SSC - Concurrency and Multi-threading

SSC - Concurrency and Multi-threadingThread Coordination (I)

Shan He

School for Computational ScienceUniversity of Birmingham

Module 06-19321: SSC

SSC - Concurrency and Multi-threading

Outline

Outline of Topics

Review what we learned

Thread signalling

Coordination toolsGuarded blockCountdownLatchCyclicBarrierSemaphore

SSC - Concurrency and Multi-threading

Thread signalling

Thread signalling

I Threads often have to coordinate their actions:I several threads start at the same time;I a thread waits for other threads to finish

I To coordinate, they need to signal each otherI Two types of threads signals

I Synchronous (what we are dealing):I Occur as a direct result of thread executionI Should be delivered to currently executing thread

I Asynchronous:I Occur due to an event typically unrelated to the current

instructionI Threading library must determine each signals recipient so

that asynchronous signals are delivered properly

I Each thread might receive a set of synchronous signals but itcan mask all signals except those that it wishes to receive

SSC - Concurrency and Multi-threading

Thread signalling

Thread signalling in Java

I For threads who use locks for synchronisation, the mainpurpose of signalling is communicate the lock status of aresource

I Three methods can be used:I wait() : Causes the current thread to wait (suspend and

also release its lock) until another thread invokes the notify()method or the notifyAll() method for this object.

I notify() : Wakes up a single thread that is waiting on this

object’s lock.I notifyAll() : Wakes up all threads that are waiting on this

object’s lock.

I Question: What class shall these methods belong to?

I There are idioms or tools in Java for coordinating theexecution of multiple threads

SSC - Concurrency and Multi-threading

Coordination tools

Guarded block

Guarded blockI Guarded block: The most common coordination idiomI Basic idea: wait for a particular condition to become true and

only in that case the actual execution of the thread resumes.I A simple example: a method that must not proceed until a

shared boolean variable has been set by another thread.I A naive implementation:

public void guardedWaitingTransaction() {while(!PaidFlag) {System.out.println("Pay me!!");

}System.out.println("Transaction finished!!");

}I What’s wrong with the above example?

SSC - Concurrency and Multi-threading

Coordination tools

Guarded block

Guarded block: using thread signallingI The above naive implementation is non-synchronized guarded

blockI Use a blank loop until the condition becomes true −→

wasting the precious CPU timeI We should use synchronized guarded block:

I Current thread is suspended to wait for the condition becomestrue

I It releases the acquired lock on that object −→ leaves theprocessor to be used by other threads

I We can use Java thread signalling methods to achieve thisI Steps:

I Invoke wait inside a loop that tests for the condition beingwaited for, also release the lock

I Another thread who acquires the same lock invokesnotifyAll to informing all threads waiting on that lock

that something important has happened.

SSC - Concurrency and Multi-threading

Coordination tools

Guarded block

Guarded block: example

I A more efficient example:public synchronized void guardedWaitingTransaction() {while(!PaidFlag) {try {wait();

} catch (InterruptedException e) {}}System.out.println("Transaction finished!!");

}I Q: Why used synchronized ?

I A: This ensures the current thread must own this object’s lock

SSC - Concurrency and Multi-threading

Coordination tools

Guarded block

Guarded block: example

I To notify the thread(s) who is waiting for the object’s lock:

public synchronized void paymoney(Integer value) {PaidFlag = true;

sum+= value;

notifyAll();

}

I Note 1: make sure notify() and notifyAll() should

be used with synchronized keyword

I Note 2: notify() only wakes up a single thread and alsodoes not specify which thread to be woken up.

I Rule of thumb: use notifyAll()

SSC - Concurrency and Multi-threading

Coordination tools

CountdownLatch

What is a countdown latch?

I Latch: a type of coordination tool that can be seen as a”switch” or ”trigger” in concurrent programming

I A countdown latch usually associated with a count value

I The latch is capable to count down the count value

I A thread or threads waits for the count value to reach zerobefore continuing to perform some process

I One-off process: Once the count value reaches 0, you cannotreset

SSC - Concurrency and Multi-threading

Coordination tools

CountdownLatch

How to use countdown latch?

I Steps:I Step 1: Create a CountDownLatch object with the initial

count valueI Step 2: Call countDown() to decrease the count by 1;

I Stpe 3: Use await() method in thread to wait for thecount to reach zero, or proceed immediately if the countalready reached zero.

I Usages: A CountDownLatch initialized to N can be used tomake one thread

I wait until N threads have completed some action, orI wait until some action has been completed N times.

SSC - Concurrency and Multi-threading

Coordination tools

CountdownLatch

What is a countdown latch?

3

3

2

1

0

0

Cnt=3

Ta await()

T1

T2

T3

countdown()

countdown()

countdown()

Ta awaiting...

Continue

Continue

Continue

Cnt=0

Resume Ta

Ta Resume

SSC - Concurrency and Multi-threading

Coordination tools

CyclicBarrier

What is a CyclicBarrier?

I CyclicBarrier: a coordination tool “that allows a set of threadsto all wait for each other to reach a common barrier point”,

I CyclicBarrier: a barrier that all threads must wait at, nothread can continue until all threads reach the barrier.

I Cyclic: because the barrier can be re-used after the waitingthreads are released.

I How to use CyclicBarrier :

I Step 1: Create a CyclicBarrier object with the 1).

number of parties (threads) are waiting upon it; and 2) abarrier action when all threads arrive at the barrier.

I Step 2: The threads wait for each other by calling the await()method on the CyclicBarrier .

SSC - Concurrency and Multi-threading

Coordination tools

CyclicBarrier

What is a CyclicBarrier?

Thread 1 Thread N

CyclicBarrier 1

CyclicBarrier 2

Wait Wait

Wait Wait

Continue Continue

Barrier action

SSC - Concurrency and Multi-threading

Coordination tools

Semaphore

What is a Semaphore?

I Semaphore: a variable or abstract data type that is used forcontrolling access, by multiple processes or threads, to acommon resource in concurrent programming

I Very simple idea: if the Semaphore value is 0, an attempt todecrement this value will cause the calling thread to wait untilsome other thread increments it.

I Invented by the famous Dutch computer scientist EdsgerDijkstra in 1965

I In Java, it is called counting semaphore, which maintains a setof permits (Semaphore value).

I Usage:I to restrict the number of threads than can access some

resource.I to send signals between threads.

SSC - Concurrency and Multi-threading

Coordination tools

Semaphore

How Semaphore works?

2

1

0

0

0

1

Cnt=2

T1 acquire()

T2

T3

T4

acquire()

acquire()

acquire()

continue

continue

0

1

0

1

2

awaiting...

awaiting...

T2release()continue

continueT4

T4release()continue

T3continue

T1release()continue

T3release()continue

SSC - Concurrency and Multi-threading

Coordination tools

Semaphore

How to use Semaphore?

I Steps:I Step 1: Create a Semaphore with number of permits N

Semaphore semaphore = new Semaphore(N);

I Step 2: Use acquire() and release() to define the criticalsection:semaphore.acquire();

//define your critical section

semaphore.release();

I Fairness: No guarantee that the first thread to call acquire()is also the first thread to obtain a permit.

I To enforce fairness, pass a boolean argumentboolean fair in the Semaphore class constructor:

Semaphore semaphore = new Semaphore(1, true);