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;

/**
 * A set of variables that together maintain a sum.  When updates
 * (method {@link #add}) are contended across threads, this set of
 * adder variables may grow dynamically 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
 * 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.
 *
 * <p>A StripedAdder may optionally be constructed with a given
 * expected contention level; i.e., the number of threads that are
 * expected to concurrently update the sum. Supplying an accurate
 * value may improve performance by reducing the need for dynamic
 * adjustment.
 *
 * @author Doug Lea
 */
public class StripedAdder implements Serializable {
    private static final long serialVersionUID = 7249069246863182397L;

    /*
     * A StripedAdder maintains a table of Atomic long variables. The
     * table is indexed by per-thread hash codes.
     *
     * Table entries are of class Adder; 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.
     *
     * Because Adders are relatively large, we avoid creating them
     * until they are needed. On the other hand, we try to create them
     * on any sign of contention.
     *
     * Per-thread hash codes are initialized to random values.
     * 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 lock. If a hashed slot is empty, and
     * lock is available, a new Adder 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.
     *
     * By default, the table is lazily initialized.  Upon first use,
     * the table is set to size 1, and contains a single Adder. The
     * maximum table size is bounded by nearest power of two >= the
     * number of CPUS.  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 failures only become known via CAS failures, convergence
     * will be slow, and because threads are typically not bound to
     * CPUS forever, may not occur at all. However, despite these
     * limitations, observed contention is typically low in these
     * cases.
     *
     * A single spinlock is used for resizing the table as well as
     * populating slots with new Adders. After initialization, there
     * is no need for a blocking lock: Upon lock contention, threads
     * try other slots rather than blocking. After initialization, at
     * least one slot exists, so retries will eventually find a
     * candidate Adder.  During these retries, there is increased
     * contention and reduced locality, which is still better than
     * alternatives.
     */

    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 Adder {
        volatile long p0, p1, p2, p3, p4, p5, p6;
        volatile long value;
        volatile long q0, q1, q2, q3, q4, q5, q6;
        Adder(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, because of 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 adders. When non-null, size is a power of 2.
     */
    private transient volatile Adder[] adders;

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

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

    /**
     * Creates a new adder with zero sum, and with stripes presized
     * for the given expected contention level.
     *
     * @param expectedContention the expected number of threads that
     * will concurrently update the sum.
     */
    public StripedAdder(int expectedContention) {
        int cap = (expectedContention < NCPU) ? expectedContention : NCPU;
        int size = 1;
        while (size < cap)
            size <<= 1;
        Adder[] as = new Adder[size];
        for (int i = 0; i < size; ++i)
            as[i] = new Adder(0);
        this.adders = as;
    }

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

    /**
     * Handle cases of add involving initialization, resizing,
     * creating new Adders, 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 (;;) {
            Adder[] as; Adder a; int n;
            if ((as = adders) != null && (n = as.length) > 0) {
                if ((a = as[(n - 1) & h]) == null) {
                    if (busy == 0 &&            // Try to attach new Adder
                        UNSAFE.compareAndSwapInt(this, busyOffset, 0, 1)) {
                        boolean created = false;
                        try {                   // Recheck under lock
                            Adder[] rs; int m, j;
                            if ((rs = adders) != null && (m = rs.length) > 0 &&
                                rs[j = (m - 1) & h] == null) {
                                rs[j] = new Adder(x);
                                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 || adders != as)
                        collide = false;        // Don't expand
                    else if (busy == 0 &&
                             UNSAFE.compareAndSwapInt(this, busyOffset, 0, 1)) {
                        collide = false;
                        try {
                            if (adders == as) { // Expand table
                                Adder[] rs = new Adder[n << 1];
                                for (int i = 0; i < n; ++i)
                                    rs[i] = as[i];
                                adders = rs;
                            }
                        } finally {
                            busy = 0;
                        }
                        continue;
                    }
                }
                h ^= h << 13;                   // Rehash
                h ^= h >>> 17;
                h ^= h << 5;
            }
            else if (adders == as) {            // Try to default-initialize
                Adder[] rs = new Adder[1];
                rs[0] = new Adder(x);
                boolean init = false;
                while (adders == as) {
                    if (UNSAFE.compareAndSwapInt(this, busyOffset, 0, 1)) {
                        try {
                            if (adders == as) {
                                adders = rs;
                                init = true;
                            }
                        } finally {
                            busy = 0;
                        }
                        break;
                    }
                    if (adders != as)
                        break;
                    Thread.yield();              // Back off
                }
                if (init)
                    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 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 = 0L;
        Adder[] as = adders;
        if (as != null) {
            int n = as.length;
            for (int i = 0; i < n; ++i) {
                Adder a = as[i];
                if (a != null)
                    sum += a.value;
            }
        }
        return sum;
    }

    /**
     * Resets each of the variables to zero, returning the estimated
     * previous sum. This is effective in fully resetting the sum only
     * if there are no concurrent updates.
     *
     * @return the estimated previous sum
     */
    public long reset() {
        long sum = 0L;
        Adder[] as = adders;
        if (as != null) {
            int n = as.length;
            for (int i = 0; i < n; ++i) {
                Adder 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;
        add(s.readLong());
    }

    // Unsafe mechanics
    private static final sun.misc.Unsafe UNSAFE;
    private static final long busyOffset;
    private static final long valueOffset;
    static {
        try {
            UNSAFE = getUnsafe();
            Class<?> sk = StripedAdder.class;
            busyOffset = UNSAFE.objectFieldOffset
                (sk.getDeclaredField("busy"));
            Class<?> ak = Adder.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:	1.10
Committed:	Fri Jul 29 13:50:54 2011 UTC (12 years, 10 months ago) by dl
Branch:	MAIN
Changes since 1.9:	+74 -66 lines
Log Message:	Reduce expansion under low contention
#	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			* A set of variables that together maintain a sum. When updates
18	dl	1.3	* (method {@link #add}) are contended across threads, this set of
19			* adder variables may grow dynamically to reduce contention. Method
20			* {@link #sum} returns the current combined total across these
21			* adders. This value is <em>NOT</em> an atomic snapshot (concurrent
22			* updates may occur while the sum is being calculated), and so cannot
23			* be used alone for fine-grained synchronization control.
24	jsr166	1.2	*
25	dl	1.1	* <p> This class may be applicable when many threads frequently
26			* update a common sum that is used for purposes such as collecting
27			* statistics. In this case, performance may be significantly faster
28			* than using a shared {@link AtomicLong}, at the expense of using
29	dl	1.10	* more space. On the other hand, if it is known that only one thread
30			* can ever update the sum, performance may be significantly slower
31			* than just updating a local variable.
32	dl	1.1	*
33	dl	1.3	* <p>A StripedAdder may optionally be constructed with a given
34			* expected contention level; i.e., the number of threads that are
35			* expected to concurrently update the sum. Supplying an accurate
36			* value may improve performance by reducing the need for dynamic
37			* adjustment.
38			*
39	jsr166	1.2	* @author Doug Lea
40	dl	1.1	*/
41			public class StripedAdder implements Serializable {
42			private static final long serialVersionUID = 7249069246863182397L;
43
44			/*
45	dl	1.4	* A StripedAdder maintains a table of Atomic long variables. The
46	dl	1.6	* table is indexed by per-thread hash codes.
47	dl	1.3	*
48	dl	1.8	* Table entries are of class Adder; a variant of AtomicLong
49			* padded to reduce cache contention on most processors. Padding
50			* is overkill for most Atomics because they are usually
51			* irregularly scattered in memory and thus don't interfere much
52			* with each other. But Atomic objects residing in arrays will
53			* tend to be placed adjacent to each other, and so will most
54			* often share cache lines (with a huge negative performance
55			* impact) without this precaution.
56			*
57			* Because Adders are relatively large, we avoid creating them
58			* until they are needed. On the other hand, we try to create them
59			* on any sign of contention.
60	dl	1.6	*
61			* Per-thread hash codes are initialized to random values.
62			* Collisions are indicated by failed CASes when performing an add
63			* operation (see method retryAdd). Upon a collision, if the table
64			* size is less than the capacity, it is doubled in size unless
65			* some other thread holds lock. If a hashed slot is empty, and
66			* lock is available, a new Adder is created. Otherwise, if the
67			* slot exists, a CAS is tried. Retries proceed by "double
68			* hashing", using a secondary hash (Marsaglia XorShift) to try to
69			* find a free slot.
70			*
71	dl	1.8	* By default, the table is lazily initialized. Upon first use,
72	dl	1.10	* the table is set to size 1, and contains a single Adder. The
73			* maximum table size is bounded by nearest power of two >= the
74			* number of CPUS. The table size is capped because, when there
75			* are more threads than CPUs, supposing that each thread were
76			* bound to a CPU, there would exist a perfect hash function
77			* mapping threads to slots that eliminates collisions. When we
78			* reach capacity, we search for this mapping by randomly varying
79			* the hash codes of colliding threads. Because search is random,
80			* and failures only become known via CAS failures, convergence
81			* will be slow, and because threads are typically not bound to
82			* CPUS forever, may not occur at all. However, despite these
83			* limitations, observed contention is typically low in these
84			* cases.
85	dl	1.3	*
86			* A single spinlock is used for resizing the table as well as
87	dl	1.8	* populating slots with new Adders. After initialization, there
88			* is no need for a blocking lock: Upon lock contention, threads
89	dl	1.4	* try other slots rather than blocking. After initialization, at
90			* least one slot exists, so retries will eventually find a
91	dl	1.6	* candidate Adder. During these retries, there is increased
92	dl	1.3	* contention and reduced locality, which is still better than
93			* alternatives.
94	dl	1.1	*/
95
96	dl	1.6	private static final int NCPU = Runtime.getRuntime().availableProcessors();
97	dl	1.5
98			/**
99	dl	1.8	* Padded variant of AtomicLong. The value field is placed
100			* between pads, hoping that the JVM doesn't reorder them.
101			* Updates are via inlined CAS in methods add and retryAdd.
102			*/
103			static final class Adder {
104			volatile long p0, p1, p2, p3, p4, p5, p6;
105			volatile long value;
106			volatile long q0, q1, q2, q3, q4, q5, q6;
107			Adder(long x) { value = x; }
108			}
109	dl	1.1
110	jsr166	1.2	/**
111	dl	1.6	* Holder for the thread-local hash code. The code is initially
112			* random, but may be set to a different value upon collisions.
113	dl	1.1	*/
114			static final class HashCode {
115	dl	1.6	static final Random rng = new Random();
116	dl	1.1	int code;
117	dl	1.6	HashCode() {
118	dl	1.8	int h = rng.nextInt(); // Avoid zero, because of xorShift rehash
119			code = (h == 0) ? 1 : h;
120	dl	1.6	}
121	dl	1.1	}
122
123			/**
124			* The corresponding ThreadLocal class
125			*/
126			static final class ThreadHashCode extends ThreadLocal<HashCode> {
127	dl	1.6	public HashCode initialValue() { return new HashCode(); }
128	dl	1.1	}
129
130			/**
131			* Static per-thread hash codes. Shared across all StripedAdders
132	dl	1.6	* to reduce ThreadLocal pollution and because adjustments due to
133			* collisions in one table are likely to be appropriate for
134			* others.
135	dl	1.1	*/
136			static final ThreadHashCode threadHashCode = new ThreadHashCode();
137
138			/**
139	dl	1.10	* Table of adders. When non-null, size is a power of 2.
140	dl	1.1	*/
141			private transient volatile Adder[] adders;
142
143			/**
144	dl	1.8	* Spinlock (locked via CAS) used when resizing and/or creating Adders.
145	dl	1.1	*/
146	dl	1.8	private volatile int busy;
147	dl	1.1
148			/**
149	dl	1.3	* Creates a new adder with zero sum.
150	dl	1.1	*/
151			public StripedAdder() {
152	dl	1.3	}
153
154			/**
155			* Creates a new adder with zero sum, and with stripes presized
156			* for the given expected contention level.
157			*
158			* @param expectedContention the expected number of threads that
159			* will concurrently update the sum.
160			*/
161			public StripedAdder(int expectedContention) {
162	dl	1.8	int cap = (expectedContention < NCPU) ? expectedContention : NCPU;
163	dl	1.10	int size = 1;
164	dl	1.8	while (size < cap)
165			size <<= 1;
166			Adder[] as = new Adder[size];
167			for (int i = 0; i < size; ++i)
168			as[i] = new Adder(0);
169			this.adders = as;
170	dl	1.1	}
171
172			/**
173			* Adds the given value.
174			*
175			* @param x the value to add
176			*/
177			public void add(long x) {
178	dl	1.8	Adder[] as; Adder a; int n; // locals to hold volatile reads
179	dl	1.1	HashCode hc = threadHashCode.get();
180	dl	1.6	int h = hc.code;
181	dl	1.10	boolean contended;
182	dl	1.8	if ((as = adders) != null && (n = as.length) > 0 &&
183			(a = as[(n - 1) & h]) != null) {
184			long v = a.value;
185			if (UNSAFE.compareAndSwapLong(a, valueOffset, v, v + x))
186			return;
187	dl	1.10	contended = true;
188	dl	1.8	}
189			else
190	dl	1.10	contended = false;
191			retryAdd(x, hc, contended);
192	dl	1.4	}
193
194			/**
195			* Handle cases of add involving initialization, resizing,
196	dl	1.6	* creating new Adders, and/or contention. See above for
197	dl	1.8	* explanation. This method suffers the usual non-modularity
198			* problems of optimistic retry code, relying on rechecked sets of
199			* reads.
200	dl	1.10	*
201			* @param x the value to add
202			* @param hc the hash code holder
203			* @param precontended true if CAS failed before call
204	dl	1.4	*/
205	dl	1.10	private void retryAdd(long x, HashCode hc, boolean precontended) {
206	dl	1.4	int h = hc.code;
207	dl	1.10	boolean collide = false; // true if last slot nonempty
208	dl	1.6	for (;;) {
209	dl	1.8	Adder[] as; Adder a; int n;
210			if ((as = adders) != null && (n = as.length) > 0) {
211	dl	1.10	if ((a = as[(n - 1) & h]) == null) {
212			if (busy == 0 && // Try to attach new Adder
213	dl	1.8	UNSAFE.compareAndSwapInt(this, busyOffset, 0, 1)) {
214			boolean created = false;
215			try { // Recheck under lock
216			Adder[] rs; int m, j;
217			if ((rs = adders) != null && (m = rs.length) > 0 &&
218			rs[j = (m - 1) & h] == null) {
219			rs[j] = new Adder(x);
220			created = true;
221			}
222			} finally {
223			busy = 0;
224			}
225			if (created)
226			break;
227			continue; // Slot is now non-empty
228	dl	1.6	}
229	dl	1.8	collide = false;
230	dl	1.6	}
231	dl	1.10	else if (precontended) // CAS already known to fail
232			precontended = false; // Continue after rehash
233			else {
234			long v = a.value;
235			if (UNSAFE.compareAndSwapLong(a, valueOffset, v, v + x))
236			break;
237			if (!collide)
238			collide = true;
239			else if (n >= NCPU \|\| adders != as)
240			collide = false; // Don't expand
241			else if (busy == 0 &&
242			UNSAFE.compareAndSwapInt(this, busyOffset, 0, 1)) {
243			collide = false;
244			try {
245			if (adders == as) { // Expand table
246			Adder[] rs = new Adder[n << 1];
247			for (int i = 0; i < n; ++i)
248			rs[i] = as[i];
249			adders = rs;
250			}
251			} finally {
252			busy = 0;
253			}
254			continue;
255			}
256			}
257	dl	1.8	h ^= h << 13; // Rehash
258			h ^= h >>> 17;
259			h ^= h << 5;
260	dl	1.6	}
261	dl	1.10	else if (adders == as) { // Try to default-initialize
262			Adder[] rs = new Adder[1];
263			rs[0] = new Adder(x);
264			boolean init = false;
265			while (adders == as) {
266			if (UNSAFE.compareAndSwapInt(this, busyOffset, 0, 1)) {
267			try {
268			if (adders == as) {
269			adders = rs;
270			init = true;
271			}
272			} finally {
273			busy = 0;
274	dl	1.8	}
275	dl	1.10	break;
276	dl	1.5	}
277	dl	1.10	if (adders != as)
278	dl	1.6	break;
279	dl	1.10	Thread.yield(); // Back off
280	dl	1.1	}
281	dl	1.10	if (init)
282			break;
283	dl	1.5	}
284	dl	1.1	}
285	dl	1.8	hc.code = h; // Record index for next time
286	dl	1.1	}
287
288			/**
289	dl	1.9	* Equivalent to {@code add(1)}.
290			*/
291			public void increment() {
292			add(1L);
293			}
294
295			/**
296			* Equivalent to {@code add(-1)}.
297			*/
298			public void decrement() {
299			add(-1L);
300			}
301
302			/**
303	dl	1.1	* Returns an estimate of the current sum. The result is
304			* calculated by summing multiple variables, so may not be
305			* accurate if updates occur concurrently with this method.
306	jsr166	1.2	*
307	dl	1.1	* @return the estimated sum
308			*/
309			public long sum() {
310	dl	1.4	long sum = 0L;
311	dl	1.1	Adder[] as = adders;
312	dl	1.4	if (as != null) {
313			int n = as.length;
314			for (int i = 0; i < n; ++i) {
315			Adder a = as[i];
316			if (a != null)
317	dl	1.8	sum += a.value;
318	dl	1.4	}
319	dl	1.1	}
320			return sum;
321			}
322
323			/**
324	dl	1.9	* Resets each of the variables to zero, returning the estimated
325			* previous sum. This is effective in fully resetting the sum only
326			* if there are no concurrent updates.
327	dl	1.1	*
328	dl	1.9	* @return the estimated previous sum
329	dl	1.1	*/
330	dl	1.9	public long reset() {
331	dl	1.4	long sum = 0L;
332	dl	1.1	Adder[] as = adders;
333	dl	1.4	if (as != null) {
334			int n = as.length;
335			for (int i = 0; i < n; ++i) {
336			Adder a = as[i];
337			if (a != null) {
338	dl	1.8	sum += a.value;
339			a.value = 0L;
340	dl	1.4	}
341	dl	1.1	}
342			}
343			return sum;
344			}
345
346			private void writeObject(java.io.ObjectOutputStream s)
347			throws java.io.IOException {
348			s.defaultWriteObject();
349			s.writeLong(sum());
350			}
351
352			private void readObject(ObjectInputStream s)
353			throws IOException, ClassNotFoundException {
354			s.defaultReadObject();
355	dl	1.8	busy = 0;
356	dl	1.4	add(s.readLong());
357	dl	1.1	}
358
359	dl	1.8	// Unsafe mechanics
360			private static final sun.misc.Unsafe UNSAFE;
361			private static final long busyOffset;
362			private static final long valueOffset;
363			static {
364			try {
365			UNSAFE = getUnsafe();
366			Class<?> sk = StripedAdder.class;
367			busyOffset = UNSAFE.objectFieldOffset
368			(sk.getDeclaredField("busy"));
369			Class<?> ak = Adder.class;
370			valueOffset = UNSAFE.objectFieldOffset
371			(ak.getDeclaredField("value"));
372			} catch (Exception e) {
373			throw new Error(e);
374			}
375			}
376
377			/**
378			* Returns a sun.misc.Unsafe. Suitable for use in a 3rd party package.
379			* Replace with a simple call to Unsafe.getUnsafe when integrating
380			* into a jdk.
381			*
382			* @return a sun.misc.Unsafe
383			*/
384			private static sun.misc.Unsafe getUnsafe() {
385			try {
386			return sun.misc.Unsafe.getUnsafe();
387			} catch (SecurityException se) {
388			try {
389			return java.security.AccessController.doPrivileged
390			(new java.security
391			.PrivilegedExceptionAction<sun.misc.Unsafe>() {
392			public sun.misc.Unsafe run() throws Exception {
393			java.lang.reflect.Field f = sun.misc
394			.Unsafe.class.getDeclaredField("theUnsafe");
395			f.setAccessible(true);
396			return (sun.misc.Unsafe) f.get(null);
397			}});
398			} catch (java.security.PrivilegedActionException e) {
399			throw new RuntimeException("Could not initialize intrinsics",
400			e.getCause());
401			}
402			}
403			}
404
405	dl	1.1	}