ECM2414 Concurrency

Question 1

Concurrency concept

Answer 1

To be able to perform multiple processes at the same time - including the potential interaction between them
- Multiple tasks at the same time is sometimes called parallelism

Question 2

Why concurrency

Answer 2

• Allowing programs to interact with other systems/users
• Multi-core systems
• Systems can achieve more processes in the same time length
• Speed is key
• Productivity increases
• Consistency - real time responses

Question 3

Threads

Answer 3

Can be treated as lightweight processes
Threads exist within a process
Processes can be considered as these big boxes. Many threads running at the same time in a process.

Question 4

Why threads and not separate programs?

Answer 4

Shared memory space, so threads can access different segments of a program/process so said process is executed faster. A way to give a class or object the right of usage of a particular resource at a given time

Question 5

The OS can run threads in parallel, how..

Answer 5

• Virtually on a single processor
  ○ On core of the system running numerous threads, not visible to users, its very fast.
  ○ IPC communication schedules what comes first between all the threads on the same processors
• Actually on a multi-core processor

Question 6

Nondeterministic processes

Answer 6

in sequential task management, tasks can happen one after the other
Problems: Time issues, Complexity, if the ensuing task,
memory address issues: everything relies on memory
We cannot accurately say our system is efficient as memory usage and complexity cannot be determined

Question 7

Threads in Java
Can be created in 2 ways

Answer 7

• Using and extending the Thread class
• Using the Runnable interface

Question 8

New thing to worry about - liveness

Answer 8

• Concerned that code performs correctly in time - i.e that good things do happen eventually
• What level of “eventually” is acceptable

Question 9

current use thread lifecycle methods

Answer 9

Package java.lang
public class Thread implements Runnable

start
run
sleep
isAlive
interrupt
isInterrupted
interrupted
join

Question 10

How do i stop a thread

Answer 10

Setting a flag:
Loop will exit (and thread will stop) on the next iteration after done is set to tru

Interrupting:
- As above, when isInterrupted() is called for this thread, it will stop
- Except that if the thread is sleeping/waiting will throw Interrupted Exception and immediately return from run()

Question 11

Race conditions

Answer 11

Data synchronisation between threads can be an issue
When two threads try to access/change data at the same time it’s
known as a race condition

Question 12

Synchronisation

Answer 12

Simply add the synchronize keyword to the 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 the first thread is done with the object.

Question 13

Atomic actions

Answer 13

In programming, atomic actions are self-contained, they cannot be stopped in the middle, they either happen or they don’t

Atomic access can help here:
- Java primitive data types: byte, short, int, long, float, double, boolean, char
- reads and writes are atomic for most primitive variables (all types except long and double)
- reads and writes are atomic for all variables declared volatile, including long and double variables

Question 14

Deadlock

Answer 14

Thread 1 needs the resource held by thread 2
Deadlock, having a thread lock its resources, using keyword lock to shield off its resources

Question 15

Starvation

Answer 15

Where a thread can’t gain regular access to shared resources because another thread is frequently calling the object’s methods

Question 16

Livelock

Answer 16

A little like deadlock except that it occurs when two threads are too busy responding to one another to do work

Question 17

Sleep()

Answer 17

used to pause the execution of the current thread for a specified time in milliseconds

Question 18

yield()

Answer 18

a static method of Thread class and it can stop the currently executing thread and will give a chance to other waiting threads of the same priority

Question 19

join()

Answer 19

allows one thread to wait until another thread completes its execution. In simpler words, it means it waits for the other thread to die

Question 20

thread class thread management methods

Answer 20

sleep
yield
join

Question 21

object class thread management methods

Answer 21

wait
notify
notifyAll

Question 22

wait()

Answer 22

Waits for a condition to occur, must call from synchronised method/block

Question 23

wait(long timeStep)

Answer 23

Waits for a condition for timeStep milliseconds, then returns, must call from synchronised method/block (also wait (long timeStep, int nanos))

Question 24

notify()

Answer 24

○ Notifies a waiting thread a condition has occurred, must call from synchronised method/block (also void notifyAll();)
○ Allows thread communication

Question 25

Wait vs sleep

Answer 25

Wait
- must occur in a block synchronised on the monitor object
- releases the monitor (lock) when called and doesn’t sacrifice the remainder of its timeslice
- will wake up if notified via a notify call
- condition controlled

Sleep
- does not need to occur in a synchronised block
- retains the monitor (lock) when called and sacrifices the remainder of its timeslice
- A sleeping thread can only be woken up by an interruption (not by notify)
- counter controlled

Question 26

Thread lifecycle

Answer 26

new -> Runnable <-> Running -> Dead
^waiting, sleeping, blocking^

Question 27

yield()

Answer 27

yield() causes the currently executing thread object to pause
and allow other threads to execute.

If no other threads of the same priority need to execute,
execution of this thread continues

Identifies the current thread as doing something not particularly
important
Specify when a specific thread should yield for another

Question 28

Join()

Answer 28

Simple function allows one thread to wait for the completion of
another.
t.join()

Like sleep, join responds to an interrupt by exiting with an
InterruptedException

Question 29

Explicit locking: the lock interface

Answer 29

In synchronized, the lock exists but isnt seen
It is possible to create move and destroy locks through the lock object
Using the lock interface we can manage locks and:
- Two or more objects can share a lock
- One object can have multiple locks

Question 30

Atomic variables

Answer 30

• Even a simple increment consists of get, change and set operations
• Java uses java.util.concurrent.atomic to provide variables that support atomic operations on single variables
• Atomic variables contain methods to deal with this, supporting atomic operations ensureing that increments, decrements, or reads happen as a single uninterruptable operation
• AtomicInteger, AtomicLong, AtomicBoolean

Atomicity ensures the data is fresh

Question 31

atomic access functions

Answer 31

set()
get()
getAndSet()
compareAndSet()
weakCompareAndSet()

atomicInteger and atomicLong also implement:
- incrementAndGet()
- decrementAndGet()
- getAndIncrement()
○ XAndGet atomically performs X and returns updated value
○ getAndX atomically performs X and returns original value
- getAndDecrement()
- addAndGet()
- getAndAdd()

Question 32

Volatile variables

Answer 32

Declaring a variable volatile tells the compiler not to optimise access and forces the next read to see the last write
Waits for data to be thread before continuing

Question 33

Publishing & Escape

Answer 33

Publishing an object is when you make it available outside its current scope
- e.g. storing a reference to it where other code can find it, it may be a non-private attribute

By default, we shouldn’t publish an object unless we have to
- If we do, we may need to confirm the thread-safe nature of the publishing

An object published when it should not have been is said to have escaped (e.g. accessing an initialised state variable)

Any object that is reachable from a published object by some chain of non-private field references has also been published

Question 34

Thread confinement

Answer 34

One of the simplest ways to ensure thread-safety
If the data doesn’t need to be shared, confine it to a single thread
If data is only used by a single thread then it is said to be confined
Thread confinement is an element of the program’s design

Question 35

Lamdba Expression

Answer 35

Java 8.0 introduced Lambda Expressions
- Introduced aspects of functional programming to the language for the first time

Single largest upgrade to the language ever (and this includes generics that we’ll cover later in the module)
- Mark Reinhold, Chief Architect: Java Platform Group, Oracle

Java 8.0 was released in 2014 so should be available across machines

A lambda expression is essentially a block of code that you can pass around between objects (previously you could only pass objects or primitives between objects)

Question 36

lambda expression syntax and distinctions

Answer 36

Syntax is (arguments) -> {body}
- Can have 0 or more arguments
- Argument type can be declared or inferred
- With only a single inferred argument, no parentheses are needed
- A body with a single expression does not require curly brackets