src/jsr166e/LongAdder.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 low update contention, the two classes have similar
 * characteristics. But under high contention, expected throughput of
 * this class is significantly higher, at the expense of higher space
 * consumption.
 *
 * <p> Method {@link #sum} returns the current combined total across
 * the variables maintaining the sum.  This value is <em>NOT</em> an
 * atomic snapshot: Invocation of {@code sum} in the absence of
 * concurrent updates returns an accurate result, but concurrent
 * updates that occur while the sum is being calculated might not be
 * incorporated.  The sum may also be {@code reset} 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 LongAdder implements Serializable {
    private static final long serialVersionUID = 7249069246863182397L;

    /*
     * A LongAdder 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.
     *
     * JVM intrinsics note: It would be possible to use a release-only
     * form of CAS here, if it were provided.
     */

    /**
     * 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 LongAdders
     * 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();

    /** Number of CPUS, to place bound on table size */
    private static final int NCPU = Runtime.getRuntime().availableProcessors();

    /**
     * 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 initialization 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 LongAdder() {
    }

    /**
     * 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;
        if ((as = cells) != null ||
            !UNSAFE.compareAndSwapLong(this, baseOffset, v = base, v + x)) {
            boolean uncontended = true;
            int h = (hc = threadHashCode.get()).code;
            if (as == null || (n = as.length) < 1 ||
                (a = as[(n - 1) & h]) == null ||
                !(uncontended = UNSAFE.compareAndSwapLong(a, valueOffset,
                                                          v = a.value, v + x)))
                retryAdd(x, hc, uncontended);
        }
    }

    /**
     * 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 wasUncontended false if CAS failed before call
     */
    private void retryAdd(long x, HashCode hc, boolean wasUncontended) {
        int h = hc.code;
        boolean collide = false;                // True if last slot nonempty
        for (;;) {
            Cell[] as; Cell a; int n; long v;
            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 (!wasUncontended)       // CAS already known to fail
                    wasUncontended = true;      // Continue after rehash
                else if (UNSAFE.compareAndSwapLong(a, valueOffset,
                                                   v = a.value, v + x))
                    break;
                else if (n >= NCPU || cells != as)
                    collide = false;            // At max size or stale
                else if (!collide)
                    collide = true;
                else if (busy == 0 &&           // Try to expand table
                         UNSAFE.compareAndSwapInt(this, busyOffset, 0, 1)) {
                    try {
                        if (cells == as) {
                            Cell[] rs = new Cell[n << 1];
                            for (int i = 0; i < n; ++i)
                                rs[i] = as[i];
                            cells = rs;
                        }
                    } finally {
                        busy = 0;
                    }
                    collide = false;
                    continue;                   // Retry with expanded table
                }
                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 table
                    if (cells == as) {
                        Cell[] rs = new Cell[2];
                        rs[h & 1] = new Cell(x);
                        cells = rs;
                        init = true;
                    }
                } finally {
                    busy = 0;
                }
                if (init)
                    break;
            }
            else if (UNSAFE.compareAndSwapLong(this, baseOffset,
                                               v = base, v + x))
                break;                          // Fall back on using base
        }
        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 = LongAdder.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:	1.4
Committed:	Mon Aug 1 18:54:15 2011 UTC (12 years, 9 months ago) by jsr166
Branch:	MAIN
Changes since 1.3:	+2 -2 lines
Log Message:	<code> => @code
#	User	Rev	Content
1	dl	1.1	/*
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 low update contention, the two classes have similar
25			* characteristics. But under high contention, expected throughput of
26			* this class is significantly higher, at the expense of higher space
27			* consumption.
28			*
29			* <p> Method {@link #sum} returns the current combined total across
30			* the variables maintaining the sum. This value is <em>NOT</em> an
31	jsr166	1.4	* atomic snapshot: Invocation of {@code sum} in the absence of
32	dl	1.1	* concurrent updates returns an accurate result, but concurrent
33			* updates that occur while the sum is being calculated might not be
34	jsr166	1.4	* incorporated. The sum may also be {@code reset} to zero, as
35	dl	1.1	* an alternative to creating a new adder. However, method {@link
36			* #reset} is intrinsically racy, so should only be used when it is
37			* known that no threads are concurrently updating the sum.
38			*
39			* <p><em>jsr166e note: This class is targeted to be placed in
40			* java.util.concurrent.atomic<em>
41			*
42			* @author Doug Lea
43			*/
44			public class LongAdder implements Serializable {
45			private static final long serialVersionUID = 7249069246863182397L;
46
47			/*
48			* A LongAdder maintains a lazily-initialized table of atomically
49			* updated variables, plus an extra "base" field. The table size
50			* is a power of two. Indexing uses masked per-thread hash codes
51			*
52			* Table entries are of class Cell; a variant of AtomicLong padded
53			* to reduce cache contention on most processors. Padding is
54			* overkill for most Atomics because they are usually irregularly
55			* scattered in memory and thus don't interfere much with each
56			* other. But Atomic objects residing in arrays will tend to be
57			* placed adjacent to each other, and so will most often share
58			* cache lines (with a huge negative performance impact) without
59			* this precaution.
60			*
61			* In part because Cells are relatively large, we avoid creating
62			* them until they are needed. When there is no contention, all
63			* updates are made to the base field. Upon first contention (a
64			* failed CAS on base update), the table is initialized to size 2.
65			* The table size is doubled upon further contention until
66			* reaching the nearest power of two greater than or equal to the
67			* number of CPUS.
68			*
69			* Per-thread hash codes are initialized to random values.
70			* Contention and/or table collisions are indicated by failed
71			* CASes when performing an add operation (see method
72			* retryAdd). Upon a collision, if the table size is less than the
73			* capacity, it is doubled in size unless some other thread holds
74			* the lock. If a hashed slot is empty, and lock is available, a
75			* new Cell is created. Otherwise, if the slot exists, a CAS is
76			* tried. Retries proceed by "double hashing", using a secondary
77			* hash (Marsaglia XorShift) to try to find a free slot.
78			*
79			* The table size is capped because, when there are more threads
80			* than CPUs, supposing that each thread were bound to a CPU,
81			* there would exist a perfect hash function mapping threads to
82			* slots that eliminates collisions. When we reach capacity, we
83			* search for this mapping by randomly varying the hash codes of
84			* colliding threads. Because search is random, and collisions
85			* only become known via CAS failures, convergence can be slow,
86			* and because threads are typically not bound to CPUS forever,
87			* may not occur at all. However, despite these limitations,
88			* observed contention rates are typically low in these cases.
89			*
90			* A single spinlock is used for initializing and resizing the
91			* table, as well as populating slots with new Cells. There is no
92			* need for a blocking lock: Upon lock contention, threads try
93			* other slots (or the base) rather than blocking. During these
94			* retries, there is increased contention and reduced locality,
95			* which is still better than alternatives.
96			*
97			* It is possible for a Cell to become unused when threads that
98			* once hashed to it terminate, as well as in the case where
99			* doubling the table causes no thread to hash to it under
100			* expanded mask. We do not try to detect or remove such cells,
101			* under the assumption that for long-running adders, observed
102			* contention levels will recur, so the cells will eventually be
103			* needed again; and for short-lived ones, it does not matter.
104			*
105			* JVM intrinsics note: It would be possible to use a release-only
106			* form of CAS here, if it were provided.
107			*/
108
109			/**
110			* Padded variant of AtomicLong. The value field is placed
111			* between pads, hoping that the JVM doesn't reorder them.
112			* Updates are via inlined CAS in methods add and retryAdd.
113			*/
114			static final class Cell {
115			volatile long p0, p1, p2, p3, p4, p5, p6;
116			volatile long value;
117			volatile long q0, q1, q2, q3, q4, q5, q6;
118			Cell(long x) { value = x; }
119			}
120
121			/**
122			* Holder for the thread-local hash code. The code is initially
123			* random, but may be set to a different value upon collisions.
124			*/
125			static final class HashCode {
126			static final Random rng = new Random();
127			int code;
128			HashCode() {
129			int h = rng.nextInt(); // Avoid zero to allow xorShift rehash
130			code = (h == 0) ? 1 : h;
131			}
132			}
133	jsr166	1.2
134	dl	1.1	/**
135			* The corresponding ThreadLocal class
136			*/
137			static final class ThreadHashCode extends ThreadLocal<HashCode> {
138			public HashCode initialValue() { return new HashCode(); }
139			}
140
141			/**
142			* Static per-thread hash codes. Shared across all LongAdders
143			* to reduce ThreadLocal pollution and because adjustments due to
144			* collisions in one table are likely to be appropriate for
145			* others.
146			*/
147			static final ThreadHashCode threadHashCode = new ThreadHashCode();
148
149	jsr166	1.3	/** Number of CPUS, to place bound on table size */
150	dl	1.1	private static final int NCPU = Runtime.getRuntime().availableProcessors();
151
152			/**
153			* Table of cells. When non-null, size is a power of 2.
154			*/
155			private transient volatile Cell[] cells;
156
157			/**
158			* Base sum, used mainly when there is no contention, but also as
159	jsr166	1.3	* a fallback during table initialization races. Updated via CAS.
160	dl	1.1	*/
161			private transient volatile long base;
162
163			/**
164			* Spinlock (locked via CAS) used when resizing and/or creating Cells.
165			*/
166			private transient volatile int busy;
167
168			/**
169			* Creates a new adder with initial sum of zero.
170			*/
171			public LongAdder() {
172			}
173
174			/**
175			* Adds the given value.
176			*
177			* @param x the value to add
178			*/
179			public void add(long x) {
180			Cell[] as; long v; HashCode hc; Cell a; int n;
181			if ((as = cells) != null \|\|
182			!UNSAFE.compareAndSwapLong(this, baseOffset, v = base, v + x)) {
183			boolean uncontended = true;
184			int h = (hc = threadHashCode.get()).code;
185			if (as == null \|\| (n = as.length) < 1 \|\|
186			(a = as[(n - 1) & h]) == null \|\|
187			!(uncontended = UNSAFE.compareAndSwapLong(a, valueOffset,
188			v = a.value, v + x)))
189			retryAdd(x, hc, uncontended);
190			}
191			}
192
193			/**
194			* Handle cases of add involving initialization, resizing,
195			* creating new Cells, and/or contention. See above for
196			* explanation. This method suffers the usual non-modularity
197			* problems of optimistic retry code, relying on rechecked sets of
198			* reads.
199			*
200			* @param x the value to add
201			* @param hc the hash code holder
202			* @param wasUncontended false if CAS failed before call
203			*/
204			private void retryAdd(long x, HashCode hc, boolean wasUncontended) {
205			int h = hc.code;
206			boolean collide = false; // True if last slot nonempty
207			for (;;) {
208			Cell[] as; Cell a; int n; long v;
209			if ((as = cells) != null && (n = as.length) > 0) {
210			if ((a = as[(n - 1) & h]) == null) {
211			if (busy == 0) { // Try to attach new Cell
212			Cell r = new Cell(x); // Optimistically create
213			if (busy == 0 &&
214			UNSAFE.compareAndSwapInt(this, busyOffset, 0, 1)) {
215			boolean created = false;
216			try { // Recheck under lock
217			Cell[] rs; int m, j;
218			if ((rs = cells) != null &&
219			(m = rs.length) > 0 &&
220			rs[j = (m - 1) & h] == null) {
221			rs[j] = r;
222			created = true;
223			}
224			} finally {
225			busy = 0;
226			}
227			if (created)
228			break;
229			continue; // Slot is now non-empty
230			}
231			}
232			collide = false;
233			}
234			else if (!wasUncontended) // CAS already known to fail
235			wasUncontended = true; // Continue after rehash
236	jsr166	1.2	else if (UNSAFE.compareAndSwapLong(a, valueOffset,
237	dl	1.1	v = a.value, v + x))
238			break;
239			else if (n >= NCPU \|\| cells != as)
240			collide = false; // At max size or stale
241			else if (!collide)
242			collide = true;
243			else if (busy == 0 && // Try to expand table
244			UNSAFE.compareAndSwapInt(this, busyOffset, 0, 1)) {
245			try {
246			if (cells == as) {
247			Cell[] rs = new Cell[n << 1];
248			for (int i = 0; i < n; ++i)
249			rs[i] = as[i];
250			cells = rs;
251			}
252			} finally {
253			busy = 0;
254			}
255			collide = false;
256			continue; // Retry with expanded table
257			}
258			h ^= h << 13; // Rehash
259			h ^= h >>> 17;
260			h ^= h << 5;
261			}
262			else if (busy == 0 && cells == as &&
263			UNSAFE.compareAndSwapInt(this, busyOffset, 0, 1)) {
264			boolean init = false;
265			try { // Initialize table
266			if (cells == as) {
267			Cell[] rs = new Cell[2];
268			rs[h & 1] = new Cell(x);
269			cells = rs;
270			init = true;
271			}
272			} finally {
273			busy = 0;
274			}
275			if (init)
276			break;
277			}
278	jsr166	1.2	else if (UNSAFE.compareAndSwapLong(this, baseOffset,
279	dl	1.1	v = base, v + x))
280			break; // Fall back on using base
281			}
282			hc.code = h; // Record index for next time
283			}
284
285			/**
286			* Equivalent to {@code add(1)}.
287			*/
288			public void increment() {
289			add(1L);
290			}
291
292			/**
293			* Equivalent to {@code add(-1)}.
294			*/
295			public void decrement() {
296			add(-1L);
297			}
298
299			/**
300			* Returns the current sum. The result is only guaranteed to be
301			* accurate in the absence of concurrent updates. Otherwise, it
302			* may fail to reflect one or more updates occuring while
303			* calculating the result.
304			*
305			* @return the sum
306			*/
307			public long sum() {
308			Cell[] as = cells;
309			long sum = base;
310			if (as != null) {
311			int n = as.length;
312			for (int i = 0; i < n; ++i) {
313			Cell a = as[i];
314			if (a != null)
315			sum += a.value;
316			}
317			}
318			return sum;
319			}
320
321			/**
322			* Resets variables maintaining the sum to zero. This is
323			* effective in setting the sum to zero only if there are no
324			* concurrent updates.
325			*/
326			public void reset() {
327			Cell[] as = cells;
328			base = 0L;
329			if (as != null) {
330			int n = as.length;
331			for (int i = 0; i < n; ++i) {
332			Cell a = as[i];
333			if (a != null)
334			a.value = 0L;
335			}
336			}
337			}
338
339			/**
340			* Equivalent in effect to {@link #sum} followed by {@link
341			* #reset}. This method may apply for example during quiescent
342			* points between multithreaded computations. If there are
343			* updates concurrent with this method, the returned value is
344			* <em>not</em> guaranteed to be the final sum occurring before
345			* the reset.
346			*
347			* @return the sum
348			*/
349			public long sumThenReset() {
350			Cell[] as = cells;
351			long sum = base;
352			base = 0L;
353			if (as != null) {
354			int n = as.length;
355			for (int i = 0; i < n; ++i) {
356			Cell a = as[i];
357			if (a != null) {
358			sum += a.value;
359			a.value = 0L;
360			}
361			}
362			}
363			return sum;
364			}
365
366			private void writeObject(java.io.ObjectOutputStream s)
367			throws java.io.IOException {
368			s.defaultWriteObject();
369			s.writeLong(sum());
370			}
371
372			private void readObject(ObjectInputStream s)
373			throws IOException, ClassNotFoundException {
374			s.defaultReadObject();
375			busy = 0;
376			cells = null;
377			base = s.readLong();
378			}
379
380			// Unsafe mechanics
381			private static final sun.misc.Unsafe UNSAFE;
382			private static final long baseOffset;
383			private static final long busyOffset;
384			private static final long valueOffset;
385			static {
386			try {
387			UNSAFE = getUnsafe();
388			Class<?> sk = LongAdder.class;
389			baseOffset = UNSAFE.objectFieldOffset
390			(sk.getDeclaredField("base"));
391			busyOffset = UNSAFE.objectFieldOffset
392			(sk.getDeclaredField("busy"));
393			Class<?> ak = Cell.class;
394			valueOffset = UNSAFE.objectFieldOffset
395			(ak.getDeclaredField("value"));
396			} catch (Exception e) {
397			throw new Error(e);
398			}
399			}
400
401			/**
402			* Returns a sun.misc.Unsafe. Suitable for use in a 3rd party package.
403			* Replace with a simple call to Unsafe.getUnsafe when integrating
404			* into a jdk.
405			*
406			* @return a sun.misc.Unsafe
407			*/
408			private static sun.misc.Unsafe getUnsafe() {
409			try {
410			return sun.misc.Unsafe.getUnsafe();
411			} catch (SecurityException se) {
412			try {
413			return java.security.AccessController.doPrivileged
414			(new java.security
415			.PrivilegedExceptionAction<sun.misc.Unsafe>() {
416			public sun.misc.Unsafe run() throws Exception {
417			java.lang.reflect.Field f = sun.misc
418			.Unsafe.class.getDeclaredField("theUnsafe");
419			f.setAccessible(true);
420			return (sun.misc.Unsafe) f.get(null);
421			}});
422			} catch (java.security.PrivilegedActionException e) {
423			throw new RuntimeException("Could not initialize intrinsics",
424			e.getCause());
425			}
426			}
427			}
428
429			}