Week 2 Flashcards
What is Concurrency?
To be able to perform multiple processes at the same time - including the potential interaction between them
What is multiple tasks at the same time sometimes called?
Parallelism
Why are we interested in Concurrency (2 reasons)?
- Allowing programs to interact with other systems/ users
- Multi-core Systems
What are Threads (in terms of processes)?
Treated as Lightweight Processes
Threads exist within a process
Many threads running at same time in a particular process
What is Multi-Threading (hint: virtually, actually)?
OS can run threads in parallel:
Virtually on a Single Core Processor
Actually on a Multi Core Processor
What is a problem with Threads?
Liveness
Liveness threads refer to a thread safety issue that occurs when a program doesn’t produce a final result
Liveness issues can have serious consequences, such as deadlocks that prevent a program from running further
What are the 2 ways to create Threads in Java?
- Using (and extending) the Thread class
- Using the Runnable interface
What are the 2 Ways to Stop a Thread in Java?
- Setting a flag:
public void run() {
while (!done) {
//…
}
}
Loop will exit (and thread will stop) on the next iteration after done is set to true
- Interrupting:
public void run() {
while (!isInterrupted() ) {
//…
}
}
When isInterrupted() is called for this thread, it will stop
Except that if the thread is sleeping/ waiting will throw InterruptedException and immediately return from run()
What is a Race Condition?
Issue with Threads
When 2 or more Threads try to access same resources it is called a Race Condition
How do you fix Race Conditions?
Synchronisation
Simply add the synchronize keyword to methods in the class
When one Thread is executing a synchronized method for an object, all other Threads that invoke synchronized methods for the same object block suspend execution until first thread is done with the object
What is Statement Synchronisation (simple)?
Where you synchronise a set of statements, not necessarily the whole method
What are Atomic Actions?
Self contained
We cannot interrupt an atomic process at any given time
What is Deadlock (hint:release)?
Deadlock is a situation where two or more processes are unable to proceed because each is waiting for the other to release a resource
This creates a cycle of dependency that halts all progress, causing the system or the involved processes to become stuck indefinitely
What is Starvation?
Where a thread can’t gain regular access to shared resources because another thread is frequently calling the object’s methods
Greedy Thread and Starved Thread
What is Livelock (hint: chatting)?
Like Deadlock
Except it occurs when 2 Threads are too busy responding to one another to do work
What does “void wait();” do?
waits for condition to occur, must call from synchronised method/ block
What does “void wait(long timeStep);” do?
Waits for condition for timeStep milleseconds, then returns
must call from synchronised method/ block
What does “void notify();” do?
Notifies a waiting thread a condition has occurred, must call from synchronised method/ block
Allows Thread communication
What does the wait() method do?
The wait() function is commonly used in programming to pause the execution of a program or thread for a specified amount of time or until a particular condition is met.
What does the sleep() method do?
Allows the Thread to go into inactive mode for a specific length of time
What is the difference between wait and sleep?
wait:
Must occur in block synchronised to monitors object
Releases the monitor(lock) when called and doesnt sacrifice remainder of its timeslice
Will wake up when notified via a notify call
sleep:
Does not need to occur in a synchronised block
Retains the monitor(lock) when called and sacrifices the remainder of its timeslice
Sleeping thread can only be woken up by an interruption (not by notify)
What does the yield() method do (hint: YIEELLDDDDDD)?
causes currently executing thread object to pause and allow other threads to execute
If no other threads of same priority need to execute, execution of this thread continues
Identifies current thread as doing something not particulary important
What does the join() method do?
Simple
Allows one thread to wait for the completion of another
What are the properties of Locking?
In synchronised the lock exists but isn’t seen
Possible to create, move and destroy locks through the Lock object
Using Lock interface we can manage locks and:
- Two or more objects can share a lock
- One object can have multiple locks
