Locking objects in an array
Hello,
I've come accross a threading problem for which I can't find a nice
solution. (SSCCP at end of post)
I have a bidimensional array of objects (view it as a 2D lattice). I
want to make atomic operations on random square 2x2 regions of the
lattice. Thus I want to lock the 4 objects of the region, then perform
the change, then unlock those objects. Several threads should be
allowed to work on the array at the same time, if each thread is
accessing a 2x2 region which does not overlap with that of another,
they should be capable of doing the work in a parallel way.
How should I design the code to achieve this?
My solution (which may be misguided) so far is that, each object of
the lattice contains a Lock and has a lock() and unlock() method which
delegate to the lock.
So the approach is basically:
1. call lock() on each of the 4 objects (always in the same order)
2. make change
3. call unlock() on each of the 4 objects
What I don't like about this, is that
a) lock and unlock really have nothing to do in the API of the objects
in the lattice. Would it be possible to achieve the same result
without using an explicit Lock (thus no lock() method), but using the
Java synchronized() mechanism?
b) if an exception is thrown anywhere, eg. where only a part of the
lattice region has been locked, the cleanup is ugly because you can't
test if a lock is presently locked. You have to rely on unlock() to
throw.
c) An exception may leave the lattice in an inconsistent state.
Any better designs are welcome.
Thanks Phil
PS: In the code, the lattice objects are Counters and the action which
is performed atomically on the region is an increment.
---------------- Lattice.java ------------
import java.util.Random;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public class Lattice {
/**
* The size of the region of the lattice to increment
* atomically. In this case, 2 x 2.
*/
private static final int FRAGMENT_SIZE = 2;
private static final int LATTICE_X_SIZE = 10;
private static final int LATTICE_Y_SIZE = 6;
private Counter[][] lattice = new Counter[LATTICE_X_SIZE]
[LATTICE_Y_SIZE];
public Lattice() {
// filling the array
for (int i = 0; i < lattice.length; i++) {
for (int j = 0; j < lattice[0].length; j++) {
lattice[i][j] = new Counter(i,j);
}
}
}
/**
* This method increments the square region of the lattice
* which has the given indexes as top-left corner. If the
* region spans outside of the lattice, wrap around is used.
* This method should be thread-safe. It can be called
* efficiently by several threads at the same time.
* @param x index of top-left corner
* @param y index of top-left corner
*/
public void increment(int x, int y){
// System.out.println("Increm " + x + " " + y);
try{
// locking the 2 x 2 region
for(int i = x; i < x + FRAGMENT_SIZE; i++){
for(int j = y; j < y + FRAGMENT_SIZE; j++){
int indexI = i % LATTICE_X_SIZE; // wrap around
int indexJ = j % LATTICE_Y_SIZE;
lattice[indexI][indexJ].lock(); // locking in increasing
order to avoid deadlock
}
}
// Now the fragment is locked
// let's increment it
for(int i = x; i < x + FRAGMENT_SIZE; i++){
for(int j = y; j < y + FRAGMENT_SIZE; j++){
int indexI = i % LATTICE_X_SIZE;
int indexJ = j % LATTICE_Y_SIZE;
// the actual action!
lattice[indexI][indexJ].increment();
}
}
} finally{
// unlock the region
for(int i = x; i < x + FRAGMENT_SIZE; i++){
for(int j = y; j < y + FRAGMENT_SIZE; j++){
int indexI = i % LATTICE_X_SIZE;
int indexJ = j % LATTICE_Y_SIZE;
try{
lattice[indexI][indexJ].unlock(); // unlocking in
increasing order
} catch (IllegalMonitorStateException e) {
// was probably never locked (?)
}
}
}
}
}
public void printArray() {
for (int i = 0; i < lattice.length; i++) {
for (int j = 0; j < lattice[0].length; j++) {
System.out.print(lattice[i][j]);
}
System.out.print("\n");
}
}
public static void main(String[] args) throws Exception {
Lattice m = new Lattice();
RegionIncrementer fi1 = new RegionIncrementer(m);
RegionIncrementer fi2 = new RegionIncrementer(m);
RegionIncrementer fi3 = new RegionIncrementer(m);
Thread t1 = new Thread(fi1);
Thread t2 = new Thread(fi2);
Thread t3 = new Thread(fi3);
t1.start();
t2.start();
t3.start();
Thread.sleep(5000);
fi1.stopit();
fi2.stopit();
fi3.stopit();
t1.join();
t2.join();
t3.join();
m.printArray();
System.out.println(fi1);
System.out.println(fi2);
System.out.println(fi3);
}
/**
* A {@link Runnable} which repeatedly chooses
* a random region on the lattice and increments
* the counter in it.
*/
static class RegionIncrementer implements Runnable{
private static final Random rnd = new Random();
private int performedIncrementsCounter;
private final Lattice lattice;
private volatile boolean run = true;
public RegionIncrementer(Lattice lattice) {
this.lattice = lattice;
}
public void run() {
while(run){
lattice.increment(rnd.nextInt(LATTICE_X_SIZE), rnd.nextInt
(LATTICE_Y_SIZE));
performedIncrementsCounter++;
}
}
public void stopit(){
run = false;
}
public String toString() {
return "Incremented " + performedIncrementsCounter + " times";
}
}
/**
* The object which are stored in the lattice. In this case a simple
* counter.
*/
static class Counter{
private final int xPos;
private final int yPos;
private int count;
private Lock lock = new ReentrantLock();
private int contentions;
public Counter(int pos, int pos2) {
xPos = pos;
yPos = pos2;
}
public void lock(){
boolean locked = lock.tryLock();
if (!locked) { // this is used just to explicitely show
contentions
contentions++;
System.out.println(xPos + " " + yPos + " Contentions: " +
contentions);
lock.lock();
}
}
public void unlock(){
lock.unlock();
}
/**
* Need not be synchronized, because wrapping code
* is synchronizing
*/
// TODO Is this OK? Will lock() and unlock() do a memory barrier?
public void increment(){
count++;
}
public String toString() {
return String.format("%1$#" + 8 + "s", "" + count);
}
}
}