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EliminationBackoffStack.java
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EliminationBackoffStack.java
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package stack;
import util.ThreadId;
import javax.annotation.Nonnull;
import java.util.Random;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicReference;
import java.util.concurrent.atomic.AtomicReferenceArray;
/**
* Concurrent stack as described in
* <a href="http://www.cs.bgu.ac.il/~hendlerd/papers/scalable-stack.pdf">A
* Scalable Lock-free Stack Algorithm</a> by Danny Hendler, Nir Shavit and Lena Yerushalmi.
* <p/>
* This algorithms relies on unique and stable thread ids that are used as
* indices into the elimination array. These indices are obtained from
* {@link ThreadId#id()}. This severely limits practical applicability of
* this stack implementation.
*
* @param <T> Stack element type.
* @see ThreadId
*/
public final class EliminationBackoffStack<T> implements ConcurrentStack<T> {
private static class Node<T> {
final T value;
volatile Node<T> next;
Node(T value) {
this.value = value;
}
boolean push(AtomicReference<Node<T>> refTop) {
return refTop.compareAndSet(next = refTop.get(), this);
}
boolean pop(AtomicReference<Node<T>> refTop) {
return refTop.compareAndSet(this, next);
}
}
private enum Operation {
PUSH,
POP
}
private static class Cell<T> {
final int id;
final Operation operation;
volatile Node<T> node;
volatile boolean wakeup;
Cell(Operation op, Node<T> n) {
id = ThreadId.id() - 1;
operation = op;
node = n;
}
void reset() {
wakeup = false;
}
int spin() {
int x = 0;
for (int n = 0; n < 10000; n++) {
if (wakeup) {
break;
}
x = (x * 24049 + 11) % 7;
}
return x;
}
void wakeup() {
wakeup = true;
}
}
private static class EliminationArray<T> extends AtomicReferenceArray<Cell<T>> {
private final Random random;
EliminationArray(int length) {
super(length);
random = new Random();
}
/**
* Compare-and-swap operation, unlike the standard compare-and-set
* operation returns the current value instead of boolean flag.
*
* @param index Element index to swap atomically.
* @param o Expected value.
* @param n New value.
* @return Current value which is expected value if succeeded,
* or something else if failed.
*/
Cell<T> compareAndSwap(int index, Cell<T> o, Cell<T> n) {
while (true) {
Cell<T> v = get(index);
if (v == o) {
if (compareAndSet(index, o, n)) {
return v;
}
}
else {
return v;
}
}
}
}
private static final int THREADS = ThreadId.MAX;
private final AtomicReference<Node<T>> top;
private final EliminationArray<T> cells;
public EliminationBackoffStack() {
top = new AtomicReference<Node<T>>();
cells = new EliminationArray<T>(THREADS);
}
@Override
public void push(@Nonnull T e) {
if (e == null) {
throw new NullPointerException();
}
Node<T> node = new Node<T>(e);
Cell<T> cell = null;
while (true) {
if (node.push(top)) {
return;
}
if (cell == null) {
cell = new Cell<T>(Operation.PUSH, node);
}
cell.reset();
if (backoff(cell)) {
return;
}
}
}
@Override
public T peek() {
Node<T> node = top.get();
if (node == null) {
return null;
}
return node.value;
}
@Override
public T pop() {
Cell<T> cell = null;
while (true) {
Node<T> node = top.get();
if (node == null) {
return null;
}
if (node.pop(top)) {
return node.value;
}
if (cell == null) {
cell = new Cell<T>(Operation.POP, null);
}
cell.reset();
if (backoff(cell)) {
return cell.node.value;
}
}
}
private boolean backoff(Cell<T> a) {
backoff.incrementAndGet();
cells.set(a.id, a);
Cell<T> p = pickPartner(a);
Cell<T> c;
if (p != null && a.operation != p.operation) {
c = cells.compareAndSwap(a.id, a, null);
if (a != c) {
return passiveCollide(a, c);
}
else {
return activeCollide(a, p);
}
}
a.spin();
c = cells.compareAndSwap(a.id, a, null);
if (a != c) {
return passiveCollide(a, c);
}
return false;
}
private Cell<T> pickPartner(Cell<T> a) {
while (true) {
int id = cells.random.nextInt(cells.length());
if (id != a.id) {
Cell<T> p = cells.get(id);
if (p != null && p.id == id) {
return p;
}
return null;
}
}
}
private boolean activeCollide(Cell<T> a, Cell<T> p) {
if (cells.compareAndSet(p.id, p, a)) {
if (a.operation == Operation.POP) {
a.node = p.node;
p.wakeup();
}
elimination.incrementAndGet();
return true;
}
return false;
}
private boolean passiveCollide(Cell<T> p, Cell<T> a) {
if (cells.compareAndSet(p.id, a, null)) {
if (p.operation == Operation.POP) {
p.node = a.node;
}
return true;
}
return false;
}
final AtomicInteger backoff = new AtomicInteger();
final AtomicInteger elimination = new AtomicInteger();
void dump() {
System.out.println(String.format("backoffs: %d", backoff.get()));
System.out.println(String.format("eliminations: %d", elimination.get()));
}
}