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);
            }
        }

    }

    /**
     * ThreadLocal holding a single-slot int array holding hash code.
     * Unlike the JDK8 version of this class, we use a suboptimal
     * int[] representation to avoid introducing a new type that can
     * impede class-unloading when ThreadLocals are not removed.
     */
    static final ThreadLocal<int[]> threadHashCode = new ThreadLocal<int[]>();

    /**
     * Generator of new random hash codes
     */
    static final Random rng = new Random();

    /** 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, int[] hc, boolean wasUncontended) {
        int h;
        if (hc == null) {
            threadHashCode.set(hc = new int[1]); // Initialize randomly
            int r = rng.nextInt(); // Avoid zero to allow xorShift rehash
            h = hc[0] = (r == 0) ? 1 : r;
        }
        else
            h = hc[0];
        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;
                hc[0] = h;                      // Record index for next time
            }
            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
        }
    }


    /**
     * 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:	1.10
Committed:	Sun Jan 18 20:17:33 2015 UTC (9 years, 4 months ago) by jsr166
Branch:	MAIN
CVS Tags:	HEAD
Changes since 1.9:	+1 -0 lines
Log Message:	exactly one blank line before and after package statements
#	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	jsr166	1.10
9	dl	1.1	import java.util.Random;
10
11			/**
12			* A package-local class holding common representation and mechanics
13			* for classes supporting dynamic striping on 64bit values. The class
14	jsr166	1.6	* extends Number so that concrete subclasses must publicly do so.
15	dl	1.1	*/
16			abstract class Striped64 extends Number {
17			/*
18			* This class maintains a lazily-initialized table of atomically
19			* updated variables, plus an extra "base" field. The table size
20			* is a power of two. Indexing uses masked per-thread hash codes.
21			* Nearly all declarations in this class are package-private,
22			* accessed directly by subclasses.
23			*
24			* Table entries are of class Cell; a variant of AtomicLong padded
25			* to reduce cache contention on most processors. Padding is
26			* overkill for most Atomics because they are usually irregularly
27			* scattered in memory and thus don't interfere much with each
28			* other. But Atomic objects residing in arrays will tend to be
29			* placed adjacent to each other, and so will most often share
30			* cache lines (with a huge negative performance impact) without
31			* this precaution.
32			*
33			* In part because Cells are relatively large, we avoid creating
34			* them until they are needed. When there is no contention, all
35			* updates are made to the base field. Upon first contention (a
36			* failed CAS on base update), the table is initialized to size 2.
37			* The table size is doubled upon further contention until
38			* reaching the nearest power of two greater than or equal to the
39			* number of CPUS. Table slots remain empty (null) until they are
40			* needed.
41			*
42			* A single spinlock ("busy") is used for initializing and
43			* resizing the table, as well as populating slots with new Cells.
44	jsr166	1.8	* There is no need for a blocking lock; when the lock is not
45	dl	1.1	* available, threads try other slots (or the base). During these
46			* retries, there is increased contention and reduced locality,
47			* which is still better than alternatives.
48			*
49			* Per-thread hash codes are initialized to random values.
50			* Contention and/or table collisions are indicated by failed
51			* CASes when performing an update operation (see method
52			* retryUpdate). Upon a collision, if the table size is less than
53			* the capacity, it is doubled in size unless some other thread
54			* holds the lock. If a hashed slot is empty, and lock is
55			* available, a new Cell is created. Otherwise, if the slot
56			* exists, a CAS is tried. Retries proceed by "double hashing",
57			* using a secondary hash (Marsaglia XorShift) to try to find a
58			* free slot.
59			*
60			* The table size is capped because, when there are more threads
61			* than CPUs, supposing that each thread were bound to a CPU,
62			* there would exist a perfect hash function mapping threads to
63			* slots that eliminates collisions. When we reach capacity, we
64			* search for this mapping by randomly varying the hash codes of
65			* colliding threads. Because search is random, and collisions
66			* only become known via CAS failures, convergence can be slow,
67			* and because threads are typically not bound to CPUS forever,
68			* may not occur at all. However, despite these limitations,
69			* observed contention rates are typically low in these cases.
70			*
71			* It is possible for a Cell to become unused when threads that
72			* once hashed to it terminate, as well as in the case where
73			* doubling the table causes no thread to hash to it under
74			* expanded mask. We do not try to detect or remove such cells,
75			* under the assumption that for long-running instances, observed
76			* contention levels will recur, so the cells will eventually be
77			* needed again; and for short-lived ones, it does not matter.
78			*/
79
80			/**
81	jsr166	1.5	* Padded variant of AtomicLong supporting only raw accesses plus CAS.
82			* The value field is placed between pads, hoping that the JVM doesn't
83			* reorder them.
84	dl	1.1	*
85			* JVM intrinsics note: It would be possible to use a release-only
86			* form of CAS here, if it were provided.
87			*/
88			static final class Cell {
89			volatile long p0, p1, p2, p3, p4, p5, p6;
90			volatile long value;
91			volatile long q0, q1, q2, q3, q4, q5, q6;
92			Cell(long x) { value = x; }
93
94			final boolean cas(long cmp, long val) {
95			return UNSAFE.compareAndSwapLong(this, valueOffset, cmp, val);
96			}
97
98			// Unsafe mechanics
99			private static final sun.misc.Unsafe UNSAFE;
100			private static final long valueOffset;
101			static {
102			try {
103			UNSAFE = getUnsafe();
104			Class<?> ak = Cell.class;
105			valueOffset = UNSAFE.objectFieldOffset
106			(ak.getDeclaredField("value"));
107			} catch (Exception e) {
108			throw new Error(e);
109			}
110			}
111	jsr166	1.2
112	dl	1.1	}
113
114			/**
115	dl	1.9	* ThreadLocal holding a single-slot int array holding hash code.
116			* Unlike the JDK8 version of this class, we use a suboptimal
117			* int[] representation to avoid introducing a new type that can
118			* impede class-unloading when ThreadLocals are not removed.
119	dl	1.1	*/
120	dl	1.9	static final ThreadLocal<int[]> threadHashCode = new ThreadLocal<int[]>();
121	jsr166	1.2
122	dl	1.1	/**
123	dl	1.9	* Generator of new random hash codes
124	dl	1.1	*/
125	dl	1.9	static final Random rng = new Random();
126	dl	1.1
127	jsr166	1.3	/** Number of CPUS, to place bound on table size */
128	dl	1.1	static final int NCPU = Runtime.getRuntime().availableProcessors();
129
130			/**
131			* Table of cells. When non-null, size is a power of 2.
132			*/
133			transient volatile Cell[] cells;
134
135			/**
136			* Base value, used mainly when there is no contention, but also as
137	jsr166	1.3	* a fallback during table initialization races. Updated via CAS.
138	dl	1.1	*/
139			transient volatile long base;
140
141			/**
142			* Spinlock (locked via CAS) used when resizing and/or creating Cells.
143			*/
144			transient volatile int busy;
145
146			/**
147			* Package-private default constructor
148			*/
149			Striped64() {
150			}
151
152			/**
153	jsr166	1.5	* CASes the base field.
154	dl	1.1	*/
155			final boolean casBase(long cmp, long val) {
156			return UNSAFE.compareAndSwapLong(this, baseOffset, cmp, val);
157			}
158
159			/**
160	jsr166	1.5	* CASes the busy field from 0 to 1 to acquire lock.
161	dl	1.1	*/
162			final boolean casBusy() {
163			return UNSAFE.compareAndSwapInt(this, busyOffset, 0, 1);
164			}
165
166			/**
167			* Computes the function of current and new value. Subclasses
168			* should open-code this update function for most uses, but the
169			* virtualized form is needed within retryUpdate.
170			*
171			* @param currentValue the current value (of either base or a cell)
172			* @param newValue the argument from a user update call
173			* @return result of the update function
174			*/
175			abstract long fn(long currentValue, long newValue);
176
177			/**
178			* Handles cases of updates involving initialization, resizing,
179			* creating new Cells, and/or contention. See above for
180			* explanation. This method suffers the usual non-modularity
181			* problems of optimistic retry code, relying on rechecked sets of
182			* reads.
183			*
184			* @param x the value
185			* @param hc the hash code holder
186			* @param wasUncontended false if CAS failed before call
187			*/
188	dl	1.9	final void retryUpdate(long x, int[] hc, boolean wasUncontended) {
189			int h;
190			if (hc == null) {
191			threadHashCode.set(hc = new int[1]); // Initialize randomly
192			int r = rng.nextInt(); // Avoid zero to allow xorShift rehash
193			h = hc[0] = (r == 0) ? 1 : r;
194			}
195			else
196			h = hc[0];
197	dl	1.1	boolean collide = false; // True if last slot nonempty
198			for (;;) {
199			Cell[] as; Cell a; int n; long v;
200			if ((as = cells) != null && (n = as.length) > 0) {
201			if ((a = as[(n - 1) & h]) == null) {
202			if (busy == 0) { // Try to attach new Cell
203			Cell r = new Cell(x); // Optimistically create
204			if (busy == 0 && casBusy()) {
205			boolean created = false;
206			try { // Recheck under lock
207			Cell[] rs; int m, j;
208			if ((rs = cells) != null &&
209			(m = rs.length) > 0 &&
210			rs[j = (m - 1) & h] == null) {
211			rs[j] = r;
212			created = true;
213			}
214			} finally {
215			busy = 0;
216			}
217			if (created)
218			break;
219			continue; // Slot is now non-empty
220			}
221			}
222			collide = false;
223			}
224			else if (!wasUncontended) // CAS already known to fail
225			wasUncontended = true; // Continue after rehash
226			else if (a.cas(v = a.value, fn(v, x)))
227			break;
228			else if (n >= NCPU \|\| cells != as)
229			collide = false; // At max size or stale
230			else if (!collide)
231			collide = true;
232	jsr166	1.2	else if (busy == 0 && casBusy()) {
233	dl	1.1	try {
234			if (cells == as) { // Expand table unless stale
235			Cell[] rs = new Cell[n << 1];
236			for (int i = 0; i < n; ++i)
237			rs[i] = as[i];
238			cells = rs;
239			}
240			} finally {
241			busy = 0;
242			}
243			collide = false;
244			continue; // Retry with expanded table
245			}
246			h ^= h << 13; // Rehash
247			h ^= h >>> 17;
248			h ^= h << 5;
249	dl	1.9	hc[0] = h; // Record index for next time
250	dl	1.1	}
251			else if (busy == 0 && cells == as && casBusy()) {
252			boolean init = false;
253			try { // Initialize table
254			if (cells == as) {
255			Cell[] rs = new Cell[2];
256			rs[h & 1] = new Cell(x);
257			cells = rs;
258			init = true;
259			}
260			} finally {
261			busy = 0;
262			}
263			if (init)
264			break;
265			}
266			else if (casBase(v = base, fn(v, x)))
267			break; // Fall back on using base
268			}
269			}
270
271
272			/**
273	jsr166	1.5	* Sets base and all cells to the given value.
274	dl	1.1	*/
275			final void internalReset(long initialValue) {
276			Cell[] as = cells;
277			base = initialValue;
278			if (as != null) {
279			int n = as.length;
280			for (int i = 0; i < n; ++i) {
281			Cell a = as[i];
282			if (a != null)
283			a.value = initialValue;
284			}
285			}
286			}
287	jsr166	1.2
288	dl	1.1	// Unsafe mechanics
289			private static final sun.misc.Unsafe UNSAFE;
290			private static final long baseOffset;
291			private static final long busyOffset;
292			static {
293			try {
294			UNSAFE = getUnsafe();
295			Class<?> sk = Striped64.class;
296			baseOffset = UNSAFE.objectFieldOffset
297			(sk.getDeclaredField("base"));
298			busyOffset = UNSAFE.objectFieldOffset
299			(sk.getDeclaredField("busy"));
300			} catch (Exception e) {
301			throw new Error(e);
302			}
303			}
304
305			/**
306			* Returns a sun.misc.Unsafe. Suitable for use in a 3rd party package.
307			* Replace with a simple call to Unsafe.getUnsafe when integrating
308			* into a jdk.
309			*
310			* @return a sun.misc.Unsafe
311			*/
312			private static sun.misc.Unsafe getUnsafe() {
313			try {
314			return sun.misc.Unsafe.getUnsafe();
315	jsr166	1.7	} catch (SecurityException tryReflectionInstead) {}
316			try {
317			return java.security.AccessController.doPrivileged
318			(new java.security.PrivilegedExceptionAction<sun.misc.Unsafe>() {
319			public sun.misc.Unsafe run() throws Exception {
320			Class<sun.misc.Unsafe> k = sun.misc.Unsafe.class;
321			for (java.lang.reflect.Field f : k.getDeclaredFields()) {
322			f.setAccessible(true);
323			Object x = f.get(null);
324			if (k.isInstance(x))
325			return k.cast(x);
326			}
327			throw new NoSuchFieldError("the Unsafe");
328			}});
329			} catch (java.security.PrivilegedActionException e) {
330			throw new RuntimeException("Could not initialize intrinsics",
331			e.getCause());
332	dl	1.1	}
333			}
334	jsr166	1.2	}