chapter 4 Flashcards
(27 cards)
What does the Happens Before Guarantee for Reads of volatile Variables state?
A read of a volatile variable will happen before any subsequent reads of volatile and non-volatile variables
What can happen if two or more threads are sharing an object without proper synchronization or volatile declarations?
Updates to the shared object made by one thread may not be visible to other threads.
What happens if a thread running on CPU one changes the shared object in its CPU cache before flushing it back to main memory?
The changed version of the shared object is not visible to threads running on other CPUs.
Give an example of a race condition (train ticket booking)
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.
What are special hardware instructions that allow us to modify the content of a word or swap the contents of two words atomically?
Test-and-Set instruction and Compare-and-Swap instruction are special hardware instructions.
How can you solve the problem of race condition with volatile variables?
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.
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?
The variable count will be incremented twice, once in each CPU cache.
Do volatile variables guarantee solving the synchronization problem for race condition?
No, volatile variables do not guarantee solving the synchronization problem for race condition.
What is the purpose of the AtomicBoolean variable in the counter example?
The AtomicBoolean variable is used as a lock to provide synchronization.
What is the purpose of a memory barrier instruction?
To ensure that all loads and stores are completed before any subsequent load or store operations are performed.
What can a memory barrier instruction prevent?
Reordered instructions and ensure store operations are completed in memory and visible to other processors before future load or store operations are performed.
What can happen in multithreading applications without proper synchronization?
Reorder instruction problems, where instructions can be reordered leading to unexpected behavior.
What do computer architectures provide to ensure memory modifications are visible to threads running on other processors?
Memory barriers or memory fences.
Why can’t kernel developers make assumptions about memory modifications on a shared-memory multiprocessor?
Memory models vary by processor type.
What is a weakly ordered memory model?
Where a memory modification of one processor may not be immediately visible to all other processors.
What happens if synchronization is violated?
Multiple threads may access shared resources simultaneously, leading to unpredictable results.
How can static synchronization be achieved in Java?
By applying the synchronized keyword to static methods or using a synchronized block with a static object.
When does a thread acquire the monitor and continue executing the monitor region?
If no other thread is waiting in the entry set and no other thread currently owns the monitor.
What happens if a thread arrives at the beginning of a monitor region that is already owned by another thread?
The newly arrived thread must wait in the entry set.
What happens when the current owner of the monitor exits the monitor?
The newly arrived thread must compete with any other threads waiting in the entry set.
When is cooperation important in Java monitors?
When one thread needs some data to be in a particular state and another thread is responsible for getting the data into that state.
What should you do if you need to guarantee the exact sequence of thread access to a synchronized block?
implement Fairness yourself.
How does static synchronization work?
It places the synchronized keyword before a static method, ensuring that the lock access is on the class, not on the object or method.
What is static synchronization?
It is a way to ensure that multiple threads do not simultaneously enter a synchronized block or method with separate locks.
