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.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;

/**
 * One or more variables that together maintain an initially zero sum.
 * When updates (method {@link #add}) are contended across threads,
 * the set of variables may grow dynamically to reduce contention.
 *
 * <p> This class is usually preferable to {@link AtomicLong} when
 * multiple threads update a common sum that is used for purposes such
 * as collecting statistics, not for fine-grained synchronization
 * control.  Under high update contention, throughput of this class is
 * expected to be significantly higher, at the expense of higher space
 * consumption.  Under low contention, this class imposes very little
 * time and space overhead compared to AtomicLong.  On the other hand,
 * in contexts where it is statically known that only one thread can
 * ever update a sum, time and space overhead is noticeably greater
 * than just updating a local variable.
 *
 * <p> Method {@link #sum} returns the current combined total across
 * the variables maintaining the sum.  This value is <em>NOT</em> an
 * atomic snapshot: Concurrent updates may occur while the sum is
 * being calculated.  However, updates cannot be "lost", so invocation
 * of <code>sum</code> in the absence of concurrent updates always
 * returns an accurate result.  The sum may also be <code>reset</code>
 * to zero, as an alternative to creating a new adder.  However,
 * method {@link #reset} is intrinsically racy, so should only be used
 * when it is known that no threads are concurrently updating the sum.
 *
 * <p><em>jsr166e note: This class is targeted to be placed in
 * java.util.concurrent.atomic<em>
 *
 * @author Doug Lea
 */
public class StripedAdder implements Serializable {
    private static final long serialVersionUID = 7249069246863182397L;

    /*
     * A StripedAdder maintains a lazily-initialized table of
     * atomically updated variables, plus an extra "base" field. The
     * table size is a power of two. Indexing uses masked per-thread
     * hash codes
     *
     * Table entries are of class Cell; a variant of AtomicLong padded
     * to reduce cache contention on most processors. Padding is
     * overkill for most Atomics because they are usually irregularly
     * scattered in memory and thus don't interfere much with each
     * other. But Atomic objects residing in arrays will tend to be
     * placed adjacent to each other, and so will most often share
     * cache lines (with a huge negative performance impact) without
     * this precaution.
     *
     * In part because Cells are relatively large, we avoid creating
     * them until they are needed.  When there is no contention, all
     * updates are made to the base field.  Upon first contention (a
     * failed CAS on base update), the table is initialized to size 2.
     * The table size is doubled upon further contention until
     * reaching the nearest power of two greater than or equal to the
     * number of CPUS.
     *
     * Per-thread hash codes are initialized to random values.
     * Contention and/or table collisions are indicated by failed
     * CASes when performing an add operation (see method
     * retryAdd). Upon a collision, if the table size is less than the
     * capacity, it is doubled in size unless some other thread holds
     * the lock. If a hashed slot is empty, and lock is available, a
     * new Cell is created. Otherwise, if the slot exists, a CAS is
     * tried.  Retries proceed by "double hashing", using a secondary
     * hash (Marsaglia XorShift) to try to find a free slot.
     *
     * The table size is capped because, when there are more threads
     * than CPUs, supposing that each thread were bound to a CPU,
     * there would exist a perfect hash function mapping threads to
     * slots that eliminates collisions. When we reach capacity, we
     * search for this mapping by randomly varying the hash codes of
     * colliding threads.  Because search is random, and collisions
     * only become known via CAS failures, convergence can be slow,
     * and because threads are typically not bound to CPUS forever,
     * may not occur at all. However, despite these limitations,
     * observed contention rates are typically low in these cases.
     *
     * A single spinlock is used for initializing and resizing the
     * table, as well as populating slots with new Cells.  There is no
     * need for a blocking lock: Upon lock contention, threads try
     * other slots (or the base) rather than blocking.  During these
     * retries, there is increased contention and reduced locality,
     * which is still better than alternatives.
     *
     * It is possible for a Cell to become unused when threads that
     * once hashed to it terminate, as well as in the case where
     * doubling the table causes no thread to hash to it under
     * expanded mask.  We do not try to detect or remove such cells,
     * under the assumption that for long-running adders, observed
     * contention levels will recur, so the cells will eventually be
     * needed again; and for short-lived ones, it does not matter.
     *
     */

    private static final int NCPU = Runtime.getRuntime().availableProcessors();

    /**
     * Padded variant of AtomicLong.  The value field is placed
     * between pads, hoping that the JVM doesn't reorder them.
     * Updates are via inlined CAS in methods add and retryAdd.
     */
    static final class Cell {
        volatile long p0, p1, p2, p3, p4, p5, p6;
        volatile long value;
        volatile long q0, q1, q2, q3, q4, q5, q6;
        Cell(long x) { value = x; }
    }

    /**
     * Holder for the thread-local hash code. The code is initially
     * random, but may be set to a different value upon collisions.
     */
    static final class HashCode {
        static final Random rng = new Random();
        int code;
        HashCode() {
            int h = rng.nextInt(); // Avoid zero to allow xorShift rehash
            code = (h == 0) ? 1 : h;
        }
    }

    /**
     * The corresponding ThreadLocal class
     */
    static final class ThreadHashCode extends ThreadLocal<HashCode> {
        public HashCode initialValue() { return new HashCode(); }
    }

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

    /**
     * Table of cells. When non-null, size is a power of 2.
     */
    private transient volatile Cell[] cells;

    /**
     * Base sum, used mainly when there is no contention, but also as
     * a fallback during table initializion races. Updated via CAS.
     */
    private transient volatile long base;

    /**
     * Spinlock (locked via CAS) used when resizing and/or creating Cells.
     */
    private transient volatile int busy;

    /**
     * Creates a new adder with initial sum of zero.
     */
    public StripedAdder() {
    }

    /**
     * Adds the given value.
     *
     * @param x the value to add
     */
    public void add(long x) {
        Cell[] as; long v; HashCode hc; Cell a; int n; boolean contended;
        if ((as = cells) != null ||
            !UNSAFE.compareAndSwapLong(this, baseOffset, v = base, v + x)) {
            int h = (hc = threadHashCode.get()).code;
            if (as != null && (n = as.length) > 0 &&
                (a = as[(n - 1) & h]) != null) {
                if (UNSAFE.compareAndSwapLong(a, valueOffset,
                                              v = a.value, v + x))
                    return;
                contended = true;
            }
            else
                contended = false;
            retryAdd(x, hc, contended);
        }
    }

    /**
     * Handle cases of add involving initialization, resizing,
     * creating new Cells, and/or contention. See above for
     * explanation. This method suffers the usual non-modularity
     * problems of optimistic retry code, relying on rechecked sets of
     * reads.
     *
     * @param x the value to add
     * @param hc the hash code holder
     * @param precontended true if CAS failed before call
     */
    private void retryAdd(long x, HashCode hc, boolean precontended) {
        int h = hc.code;
        boolean collide = false; // true if last slot nonempty
        for (;;) {
            Cell[] as; Cell a; int n;
            if ((as = cells) != null && (n = as.length) > 0) {
                if ((a = as[(n - 1) & h]) == null) {
                    if (busy == 0) {            // Try to attach new Cell
                        Cell r = new Cell(x);   // Optimistically create
                        if (busy == 0 &&
                            UNSAFE.compareAndSwapInt(this, busyOffset, 0, 1)) {
                            boolean created = false;
                            try {               // Recheck under lock
                                Cell[] rs; int m, j;
                                if ((rs = cells) != null &&
                                    (m = rs.length) > 0 &&
                                    rs[j = (m - 1) & h] == null) {
                                    rs[j] = r;
                                    created = true;
                                }
                            } finally {
                                busy = 0;
                            }
                            if (created)
                                break;
                            continue;           // Slot is now non-empty
                        }
                    }
                    collide = false;
                }
                else if (precontended)          // CAS already known to fail
                    precontended = false;       // Continue after rehash
                else {
                    long v = a.value;
                    if (UNSAFE.compareAndSwapLong(a, valueOffset, v, v + x))
                        break;
                    if (!collide)
                        collide = true;
                    else if (n >= NCPU || cells != as)
                        collide = false;        // Can't expand
                    else if (busy == 0 &&
                             UNSAFE.compareAndSwapInt(this, busyOffset, 0, 1)) {
                        collide = false;
                        try {
                            if (cells == as) { // Expand table
                                Cell[] rs = new Cell[n << 1];
                                for (int i = 0; i < n; ++i)
                                    rs[i] = as[i];
                                cells = rs;
                            }
                        } finally {
                            busy = 0;
                        }
                        continue;
                    }
                }
                h ^= h << 13;                   // Rehash
                h ^= h >>> 17;
                h ^= h << 5;
            }
            else if (busy == 0 && cells == as &&
                     UNSAFE.compareAndSwapInt(this, busyOffset, 0, 1)) {
                boolean init = false;
                try {                           // Initialize
                    if (cells == as) {
                        Cell r = new Cell(x);
                        Cell[] rs = new Cell[2];
                        rs[h & 1] = r;
                        cells = rs;
                        init = true;
                    }
                } finally {
                    busy = 0;
                }
                if (init)
                    break;
            }
            else {                              // Lost initialization race
                long b = base;                  // Fall back on using base
                if (UNSAFE.compareAndSwapLong(this, baseOffset, b, b + x))
                    break;
            }
        }
        hc.code = h;                            // Record index for next time
    }

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

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

    /**
     * Returns the current sum.  The result is only guaranteed to be
     * accurate in the absence of concurrent updates. Otherwise, it
     * may fail to reflect one or more updates occuring while
     * calculating the result.
     *
     * @return the sum
     */
    public long sum() {
        Cell[] as = cells;
        long sum = base;
        if (as != null) {
            int n = as.length;
            for (int i = 0; i < n; ++i) {
                Cell a = as[i];
                if (a != null)
                    sum += a.value;
            }
        }
        return sum;
    }

    /**
     * Resets variables maintaining the sum to zero.  This is
     * effective in setting the sum to zero only if there are no
     * concurrent updates.
     */
    public void reset() {
        Cell[] as = cells;
        base = 0L;
        if (as != null) {
            int n = as.length;
            for (int i = 0; i < n; ++i) {
                Cell a = as[i];
                if (a != null)
                    a.value = 0L;
            }
        }
    }

    /**
     * Equivalent in effect to {@link #sum} followed by {@link
     * #reset}. This method may apply for example during quiescent
     * points between multithreaded computations.  If there are
     * updates concurrent with this method, the returned value is
     * <em>not</em> guaranteed to be the final sum occurring before
     * the reset.
     *
     * @return the sum
     */
    public long sumThenReset() {
        Cell[] as = cells;
        long sum = base;
        base = 0L;
        if (as != null) {
            int n = as.length;
            for (int i = 0; i < n; ++i) {
                Cell a = as[i];
                if (a != null) {
                    sum += a.value;
                    a.value = 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();
        busy = 0;
        cells = null;
        base = s.readLong();
    }

    // Unsafe mechanics
    private static final sun.misc.Unsafe UNSAFE;
    private static final long baseOffset;
    private static final long busyOffset;
    private static final long valueOffset;
    static {
        try {
            UNSAFE = getUnsafe();
            Class<?> sk = StripedAdder.class;
            baseOffset = UNSAFE.objectFieldOffset
                (sk.getDeclaredField("base"));
            busyOffset = UNSAFE.objectFieldOffset
                (sk.getDeclaredField("busy"));
            Class<?> ak = Cell.class;
            valueOffset = UNSAFE.objectFieldOffset
                (ak.getDeclaredField("value"));
        } catch (Exception e) {
            throw new Error(e);
        }
    }

    /**
     * Returns a sun.misc.Unsafe.  Suitable for use in a 3rd party package.
     * Replace with a simple call to Unsafe.getUnsafe when integrating
     * into a jdk.
     *
     * @return a sun.misc.Unsafe
     */
    private static sun.misc.Unsafe getUnsafe() {
        try {
            return sun.misc.Unsafe.getUnsafe();
        } catch (SecurityException se) {
            try {
                return java.security.AccessController.doPrivileged
                    (new java.security
                     .PrivilegedExceptionAction<sun.misc.Unsafe>() {
                        public sun.misc.Unsafe run() throws Exception {
                            java.lang.reflect.Field f = sun.misc
                                .Unsafe.class.getDeclaredField("theUnsafe");
                            f.setAccessible(true);
                            return (sun.misc.Unsafe) f.get(null);
                        }});
            } catch (java.security.PrivilegedActionException e) {
                throw new RuntimeException("Could not initialize intrinsics",
                                           e.getCause());
            }
        }
    }

}
Revision:	1.13
Committed:	Sun Jul 31 14:20:01 2011 UTC (12 years, 9 months ago) by dl
Branch:	MAIN
CVS Tags:	HEAD
Changes since 1.12:	+0 -0 lines
State:	*FILE REMOVED*
Log Message:	Rename classes
#	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.Random;
9	import java.util.concurrent.atomic.AtomicInteger;
10	import java.util.concurrent.atomic.AtomicLong;
11	import java.io.IOException;
12	import java.io.Serializable;
13	import java.io.ObjectInputStream;
14	import java.io.ObjectOutputStream;
15
16	/**
17	* One or more variables that together maintain an initially zero sum.
18	* When updates (method {@link #add}) are contended across threads,
19	* the set of variables may grow dynamically to reduce contention.
20	*
21	* <p> This class is usually preferable to {@link AtomicLong} when
22	* multiple threads update a common sum that is used for purposes such
23	* as collecting statistics, not for fine-grained synchronization
24	* control. Under high update contention, throughput of this class is
25	* expected to be significantly higher, at the expense of higher space
26	* consumption. Under low contention, this class imposes very little
27	* time and space overhead compared to AtomicLong. On the other hand,
28	* in contexts where it is statically known that only one thread can
29	* ever update a sum, time and space overhead is noticeably greater
30	* than just updating a local variable.
31	*
32	* <p> Method {@link #sum} returns the current combined total across
33	* the variables maintaining the sum. This value is <em>NOT</em> an
34	* atomic snapshot: Concurrent updates may occur while the sum is
35	* being calculated. However, updates cannot be "lost", so invocation
36	* of <code>sum</code> in the absence of concurrent updates always
37	* returns an accurate result. The sum may also be <code>reset</code>
38	* to zero, as an alternative to creating a new adder. However,
39	* method {@link #reset} is intrinsically racy, so should only be used
40	* when it is known that no threads are concurrently updating the sum.
41	*
42	* <p><em>jsr166e note: This class is targeted to be placed in
43	* java.util.concurrent.atomic<em>
44	*
45	* @author Doug Lea
46	*/
47	public class StripedAdder implements Serializable {
48	private static final long serialVersionUID = 7249069246863182397L;
49
50	/*
51	* A StripedAdder maintains a lazily-initialized table of
52	* atomically updated variables, plus an extra "base" field. The
53	* table size is a power of two. Indexing uses masked per-thread
54	* hash codes
55	*
56	* Table entries are of class Cell; a variant of AtomicLong padded
57	* to reduce cache contention on most processors. Padding is
58	* overkill for most Atomics because they are usually irregularly
59	* scattered in memory and thus don't interfere much with each
60	* other. But Atomic objects residing in arrays will tend to be
61	* placed adjacent to each other, and so will most often share
62	* cache lines (with a huge negative performance impact) without
63	* this precaution.
64	*
65	* In part because Cells are relatively large, we avoid creating
66	* them until they are needed. When there is no contention, all
67	* updates are made to the base field. Upon first contention (a
68	* failed CAS on base update), the table is initialized to size 2.
69	* The table size is doubled upon further contention until
70	* reaching the nearest power of two greater than or equal to the
71	* number of CPUS.
72	*
73	* Per-thread hash codes are initialized to random values.
74	* Contention and/or table collisions are indicated by failed
75	* CASes when performing an add operation (see method
76	* retryAdd). Upon a collision, if the table size is less than the
77	* capacity, it is doubled in size unless some other thread holds
78	* the lock. If a hashed slot is empty, and lock is available, a
79	* new Cell is created. Otherwise, if the slot exists, a CAS is
80	* tried. Retries proceed by "double hashing", using a secondary
81	* hash (Marsaglia XorShift) to try to find a free slot.
82	*
83	* The table size is capped because, when there are more threads
84	* than CPUs, supposing that each thread were bound to a CPU,
85	* there would exist a perfect hash function mapping threads to
86	* slots that eliminates collisions. When we reach capacity, we
87	* search for this mapping by randomly varying the hash codes of
88	* colliding threads. Because search is random, and collisions
89	* only become known via CAS failures, convergence can be slow,
90	* and because threads are typically not bound to CPUS forever,
91	* may not occur at all. However, despite these limitations,
92	* observed contention rates are typically low in these cases.
93	*
94	* A single spinlock is used for initializing and resizing the
95	* table, as well as populating slots with new Cells. There is no
96	* need for a blocking lock: Upon lock contention, threads try
97	* other slots (or the base) rather than blocking. During these
98	* retries, there is increased contention and reduced locality,
99	* which is still better than alternatives.
100	*
101	* It is possible for a Cell to become unused when threads that
102	* once hashed to it terminate, as well as in the case where
103	* doubling the table causes no thread to hash to it under
104	* expanded mask. We do not try to detect or remove such cells,
105	* under the assumption that for long-running adders, observed
106	* contention levels will recur, so the cells will eventually be
107	* needed again; and for short-lived ones, it does not matter.
108	*
109	*/
110
111	private static final int NCPU = Runtime.getRuntime().availableProcessors();
112
113	/**
114	* Padded variant of AtomicLong. The value field is placed
115	* between pads, hoping that the JVM doesn't reorder them.
116	* Updates are via inlined CAS in methods add and retryAdd.
117	*/
118	static final class Cell {
119	volatile long p0, p1, p2, p3, p4, p5, p6;
120	volatile long value;
121	volatile long q0, q1, q2, q3, q4, q5, q6;
122	Cell(long x) { value = x; }
123	}
124
125	/**
126	* Holder for the thread-local hash code. The code is initially
127	* random, but may be set to a different value upon collisions.
128	*/
129	static final class HashCode {
130	static final Random rng = new Random();
131	int code;
132	HashCode() {
133	int h = rng.nextInt(); // Avoid zero to allow xorShift rehash
134	code = (h == 0) ? 1 : h;
135	}
136	}
137
138	/**
139	* The corresponding ThreadLocal class
140	*/
141	static final class ThreadHashCode extends ThreadLocal<HashCode> {
142	public HashCode initialValue() { return new HashCode(); }
143	}
144
145	/**
146	* Static per-thread hash codes. Shared across all StripedAdders
147	* to reduce ThreadLocal pollution and because adjustments due to
148	* collisions in one table are likely to be appropriate for
149	* others.
150	*/
151	static final ThreadHashCode threadHashCode = new ThreadHashCode();
152
153	/**
154	* Table of cells. When non-null, size is a power of 2.
155	*/
156	private transient volatile Cell[] cells;
157
158	/**
159	* Base sum, used mainly when there is no contention, but also as
160	* a fallback during table initializion races. Updated via CAS.
161	*/
162	private transient volatile long base;
163
164	/**
165	* Spinlock (locked via CAS) used when resizing and/or creating Cells.
166	*/
167	private transient volatile int busy;
168
169	/**
170	* Creates a new adder with initial sum of zero.
171	*/
172	public StripedAdder() {
173	}
174
175	/**
176	* Adds the given value.
177	*
178	* @param x the value to add
179	*/
180	public void add(long x) {
181	Cell[] as; long v; HashCode hc; Cell a; int n; boolean contended;
182	if ((as = cells) != null \|\|
183	!UNSAFE.compareAndSwapLong(this, baseOffset, v = base, v + x)) {
184	int h = (hc = threadHashCode.get()).code;
185	if (as != null && (n = as.length) > 0 &&
186	(a = as[(n - 1) & h]) != null) {
187	if (UNSAFE.compareAndSwapLong(a, valueOffset,
188	v = a.value, v + x))
189	return;
190	contended = true;
191	}
192	else
193	contended = false;
194	retryAdd(x, hc, contended);
195	}
196	}
197
198	/**
199	* Handle cases of add involving initialization, resizing,
200	* creating new Cells, and/or contention. See above for
201	* explanation. This method suffers the usual non-modularity
202	* problems of optimistic retry code, relying on rechecked sets of
203	* reads.
204	*
205	* @param x the value to add
206	* @param hc the hash code holder
207	* @param precontended true if CAS failed before call
208	*/
209	private void retryAdd(long x, HashCode hc, boolean precontended) {
210	int h = hc.code;
211	boolean collide = false; // true if last slot nonempty
212	for (;;) {
213	Cell[] as; Cell a; int n;
214	if ((as = cells) != null && (n = as.length) > 0) {
215	if ((a = as[(n - 1) & h]) == null) {
216	if (busy == 0) { // Try to attach new Cell
217	Cell r = new Cell(x); // Optimistically create
218	if (busy == 0 &&
219	UNSAFE.compareAndSwapInt(this, busyOffset, 0, 1)) {
220	boolean created = false;
221	try { // Recheck under lock
222	Cell[] rs; int m, j;
223	if ((rs = cells) != null &&
224	(m = rs.length) > 0 &&
225	rs[j = (m - 1) & h] == null) {
226	rs[j] = r;
227	created = true;
228	}
229	} finally {
230	busy = 0;
231	}
232	if (created)
233	break;
234	continue; // Slot is now non-empty
235	}
236	}
237	collide = false;
238	}
239	else if (precontended) // CAS already known to fail
240	precontended = false; // Continue after rehash
241	else {
242	long v = a.value;
243	if (UNSAFE.compareAndSwapLong(a, valueOffset, v, v + x))
244	break;
245	if (!collide)
246	collide = true;
247	else if (n >= NCPU \|\| cells != as)
248	collide = false; // Can't expand
249	else if (busy == 0 &&
250	UNSAFE.compareAndSwapInt(this, busyOffset, 0, 1)) {
251	collide = false;
252	try {
253	if (cells == as) { // Expand table
254	Cell[] rs = new Cell[n << 1];
255	for (int i = 0; i < n; ++i)
256	rs[i] = as[i];
257	cells = rs;
258	}
259	} finally {
260	busy = 0;
261	}
262	continue;
263	}
264	}
265	h ^= h << 13; // Rehash
266	h ^= h >>> 17;
267	h ^= h << 5;
268	}
269	else if (busy == 0 && cells == as &&
270	UNSAFE.compareAndSwapInt(this, busyOffset, 0, 1)) {
271	boolean init = false;
272	try { // Initialize
273	if (cells == as) {
274	Cell r = new Cell(x);
275	Cell[] rs = new Cell[2];
276	rs[h & 1] = r;
277	cells = rs;
278	init = true;
279	}
280	} finally {
281	busy = 0;
282	}
283	if (init)
284	break;
285	}
286	else { // Lost initialization race
287	long b = base; // Fall back on using base
288	if (UNSAFE.compareAndSwapLong(this, baseOffset, b, b + x))
289	break;
290	}
291	}
292	hc.code = h; // Record index for next time
293	}
294
295	/**
296	* Equivalent to {@code add(1)}.
297	*/
298	public void increment() {
299	add(1L);
300	}
301
302	/**
303	* Equivalent to {@code add(-1)}.
304	*/
305	public void decrement() {
306	add(-1L);
307	}
308
309	/**
310	* Returns the current sum. The result is only guaranteed to be
311	* accurate in the absence of concurrent updates. Otherwise, it
312	* may fail to reflect one or more updates occuring while
313	* calculating the result.
314	*
315	* @return the sum
316	*/
317	public long sum() {
318	Cell[] as = cells;
319	long sum = base;
320	if (as != null) {
321	int n = as.length;
322	for (int i = 0; i < n; ++i) {
323	Cell a = as[i];
324	if (a != null)
325	sum += a.value;
326	}
327	}
328	return sum;
329	}
330
331	/**
332	* Resets variables maintaining the sum to zero. This is
333	* effective in setting the sum to zero only if there are no
334	* concurrent updates.
335	*/
336	public void reset() {
337	Cell[] as = cells;
338	base = 0L;
339	if (as != null) {
340	int n = as.length;
341	for (int i = 0; i < n; ++i) {
342	Cell a = as[i];
343	if (a != null)
344	a.value = 0L;
345	}
346	}
347	}
348
349	/**
350	* Equivalent in effect to {@link #sum} followed by {@link
351	* #reset}. This method may apply for example during quiescent
352	* points between multithreaded computations. If there are
353	* updates concurrent with this method, the returned value is
354	* <em>not</em> guaranteed to be the final sum occurring before
355	* the reset.
356	*
357	* @return the sum
358	*/
359	public long sumThenReset() {
360	Cell[] as = cells;
361	long sum = base;
362	base = 0L;
363	if (as != null) {
364	int n = as.length;
365	for (int i = 0; i < n; ++i) {
366	Cell a = as[i];
367	if (a != null) {
368	sum += a.value;
369	a.value = 0L;
370	}
371	}
372	}
373	return sum;
374	}
375
376	private void writeObject(java.io.ObjectOutputStream s)
377	throws java.io.IOException {
378	s.defaultWriteObject();
379	s.writeLong(sum());
380	}
381
382	private void readObject(ObjectInputStream s)
383	throws IOException, ClassNotFoundException {
384	s.defaultReadObject();
385	busy = 0;
386	cells = null;
387	base = s.readLong();
388	}
389
390	// Unsafe mechanics
391	private static final sun.misc.Unsafe UNSAFE;
392	private static final long baseOffset;
393	private static final long busyOffset;
394	private static final long valueOffset;
395	static {
396	try {
397	UNSAFE = getUnsafe();
398	Class<?> sk = StripedAdder.class;
399	baseOffset = UNSAFE.objectFieldOffset
400	(sk.getDeclaredField("base"));
401	busyOffset = UNSAFE.objectFieldOffset
402	(sk.getDeclaredField("busy"));
403	Class<?> ak = Cell.class;
404	valueOffset = UNSAFE.objectFieldOffset
405	(ak.getDeclaredField("value"));
406	} catch (Exception e) {
407	throw new Error(e);
408	}
409	}
410
411	/**
412	* Returns a sun.misc.Unsafe. Suitable for use in a 3rd party package.
413	* Replace with a simple call to Unsafe.getUnsafe when integrating
414	* into a jdk.
415	*
416	* @return a sun.misc.Unsafe
417	*/
418	private static sun.misc.Unsafe getUnsafe() {
419	try {
420	return sun.misc.Unsafe.getUnsafe();
421	} catch (SecurityException se) {
422	try {
423	return java.security.AccessController.doPrivileged
424	(new java.security
425	.PrivilegedExceptionAction<sun.misc.Unsafe>() {
426	public sun.misc.Unsafe run() throws Exception {
427	java.lang.reflect.Field f = sun.misc
428	.Unsafe.class.getDeclaredField("theUnsafe");
429	f.setAccessible(true);
430	return (sun.misc.Unsafe) f.get(null);
431	}});
432	} catch (java.security.PrivilegedActionException e) {
433	throw new RuntimeException("Could not initialize intrinsics",
434	e.getCause());
435	}
436	}
437	}
438
439	}