src/jsr166e/Striped64.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;

/**
 * A package-local class holding common representation and mechanics
 * for classes supporting dynamic striping on 64bit values. The class
 * extends Number so that concrete subclasses must publicly do so.
 */
abstract class Striped64 extends Number {
    /*
     * This class 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.
     * Nearly all declarations in this class are package-private,
     * accessed directly by subclasses.
     *
     * 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. Table slots remain empty (null) until they are
     * needed.
     *
     * A single spinlock ("busy") is used for initializing and
     * resizing the table, as well as populating slots with new Cells.
     * There is no need for a blocking lock; when the lock is not
     * available, threads try other slots (or the base).  During these
     * retries, there is increased contention and reduced locality,
     * which is still better than alternatives.
     *
     * Per-thread hash codes are initialized to random values.
     * Contention and/or table collisions are indicated by failed
     * CASes when performing an update operation (see method
     * retryUpdate). 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.
     *
     * 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 instances, observed
     * contention levels will recur, so the cells will eventually be
     * needed again; and for short-lived ones, it does not matter.
     */

    /**
     * Padded variant of AtomicLong supporting only raw accesses plus CAS.
     * The value field is placed between pads, hoping that the JVM doesn't
     * reorder them.
     *
     * JVM intrinsics note: It would be possible to use a release-only
     * form of CAS here, if it were provided.
     */
    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; }

        final boolean cas(long cmp, long val) {
            return UNSAFE.compareAndSwapLong(this, valueOffset, cmp, val);
        }

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

    }

    /**
     * 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 instances 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 */
    static final int NCPU = Runtime.getRuntime().availableProcessors();

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

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

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

    /**
     * Package-private default constructor
     */
    Striped64() {
    }

    /**
     * CASes the base field.
     */
    final boolean casBase(long cmp, long val) {
        return UNSAFE.compareAndSwapLong(this, baseOffset, cmp, val);
    }

    /**
     * CASes the busy field from 0 to 1 to acquire lock.
     */
    final boolean casBusy() {
        return UNSAFE.compareAndSwapInt(this, busyOffset, 0, 1);
    }

    /**
     * Computes the function of current and new value. Subclasses
     * should open-code this update function for most uses, but the
     * virtualized form is needed within retryUpdate.
     *
     * @param currentValue the current value (of either base or a cell)
     * @param newValue the argument from a user update call
     * @return result of the update function
     */
    abstract long fn(long currentValue, long newValue);

    /**
     * Handles cases of updates 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
     * @param hc the hash code holder
     * @param wasUncontended false if CAS failed before call
     */
    final void retryUpdate(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 && casBusy()) {
                            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 (a.cas(v = a.value, fn(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 && casBusy()) {
                    try {
                        if (cells == as) {      // Expand table unless stale
                            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 && casBusy()) {
                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 (casBase(v = base, fn(v, x)))
                break;                          // Fall back on using base
        }
        hc.code = h;                            // Record index for next time
    }


    /**
     * Sets base and all cells to the given value.
     */
    final void internalReset(long initialValue) {
        Cell[] as = cells;
        base = initialValue;
        if (as != null) {
            int n = as.length;
            for (int i = 0; i < n; ++i) {
                Cell a = as[i];
                if (a != null)
                    a.value = initialValue;
            }
        }
    }

    // Unsafe mechanics
    private static final sun.misc.Unsafe UNSAFE;
    private static final long baseOffset;
    private static final long busyOffset;
    static {
        try {
            UNSAFE = getUnsafe();
            Class<?> sk = Striped64.class;
            baseOffset = UNSAFE.objectFieldOffset
                (sk.getDeclaredField("base"));
            busyOffset = UNSAFE.objectFieldOffset
                (sk.getDeclaredField("busy"));
        } 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 tryReflectionInstead) {}
        try {
            return java.security.AccessController.doPrivileged
            (new java.security.PrivilegedExceptionAction<sun.misc.Unsafe>() {
                public sun.misc.Unsafe run() throws Exception {
                    Class<sun.misc.Unsafe> k = sun.misc.Unsafe.class;
                    for (java.lang.reflect.Field f : k.getDeclaredFields()) {
                        f.setAccessible(true);
                        Object x = f.get(null);
                        if (k.isInstance(x))
                            return k.cast(x);
                    }
                    throw new NoSuchFieldError("the Unsafe");
                }});
        } catch (java.security.PrivilegedActionException e) {
            throw new RuntimeException("Could not initialize intrinsics",
                                       e.getCause());
        }
    }
}
Revision:	1.8
Committed:	Mon Jan 14 01:59:15 2013 UTC (11 years, 4 months ago) by jsr166
Branch:	MAIN
Changes since 1.7:	+1 -1 lines
Log Message:	javadoc fixes
#	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
10			/**
11			* A package-local class holding common representation and mechanics
12			* for classes supporting dynamic striping on 64bit values. The class
13	jsr166	1.6	* extends Number so that concrete subclasses must publicly do so.
14	dl	1.1	*/
15			abstract class Striped64 extends Number {
16			/*
17			* This class maintains a lazily-initialized table of atomically
18			* updated variables, plus an extra "base" field. The table size
19			* is a power of two. Indexing uses masked per-thread hash codes.
20			* Nearly all declarations in this class are package-private,
21			* accessed directly by subclasses.
22			*
23			* Table entries are of class Cell; a variant of AtomicLong padded
24			* to reduce cache contention on most processors. Padding is
25			* overkill for most Atomics because they are usually irregularly
26			* scattered in memory and thus don't interfere much with each
27			* other. But Atomic objects residing in arrays will tend to be
28			* placed adjacent to each other, and so will most often share
29			* cache lines (with a huge negative performance impact) without
30			* this precaution.
31			*
32			* In part because Cells are relatively large, we avoid creating
33			* them until they are needed. When there is no contention, all
34			* updates are made to the base field. Upon first contention (a
35			* failed CAS on base update), the table is initialized to size 2.
36			* The table size is doubled upon further contention until
37			* reaching the nearest power of two greater than or equal to the
38			* number of CPUS. Table slots remain empty (null) until they are
39			* needed.
40			*
41			* A single spinlock ("busy") is used for initializing and
42			* resizing the table, as well as populating slots with new Cells.
43	jsr166	1.8	* There is no need for a blocking lock; when the lock is not
44	dl	1.1	* available, threads try other slots (or the base). During these
45			* retries, there is increased contention and reduced locality,
46			* which is still better than alternatives.
47			*
48			* Per-thread hash codes are initialized to random values.
49			* Contention and/or table collisions are indicated by failed
50			* CASes when performing an update operation (see method
51			* retryUpdate). Upon a collision, if the table size is less than
52			* the capacity, it is doubled in size unless some other thread
53			* holds the lock. If a hashed slot is empty, and lock is
54			* available, a new Cell is created. Otherwise, if the slot
55			* exists, a CAS is tried. Retries proceed by "double hashing",
56			* using a secondary hash (Marsaglia XorShift) to try to find a
57			* free slot.
58			*
59			* The table size is capped because, when there are more threads
60			* than CPUs, supposing that each thread were bound to a CPU,
61			* there would exist a perfect hash function mapping threads to
62			* slots that eliminates collisions. When we reach capacity, we
63			* search for this mapping by randomly varying the hash codes of
64			* colliding threads. Because search is random, and collisions
65			* only become known via CAS failures, convergence can be slow,
66			* and because threads are typically not bound to CPUS forever,
67			* may not occur at all. However, despite these limitations,
68			* observed contention rates are typically low in these cases.
69			*
70			* It is possible for a Cell to become unused when threads that
71			* once hashed to it terminate, as well as in the case where
72			* doubling the table causes no thread to hash to it under
73			* expanded mask. We do not try to detect or remove such cells,
74			* under the assumption that for long-running instances, observed
75			* contention levels will recur, so the cells will eventually be
76			* needed again; and for short-lived ones, it does not matter.
77			*/
78
79			/**
80	jsr166	1.5	* Padded variant of AtomicLong supporting only raw accesses plus CAS.
81			* The value field is placed between pads, hoping that the JVM doesn't
82			* reorder them.
83	dl	1.1	*
84			* JVM intrinsics note: It would be possible to use a release-only
85			* form of CAS here, if it were provided.
86			*/
87			static final class Cell {
88			volatile long p0, p1, p2, p3, p4, p5, p6;
89			volatile long value;
90			volatile long q0, q1, q2, q3, q4, q5, q6;
91			Cell(long x) { value = x; }
92
93			final boolean cas(long cmp, long val) {
94			return UNSAFE.compareAndSwapLong(this, valueOffset, cmp, val);
95			}
96
97			// Unsafe mechanics
98			private static final sun.misc.Unsafe UNSAFE;
99			private static final long valueOffset;
100			static {
101			try {
102			UNSAFE = getUnsafe();
103			Class<?> ak = Cell.class;
104			valueOffset = UNSAFE.objectFieldOffset
105			(ak.getDeclaredField("value"));
106			} catch (Exception e) {
107			throw new Error(e);
108			}
109			}
110	jsr166	1.2
111	dl	1.1	}
112
113			/**
114			* Holder for the thread-local hash code. The code is initially
115			* random, but may be set to a different value upon collisions.
116			*/
117			static final class HashCode {
118			static final Random rng = new Random();
119			int code;
120			HashCode() {
121			int h = rng.nextInt(); // Avoid zero to allow xorShift rehash
122			code = (h == 0) ? 1 : h;
123			}
124			}
125	jsr166	1.2
126	dl	1.1	/**
127			* The corresponding ThreadLocal class
128			*/
129			static final class ThreadHashCode extends ThreadLocal<HashCode> {
130			public HashCode initialValue() { return new HashCode(); }
131			}
132
133			/**
134			* Static per-thread hash codes. Shared across all instances to
135			* reduce ThreadLocal pollution and because adjustments due to
136			* collisions in one table are likely to be appropriate for
137			* others.
138			*/
139			static final ThreadHashCode threadHashCode = new ThreadHashCode();
140
141	jsr166	1.3	/** Number of CPUS, to place bound on table size */
142	dl	1.1	static final int NCPU = Runtime.getRuntime().availableProcessors();
143
144			/**
145			* Table of cells. When non-null, size is a power of 2.
146			*/
147			transient volatile Cell[] cells;
148
149			/**
150			* Base value, used mainly when there is no contention, but also as
151	jsr166	1.3	* a fallback during table initialization races. Updated via CAS.
152	dl	1.1	*/
153			transient volatile long base;
154
155			/**
156			* Spinlock (locked via CAS) used when resizing and/or creating Cells.
157			*/
158			transient volatile int busy;
159
160			/**
161			* Package-private default constructor
162			*/
163			Striped64() {
164			}
165
166			/**
167	jsr166	1.5	* CASes the base field.
168	dl	1.1	*/
169			final boolean casBase(long cmp, long val) {
170			return UNSAFE.compareAndSwapLong(this, baseOffset, cmp, val);
171			}
172
173			/**
174	jsr166	1.5	* CASes the busy field from 0 to 1 to acquire lock.
175	dl	1.1	*/
176			final boolean casBusy() {
177			return UNSAFE.compareAndSwapInt(this, busyOffset, 0, 1);
178			}
179
180			/**
181			* Computes the function of current and new value. Subclasses
182			* should open-code this update function for most uses, but the
183			* virtualized form is needed within retryUpdate.
184			*
185			* @param currentValue the current value (of either base or a cell)
186			* @param newValue the argument from a user update call
187			* @return result of the update function
188			*/
189			abstract long fn(long currentValue, long newValue);
190
191			/**
192			* Handles cases of updates involving initialization, resizing,
193			* creating new Cells, and/or contention. See above for
194			* explanation. This method suffers the usual non-modularity
195			* problems of optimistic retry code, relying on rechecked sets of
196			* reads.
197			*
198			* @param x the value
199			* @param hc the hash code holder
200			* @param wasUncontended false if CAS failed before call
201			*/
202			final void retryUpdate(long x, HashCode hc, boolean wasUncontended) {
203			int h = hc.code;
204			boolean collide = false; // True if last slot nonempty
205			for (;;) {
206			Cell[] as; Cell a; int n; long v;
207			if ((as = cells) != null && (n = as.length) > 0) {
208			if ((a = as[(n - 1) & h]) == null) {
209			if (busy == 0) { // Try to attach new Cell
210			Cell r = new Cell(x); // Optimistically create
211			if (busy == 0 && casBusy()) {
212			boolean created = false;
213			try { // Recheck under lock
214			Cell[] rs; int m, j;
215			if ((rs = cells) != null &&
216			(m = rs.length) > 0 &&
217			rs[j = (m - 1) & h] == null) {
218			rs[j] = r;
219			created = true;
220			}
221			} finally {
222			busy = 0;
223			}
224			if (created)
225			break;
226			continue; // Slot is now non-empty
227			}
228			}
229			collide = false;
230			}
231			else if (!wasUncontended) // CAS already known to fail
232			wasUncontended = true; // Continue after rehash
233			else if (a.cas(v = a.value, fn(v, x)))
234			break;
235			else if (n >= NCPU \|\| cells != as)
236			collide = false; // At max size or stale
237			else if (!collide)
238			collide = true;
239	jsr166	1.2	else if (busy == 0 && casBusy()) {
240	dl	1.1	try {
241			if (cells == as) { // Expand table unless stale
242			Cell[] rs = new Cell[n << 1];
243			for (int i = 0; i < n; ++i)
244			rs[i] = as[i];
245			cells = rs;
246			}
247			} finally {
248			busy = 0;
249			}
250			collide = false;
251			continue; // Retry with expanded table
252			}
253			h ^= h << 13; // Rehash
254			h ^= h >>> 17;
255			h ^= h << 5;
256			}
257			else if (busy == 0 && cells == as && casBusy()) {
258			boolean init = false;
259			try { // Initialize table
260			if (cells == as) {
261			Cell[] rs = new Cell[2];
262			rs[h & 1] = new Cell(x);
263			cells = rs;
264			init = true;
265			}
266			} finally {
267			busy = 0;
268			}
269			if (init)
270			break;
271			}
272			else if (casBase(v = base, fn(v, x)))
273			break; // Fall back on using base
274			}
275			hc.code = h; // Record index for next time
276			}
277
278
279			/**
280	jsr166	1.5	* Sets base and all cells to the given value.
281	dl	1.1	*/
282			final void internalReset(long initialValue) {
283			Cell[] as = cells;
284			base = initialValue;
285			if (as != null) {
286			int n = as.length;
287			for (int i = 0; i < n; ++i) {
288			Cell a = as[i];
289			if (a != null)
290			a.value = initialValue;
291			}
292			}
293			}
294	jsr166	1.2
295	dl	1.1	// Unsafe mechanics
296			private static final sun.misc.Unsafe UNSAFE;
297			private static final long baseOffset;
298			private static final long busyOffset;
299			static {
300			try {
301			UNSAFE = getUnsafe();
302			Class<?> sk = Striped64.class;
303			baseOffset = UNSAFE.objectFieldOffset
304			(sk.getDeclaredField("base"));
305			busyOffset = UNSAFE.objectFieldOffset
306			(sk.getDeclaredField("busy"));
307			} catch (Exception e) {
308			throw new Error(e);
309			}
310			}
311
312			/**
313			* Returns a sun.misc.Unsafe. Suitable for use in a 3rd party package.
314			* Replace with a simple call to Unsafe.getUnsafe when integrating
315			* into a jdk.
316			*
317			* @return a sun.misc.Unsafe
318			*/
319			private static sun.misc.Unsafe getUnsafe() {
320			try {
321			return sun.misc.Unsafe.getUnsafe();
322	jsr166	1.7	} catch (SecurityException tryReflectionInstead) {}
323			try {
324			return java.security.AccessController.doPrivileged
325			(new java.security.PrivilegedExceptionAction<sun.misc.Unsafe>() {
326			public sun.misc.Unsafe run() throws Exception {
327			Class<sun.misc.Unsafe> k = sun.misc.Unsafe.class;
328			for (java.lang.reflect.Field f : k.getDeclaredFields()) {
329			f.setAccessible(true);
330			Object x = f.get(null);
331			if (k.isInstance(x))
332			return k.cast(x);
333			}
334			throw new NoSuchFieldError("the Unsafe");
335			}});
336			} catch (java.security.PrivilegedActionException e) {
337			throw new RuntimeException("Could not initialize intrinsics",
338			e.getCause());
339	dl	1.1	}
340			}
341	jsr166	1.2	}