer 26

It refreshes the values of all variables visible to the thread when it enters a synchronized block and commits changes to variables visible to the thread when it exits the block.