src/jsr166e/StripedAdder.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/publicdomain/zero/1.0/
 */

package jsr166e;
import java.util.Arrays;
import java.util.Random;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicLong;
import java.io.IOException;
import java.io.Serializable;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;

/**
 * A set of variables that together maintain a sum.  When updates
 * (method {@link #add}) are contended across threads, the set of
 * adders may grow to reduce contention. Method {@link #sum} returns
 * the current combined total across these adders. This value is
 * <em>NOT</em> an atomic snapshot (concurrent updates may occur while
 * the sum is being calculated), and so cannot be used alone for
 * fine-grained synchronization control.
 *
 * <p> This class may be applicable when many threads frequently
 * update a common sum that is used for purposes such as collecting
 * statistics. In this case, performance may be significantly faster
 * than using a shared {@link AtomicLong}, at the expense of using
 * significantly more space.  On the other hand, if it is known that
 * only one thread can ever update the sum, performance may be
 * significantly slower than just updating a local variable.
 *
 * @author Doug Lea
 */
public class StripedAdder implements Serializable {
    private static final long serialVersionUID = 7249069246863182397L;

    /*
     * Overview: We maintain a table of AtomicLongs (padded to reduce
     * false sharing). The table is indexed by per-thread hash codes
     * that are initialized as random values.  The table doubles in
     * size upon contention (as indicated by failed CASes when
     * performing add()), but is capped at the nearest power of two >=
     * #cpus: At that point, contention should be infrequent if each
     * thread has a unique index; so we instead adjust hash codes to
     * new random values upon contention rather than expanding. A
     * single spinlock is used for resizing the table as well as
     * populating slots with new Adders. Upon lock contention, threads
     * just try other slots rather than blocking. We guarantee that at
     * least one slot exists, so retries will eventually find a
     * candidate Adder.
     */

    /**
     * Number of processors, to place a cap on table growth.
     */
    static final int NCPU = Runtime.getRuntime().availableProcessors();

    /**
     * Version of AtomicLong padded to avoid sharing cache
     * lines on most processors
     */
    static final class Adder extends AtomicLong {
        long p0, p1, p2, p3, p4, p5, p6, p7, p8, p9, pa, pb, pc, pd;
        Adder(long x) { super(x); }
    }

    /**
     * Holder for the thread-local hash code.
     */
    static final class HashCode {
        int code;
        HashCode(int h) { code = h; }
    }

    /**
     * The corresponding ThreadLocal class
     */
    static final class ThreadHashCode extends ThreadLocal<HashCode> {
        static final Random rng = new Random();
        public HashCode initialValue() {
            int h = rng.nextInt();
            return new HashCode((h == 0) ? 1 : h); // ensure nonzero
        }
    }

    /**
     * Static per-thread hash codes. Shared across all StripedAdders
     * because adjustments due to collisions in one table are likely
     * to be appropriate for others.
     */
    static final ThreadHashCode threadHashCode = new ThreadHashCode();

    /**
     * Table of adders. Initially of size 2; grows to be at most NCPU.
     */
    private transient volatile Adder[] adders;

    /**
     * Serves as a lock when resizing and/or creating Adders.  There
     * is no need for a blocking lock: When busy, other threads try
     * other slots.
     */
    private final AtomicInteger mutex;

    /**
     * Marsaglia XorShift for rehashing on collisions
     */
    private static int xorShift(int r) {
        r ^= r << 13;
        r ^= r >>> 17;
        return r ^ (r << 5);
    }

    /**
     * Creates a new adder with initially zero sum.
     */
    public StripedAdder() {
        Adder[] as = new Adder[2];
        as[0] = new Adder(0); // ensure at least one available adder
        this.adders = as;
        this.mutex = new AtomicInteger();
    }

    /**
     * Adds the given value.
     *
     * @param x the value to add
     */
    public void add(long x) {
        HashCode hc = threadHashCode.get();
        for (int h = hc.code;;) {
            Adder[] as = adders;
            int n = as.length;
            Adder a = as[h & (n - 1)];
            if (a != null) {
                long v = a.get();
                if (a.compareAndSet(v, v + x))
                    break;
                if (n >= NCPU) {                 // Collision when table at max
                    h = hc.code = xorShift(h);   // change code
                    continue;
                }
            }
            final AtomicInteger mutex = this.mutex;
            if (mutex.get() != 0)
                h = xorShift(h);                 // Try elsewhere
            else if (mutex.compareAndSet(0, 1)) {
                boolean created = false;
                try {
                    Adder[] rs = adders;
                    if (a != null && rs == as)   // Resize table
                        rs = adders = Arrays.copyOf(as, as.length << 1);
                    int j = h & (rs.length - 1);
                    if (rs[j] == null) {         // Create adder
                        rs[j] = new Adder(x);
                        created = true;
                    }
                } finally {
                    mutex.set(0);
                }
                if (created) {
                    hc.code = h;                // Use this adder next time
                    break;
                }
            }
        }
    }

    /**
     * Returns an estimate of the current sum.  The result is
     * calculated by summing multiple variables, so may not be
     * accurate if updates occur concurrently with this method.
     *
     * @return the estimated sum
     */
    public long sum() {
        long sum = 0;
        Adder[] as = adders;
        int n = as.length;
        for (int i = 0; i < n; ++i) {
            Adder a = as[i];
            if (a != null)
                sum += a.get();
        }
        return sum;
    }

    /**
     * Resets each of the variables to zero. This is effective in
     * fully resetting the sum only if there are no concurrent
     * updates.
     */
    public void reset() {
        Adder[] as = adders;
        int n = as.length;
        for (int i = 0; i < n; ++i) {
            Adder a = as[i];
            if (a != null)
                a.set(0L);
        }
    }

    /**
     * Equivalent to {@code add(1)}.
     */
    public void increment() {
        add(1L);
    }

    /**
     * Equivalent to {@code add(-1)}.
     */
    public void decrement() {
        add(-1L);
    }

    /**
     * Equivalent to {@link #sum} followed by {@link #reset}.
     *
     * @return the estimated sum
     */
    public long sumAndReset() {
        long sum = 0;
        Adder[] as = adders;
        int n = as.length;
        for (int i = 0; i < n; ++i) {
            Adder a = as[i];
            if (a != null) {
                sum += a.get();
                a.set(0L);
            }
        }
        return sum;
    }

    private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException {
        s.defaultWriteObject();
        s.writeLong(sum());
    }

    private void readObject(ObjectInputStream s)
        throws IOException, ClassNotFoundException {
        s.defaultReadObject();
        long c = s.readLong();
        Adder[] as = new Adder[2];
        as[0] = new Adder(c);
        this.adders = as;
        mutex.set(0);
    }

}


Revision:	1.2
Committed:	Wed Jul 20 16:06:19 2011 UTC (12 years, 10 months ago) by jsr166
Branch:	MAIN
Changes since 1.1:	+7 -7 lines
Log Message:	whitespace
#	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/publicdomain/zero/1.0/
5	*/
6
7	package jsr166e;
8	import java.util.Arrays;
9	import java.util.Random;
10	import java.util.concurrent.atomic.AtomicInteger;
11	import java.util.concurrent.atomic.AtomicLong;
12	import java.io.IOException;
13	import java.io.Serializable;
14	import java.io.ObjectInputStream;
15	import java.io.ObjectOutputStream;
16
17	/**
18	* A set of variables that together maintain a sum. When updates
19	* (method {@link #add}) are contended across threads, the set of
20	* adders may grow to reduce contention. Method {@link #sum} returns
21	* the current combined total across these adders. This value is
22	* <em>NOT</em> an atomic snapshot (concurrent updates may occur while
23	* the sum is being calculated), and so cannot be used alone for
24	* fine-grained synchronization control.
25	*
26	* <p> This class may be applicable when many threads frequently
27	* update a common sum that is used for purposes such as collecting
28	* statistics. In this case, performance may be significantly faster
29	* than using a shared {@link AtomicLong}, at the expense of using
30	* significantly more space. On the other hand, if it is known that
31	* only one thread can ever update the sum, performance may be
32	* significantly slower than just updating a local variable.
33	*
34	* @author Doug Lea
35	*/
36	public class StripedAdder implements Serializable {
37	private static final long serialVersionUID = 7249069246863182397L;
38
39	/*
40	* Overview: We maintain a table of AtomicLongs (padded to reduce
41	* false sharing). The table is indexed by per-thread hash codes
42	* that are initialized as random values. The table doubles in
43	* size upon contention (as indicated by failed CASes when
44	* performing add()), but is capped at the nearest power of two >=
45	* #cpus: At that point, contention should be infrequent if each
46	* thread has a unique index; so we instead adjust hash codes to
47	* new random values upon contention rather than expanding. A
48	* single spinlock is used for resizing the table as well as
49	* populating slots with new Adders. Upon lock contention, threads
50	* just try other slots rather than blocking. We guarantee that at
51	* least one slot exists, so retries will eventually find a
52	* candidate Adder.
53	*/
54
55	/**
56	* Number of processors, to place a cap on table growth.
57	*/
58	static final int NCPU = Runtime.getRuntime().availableProcessors();
59
60	/**
61	* Version of AtomicLong padded to avoid sharing cache
62	* lines on most processors
63	*/
64	static final class Adder extends AtomicLong {
65	long p0, p1, p2, p3, p4, p5, p6, p7, p8, p9, pa, pb, pc, pd;
66	Adder(long x) { super(x); }
67	}
68
69	/**
70	* Holder for the thread-local hash code.
71	*/
72	static final class HashCode {
73	int code;
74	HashCode(int h) { code = h; }
75	}
76
77	/**
78	* The corresponding ThreadLocal class
79	*/
80	static final class ThreadHashCode extends ThreadLocal<HashCode> {
81	static final Random rng = new Random();
82	public HashCode initialValue() {
83	int h = rng.nextInt();
84	return new HashCode((h == 0) ? 1 : h); // ensure nonzero
85	}
86	}
87
88	/**
89	* Static per-thread hash codes. Shared across all StripedAdders
90	* because adjustments due to collisions in one table are likely
91	* to be appropriate for others.
92	*/
93	static final ThreadHashCode threadHashCode = new ThreadHashCode();
94
95	/**
96	* Table of adders. Initially of size 2; grows to be at most NCPU.
97	*/
98	private transient volatile Adder[] adders;
99
100	/**
101	* Serves as a lock when resizing and/or creating Adders. There
102	* is no need for a blocking lock: When busy, other threads try
103	* other slots.
104	*/
105	private final AtomicInteger mutex;
106
107	/**
108	* Marsaglia XorShift for rehashing on collisions
109	*/
110	private static int xorShift(int r) {
111	r ^= r << 13;
112	r ^= r >>> 17;
113	return r ^ (r << 5);
114	}
115
116	/**
117	* Creates a new adder with initially zero sum.
118	*/
119	public StripedAdder() {
120	Adder[] as = new Adder[2];
121	as[0] = new Adder(0); // ensure at least one available adder
122	this.adders = as;
123	this.mutex = new AtomicInteger();
124	}
125
126	/**
127	* Adds the given value.
128	*
129	* @param x the value to add
130	*/
131	public void add(long x) {
132	HashCode hc = threadHashCode.get();
133	for (int h = hc.code;;) {
134	Adder[] as = adders;
135	int n = as.length;
136	Adder a = as[h & (n - 1)];
137	if (a != null) {
138	long v = a.get();
139	if (a.compareAndSet(v, v + x))
140	break;
141	if (n >= NCPU) { // Collision when table at max
142	h = hc.code = xorShift(h); // change code
143	continue;
144	}
145	}
146	final AtomicInteger mutex = this.mutex;
147	if (mutex.get() != 0)
148	h = xorShift(h); // Try elsewhere
149	else if (mutex.compareAndSet(0, 1)) {
150	boolean created = false;
151	try {
152	Adder[] rs = adders;
153	if (a != null && rs == as) // Resize table
154	rs = adders = Arrays.copyOf(as, as.length << 1);
155	int j = h & (rs.length - 1);
156	if (rs[j] == null) { // Create adder
157	rs[j] = new Adder(x);
158	created = true;
159	}
160	} finally {
161	mutex.set(0);
162	}
163	if (created) {
164	hc.code = h; // Use this adder next time
165	break;
166	}
167	}
168	}
169	}
170
171	/**
172	* Returns an estimate of the current sum. The result is
173	* calculated by summing multiple variables, so may not be
174	* accurate if updates occur concurrently with this method.
175	*
176	* @return the estimated sum
177	*/
178	public long sum() {
179	long sum = 0;
180	Adder[] as = adders;
181	int n = as.length;
182	for (int i = 0; i < n; ++i) {
183	Adder a = as[i];
184	if (a != null)
185	sum += a.get();
186	}
187	return sum;
188	}
189
190	/**
191	* Resets each of the variables to zero. This is effective in
192	* fully resetting the sum only if there are no concurrent
193	* updates.
194	*/
195	public void reset() {
196	Adder[] as = adders;
197	int n = as.length;
198	for (int i = 0; i < n; ++i) {
199	Adder a = as[i];
200	if (a != null)
201	a.set(0L);
202	}
203	}
204
205	/**
206	* Equivalent to {@code add(1)}.
207	*/
208	public void increment() {
209	add(1L);
210	}
211
212	/**
213	* Equivalent to {@code add(-1)}.
214	*/
215	public void decrement() {
216	add(-1L);
217	}
218
219	/**
220	* Equivalent to {@link #sum} followed by {@link #reset}.
221	*
222	* @return the estimated sum
223	*/
224	public long sumAndReset() {
225	long sum = 0;
226	Adder[] as = adders;
227	int n = as.length;
228	for (int i = 0; i < n; ++i) {
229	Adder a = as[i];
230	if (a != null) {
231	sum += a.get();
232	a.set(0L);
233	}
234	}
235	return sum;
236	}
237
238	private void writeObject(java.io.ObjectOutputStream s)
239	throws java.io.IOException {
240	s.defaultWriteObject();
241	s.writeLong(sum());
242	}
243
244	private void readObject(ObjectInputStream s)
245	throws IOException, ClassNotFoundException {
246	s.defaultReadObject();
247	long c = s.readLong();
248	Adder[] as = new Adder[2];
249	as[0] = new Adder(c);
250	this.adders = as;
251	mutex.set(0);
252	}
253
254	}
255
256