src/jsr166y/RecursiveAction.java

/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/licenses/publicdomain
 */

package jsr166y;

/**
 * Recursive resultless ForkJoinTasks. This class establishes
 * conventions to parameterize resultless actions as {@code Void}
 * ForkJoinTasks. Because {@code null} is the only valid value of
 * type {@code Void}, methods such as join always return {@code null}
 * upon completion.
 *
 * <p><b>Sample Usages.</b> Here is a sketch of a ForkJoin sort that
 * sorts a given {@code long[]} array:
 *
 *  <pre> {@code
 * class SortTask extends RecursiveAction {
 *   final long[] array; final int lo; final int hi;
 *   SortTask(long[] array, int lo, int hi) {
 *     this.array = array; this.lo = lo; this.hi = hi;
 *   }
 *   protected void compute() {
 *     if (hi - lo < THRESHOLD)
 *       sequentiallySort(array, lo, hi);
 *     else {
 *       int mid = (lo + hi) >>> 1;
 *       invokeAll(new SortTask(array, lo, mid),
 *                 new SortTask(array, mid, hi));
 *       merge(array, lo, hi);
 *     }
 *   }
 * }}</pre>
 *
 * You could then sort {@code anArray} by creating {@code new
 * SortTask(anArray, 0, anArray.length-1) } and invoking it in a
 * ForkJoinPool.  As a more concrete simple example, the following
 * task increments each element of an array:
 *  <pre> {@code
 * class IncrementTask extends RecursiveAction {
 *   final long[] array; final int lo; final int hi;
 *   IncrementTask(long[] array, int lo, int hi) {
 *     this.array = array; this.lo = lo; this.hi = hi;
 *   }
 *   protected void compute() {
 *     if (hi - lo < THRESHOLD) {
 *       for (int i = lo; i < hi; ++i)
 *         array[i]++;
 *     }
 *     else {
 *       int mid = (lo + hi) >>> 1;
 *       invokeAll(new IncrementTask(array, lo, mid),
 *                 new IncrementTask(array, mid, hi));
 *     }
 *   }
 * }}</pre>
 *
 * <p>The following example illustrates some refinements and idioms
 * that may lead to better performance: RecursiveActions need not be
 * fully recursive, so long as they maintain the basic
 * divide-and-conquer approach. Here is a class that sums the squares
 * of each element of a double array, by subdividing out only the
 * right-hand-sides of repeated divisions by two, and keeping track of
 * them with a chain of {@code next} references. It uses a dynamic
 * threshold based on method {@code surplus}, but counterbalances
 * potential excess partitioning by directly performing leaf actions
 * on unstolen tasks rather than further subdividing.
 *
 *  <pre> {@code
 * double sumOfSquares(ForkJoinPool pool, double[] array) {
 *   int n = array.length;
 *   int seqSize = 1 + n / (8 * pool.getParallelism());
 *   Applyer a = new Applyer(array, 0, n, seqSize, null);
 *   pool.invoke(a);
 *   return a.result;
 * }
 *
 * class Applyer extends RecursiveAction {
 *   final double[] array;
 *   final int lo, hi, seqSize;
 *   double result;
 *   Applyer next; // keeps track of right-hand-side tasks
 *   Applyer(double[] array, int lo, int hi, int seqSize, Applyer next) {
 *     this.array = array; this.lo = lo; this.hi = hi;
 *     this.seqSize = seqSize; this.next = next;
 *   }
 *
 *   double atLeaf(int l, int r) {
 *     double sum = 0;
 *     for (int i = l; i < h; ++i) // perform leftmost base step
 *       sum += array[i] * array[i];
 *     return sum;
 *   }
 *
 *   protected void compute() {
 *     int l = lo;
 *     int h = hi;
 *     Applyer right = null;
 *     while (h - l > 1 && getSurplusQueuedTaskCount() <= 3) {
 *        int mid = (l + h) >>> 1;
 *        right = new Applyer(array, mid, h, seqSize, right);
 *        right.fork();
 *        h = mid;
 *     }
 *     double sum = atLeaf(l, h);
 *     while (right != null) {
 *        if (right.tryUnfork()) // directly calculate if not stolen
 *          sum += right.atLeaf(right.lo, right.hi);
 *       else {
 *          right.helpJoin();
 *          sum += right.result;
 *        }
 *        right = right.next;
 *      }
 *     result = sum;
 *   }
 * }}</pre>
 *
 * @since 1.7
 * @author Doug Lea
 */
public abstract class RecursiveAction extends ForkJoinTask<Void> {

    /**
     * The main computation performed by this task.
     */
    protected abstract void compute();

    /**
     * Always returns null.
     */
    public final Void getRawResult() { return null; }

    /**
     * Requires null completion value.
     */
    protected final void setRawResult(Void mustBeNull) { }

    /**
     * Implements execution conventions for RecursiveActions.
     */
    protected final boolean exec() {
        compute();
        return true;
    }

}
Revision:	1.8
Committed:	Wed Jul 29 12:05:55 2009 UTC (14 years, 9 months ago) by dl
Branch:	MAIN
Changes since 1.7:	+1 -2 lines
Log Message:	Improve drainTasksTo signature; update example
#	Content
1	/*
2	* Written by Doug Lea with assistance from members of JCP JSR-166
3	* Expert Group and released to the public domain, as explained at
4	* http://creativecommons.org/licenses/publicdomain
5	*/
6
7	package jsr166y;
8
9	/**
10	* Recursive resultless ForkJoinTasks. This class establishes
11	* conventions to parameterize resultless actions as {@code Void}
12	* ForkJoinTasks. Because {@code null} is the only valid value of
13	* type {@code Void}, methods such as join always return {@code null}
14	* upon completion.
15	*
16	* <p><b>Sample Usages.</b> Here is a sketch of a ForkJoin sort that
17	* sorts a given {@code long[]} array:
18	*
19	* <pre> {@code
20	* class SortTask extends RecursiveAction {
21	* final long[] array; final int lo; final int hi;
22	* SortTask(long[] array, int lo, int hi) {
23	* this.array = array; this.lo = lo; this.hi = hi;
24	* }
25	* protected void compute() {
26	* if (hi - lo < THRESHOLD)
27	* sequentiallySort(array, lo, hi);
28	* else {
29	* int mid = (lo + hi) >>> 1;
30	* invokeAll(new SortTask(array, lo, mid),
31	* new SortTask(array, mid, hi));
32	* merge(array, lo, hi);
33	* }
34	* }
35	* }}</pre>
36	*
37	* You could then sort {@code anArray} by creating {@code new
38	* SortTask(anArray, 0, anArray.length-1) } and invoking it in a
39	* ForkJoinPool. As a more concrete simple example, the following
40	* task increments each element of an array:
41	* <pre> {@code
42	* class IncrementTask extends RecursiveAction {
43	* final long[] array; final int lo; final int hi;
44	* IncrementTask(long[] array, int lo, int hi) {
45	* this.array = array; this.lo = lo; this.hi = hi;
46	* }
47	* protected void compute() {
48	* if (hi - lo < THRESHOLD) {
49	* for (int i = lo; i < hi; ++i)
50	* array[i]++;
51	* }
52	* else {
53	* int mid = (lo + hi) >>> 1;
54	* invokeAll(new IncrementTask(array, lo, mid),
55	* new IncrementTask(array, mid, hi));
56	* }
57	* }
58	* }}</pre>
59	*
60	* <p>The following example illustrates some refinements and idioms
61	* that may lead to better performance: RecursiveActions need not be
62	* fully recursive, so long as they maintain the basic
63	* divide-and-conquer approach. Here is a class that sums the squares
64	* of each element of a double array, by subdividing out only the
65	* right-hand-sides of repeated divisions by two, and keeping track of
66	* them with a chain of {@code next} references. It uses a dynamic
67	* threshold based on method {@code surplus}, but counterbalances
68	* potential excess partitioning by directly performing leaf actions
69	* on unstolen tasks rather than further subdividing.
70	*
71	* <pre> {@code
72	* double sumOfSquares(ForkJoinPool pool, double[] array) {
73	* int n = array.length;
74	* int seqSize = 1 + n / (8 * pool.getParallelism());
75	* Applyer a = new Applyer(array, 0, n, seqSize, null);
76	* pool.invoke(a);
77	* return a.result;
78	* }
79	*
80	* class Applyer extends RecursiveAction {
81	* final double[] array;
82	* final int lo, hi, seqSize;
83	* double result;
84	* Applyer next; // keeps track of right-hand-side tasks
85	* Applyer(double[] array, int lo, int hi, int seqSize, Applyer next) {
86	* this.array = array; this.lo = lo; this.hi = hi;
87	* this.seqSize = seqSize; this.next = next;
88	* }
89	*
90	* double atLeaf(int l, int r) {
91	* double sum = 0;
92	* for (int i = l; i < h; ++i) // perform leftmost base step
93	* sum += array[i] * array[i];
94	* return sum;
95	* }
96	*
97	* protected void compute() {
98	* int l = lo;
99	* int h = hi;
100	* Applyer right = null;
101	* while (h - l > 1 && getSurplusQueuedTaskCount() <= 3) {
102	* int mid = (l + h) >>> 1;
103	* right = new Applyer(array, mid, h, seqSize, right);
104	* right.fork();
105	* h = mid;
106	* }
107	* double sum = atLeaf(l, h);
108	* while (right != null) {
109	* if (right.tryUnfork()) // directly calculate if not stolen
110	* sum += right.atLeaf(right.lo, right.hi);
111	* else {
112	* right.helpJoin();
113	* sum += right.result;
114	* }
115	* right = right.next;
116	* }
117	* result = sum;
118	* }
119	* }}</pre>
120	*
121	* @since 1.7
122	* @author Doug Lea
123	*/
124	public abstract class RecursiveAction extends ForkJoinTask<Void> {
125
126	/**
127	* The main computation performed by this task.
128	*/
129	protected abstract void compute();
130
131	/**
132	* Always returns null.
133	*/
134	public final Void getRawResult() { return null; }
135
136	/**
137	* Requires null completion value.
138	*/
139	protected final void setRawResult(Void mustBeNull) { }
140
141	/**
142	* Implements execution conventions for RecursiveActions.
143	*/
144	protected final boolean exec() {
145	compute();
146	return true;
147	}
148
149	}