PV168

Memory Model

Retrospective

• Did you understand what was done in the last seminar?
• Was it intuitively understandable?
• Are you able to describe how it works?
• Did you find it difficult?

Monitor

• The goal is to prevent concurrent access using the critical section.
• Do not use multiple monitors on the same data.
• Each object instance and the class itself has a monitor.
  • Do not mix them.

Monitor

• Where to place synchronized?
  • in the method declaration
  • in the static method declaration
  • in its own code block

Monitor

class Counter {
    private int value = 0;
    public synchronized int getValue() {
        return value++;
    }

    public int getSameValue() {
        synchronized(this) {
            return value++;
        }
    }
}

Atomic numbers

• Useful for just-a-counter cases.
• Lower overhead
  • Monitors are quite expensive.
  • Can you say why?

Atomic numbers

class Counter {
    private AtomicInteger value = new AtomicInteger(0);
    public int getValue() {
        return value.getAndIncrement();
    }
}

But how monitors and atomics work?

Memory Model

In computing, a memory model describes the interactions of threads through memory and their shared use of the data.[1]

• Essential for any programming environment using parallelism.
• Consists of:
  • Language specification
  • Compiler specification (including optimizations)
  • Hardware (JVM in Java)

Memory Model

• Specifies: [2]
  • synchronization points and actions
  • what is affected by synchronization points
  • what applies to memory access
    • before the synchronization point
    • after the synchronization point
• Defines Sequential Consistency and Happens-before relation.

Sequential Consistency

Compilers are allowed to reorder the instructions, when this does not affect the execution of the thread in isolation. [2]

• Also, the JIT may reorder, delay, or cache accesses.
• Also, the JVM may reorder, delay, or cache accesses.
• Also, the CPU may reorder, delay, or cache accesses.

Sequential Consistency

A = B = 0

// thread 1
r2 = A;
B = 1

// thread 2
r1 = B;
A = 2;

• What are the values of r1 and r2?
  • It may happen that r1 == 1 and r2 == 2.

Synchronization Points

• Java has 4 types:
  • volatile
  • Atomic classes
  • synchronized
  • starts and joins of threads

Actions

• Action types:
  • Load
  • Store
  • Synchronization
• Loads without any store are trouble-free.
• Any store requires a proper synchronization.
  • Including all load accesses!
• Synchronization locks and unlocks the monitor.

Shared variables

• Shared variables (e.g. affected by the Memory Model):
  • instance fields
  • static fields
  • array elements
• Variables unaffected by the Memory Model:
  • local variables
  • methods arguments, catched exceptions

Happens-before

• Transitive relationship of actions.

  • If one action happens-before another, then the first is visible to and ordered before the second [3]

• Applies to all synchronization points.
• Partially applies to all final fields.
  • After construction, no further synchronization is needed.

volatile

• Implements the memory barrier:
  • accesses to volatiles cannot be reordered
  • subsequent accesses cannot be reordered before volatile load
  • prior accesses cannot be reordered after volatile store
• Numeric operations are not atomic (++, --)
  • Store of 64 bit values is done in two steps.
• DO NOT USE IT UNLESS YOU KNOW WHAT YOU ARE DOING!

volatile

class VolatileExample {
    int x = 0;
    volatile boolean flag = false;
    public void writer() {
        x = 42;
        flag = true;
    }

    public void reader() {
        if (flag == true) {
          System.out.println(x); // guaranteed to see 42.
        }
    }
}

Atomic classes

• Useful for numeric counters, gauges, and indicators.
• Thread safe for all basic numeric operations.
  • Increments, decrements, assignment
• “Synchronizes properly as you would expect.”
  • Almost the same rules applies as for volatiles.
  • Allows finer specification of memory ordering.
• DO NOT USE FOR OTHER THINGS THAN COUNTERS UNLESS YOU KNOW WHAT YOU ARE DOING!

Atomic classes

class AtomicExample {
    private int value = 0;
    private AtomicBoolean flag = new AtomicBoolean(false);
    public void writer() {
        value = 42;
        flag.set(true);
    }

    public void reader() {
        while (!flag.get()) {} // busy wait here
        System.out.println(value); // guaranteed to see 42.
    }
}

synchronized

• When used in a method signature, it uses the monitor of the instance.
  • Or monitor of the class in case the method is static.
• When used in the body of a method, it uses the monitor of the specified object.

public synchronized void doSomething() { /* ... */ }
public void doSomethingElse() {
    synchronized(this.attribute) {/* ... */}
}

Threads

• Thread start and join have similar memory semantics as synchronized:
  • Any store which happened-before the start of the thread is visible by the thread.
  • Any store which happened-before the join of the about-to-join thread is visible in the joinee thread.

Threads

class ThreadExample {
    public void run() {
        Integer value = 42;
        var t = new Thread(() -> value *= 2 );
        // value is 42
        t.start();
        t.join();
        // value is guaranteed to be 84;
    }
}

final Fields

• Special rules apply to final fields: [4]
  • Set the final fields for an object in that object’s constructor.
  • Do not write a reference to the object being constructed in a place where another thread can see it before the object’s constructor is finished.
  • If this is followed, then when the object is seen by another thread, that thread will always see the correctly constructed version of that object’s final fields.

Double-Checked Locking

Is this correct?

class Foo {
    private Helper helper = null;
    public Helper getHelper() {
        if (helper == null) {
            synchronized(this) {
                if (helper == null)
                    helper = new Helper();
            }
        }
        return helper;
    }
}

Double-Checked Locking Is Broken

And there is no way how it can be fixed.

How To Test The Correct Usage?

• Unit tests are insufficient.
  • In fact, no commonly used automatization approaches can validate the correctness.
• To prove the correctness, one must provide a formal proof.
  • This is the reason airplanes work in strictly single-threaded environments.

Without correct synchronization, very strange, confusing and counterintuitive behaviors are possible.