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Comparing jsr166/src/jsr166e/StripedAdder.java (file contents):
Revision 1.3 by dl, Fri Jul 22 13:25:12 2011 UTC vs.
Revision 1.8 by dl, Thu Jul 28 15:05:55 2011 UTC

#	Line 5 | Line 5
5		*/
6
7		package jsr166e;
8	–	import java.util.Arrays;
8		import java.util.Random;
9		import java.util.concurrent.atomic.AtomicInteger;
10		import java.util.concurrent.atomic.AtomicLong;
#	Line 27 | Line 26 | import java.io.ObjectOutputStream;
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	<	* significantly more space.  On the other hand, if it is known that
30	<	* only one thread can ever update the sum, performance may be
31	<	* significantly slower than just updating a local variable.
29	>	* much more space.  On the other hand, if it is known that only one
30	>	* thread can ever update the sum, performance may be significantly
31	>	* slower than just updating a local variable.
32		*
33		* <p>A StripedAdder may optionally be constructed with a given
34		* expected contention level; i.e., the number of threads that are
#	Line 43 | Line 42 | public class StripedAdder implements Ser
42		private static final long serialVersionUID = 7249069246863182397L;
43
44		/*
45	<	* Overview: We maintain a table of Atomic long variables. The
46	<	* table is indexed by per-thread hash codes that are initialized
48	<	* to random values.
45	>	* A StripedAdder maintains a table of Atomic long variables. The
46	>	* table is indexed by per-thread hash codes.
47		*
48	<	* The table doubles in size upon contention (as indicated by
49	<	* failed CASes when performing add()), but is capped at the
50	<	* nearest power of two >= #CPUS. This reflects the idea that,
53	<	* when there are more threads than CPUs, then if each thread were
54	<	* bound to a CPU, there would exist a perfect hash function
55	<	* mapping threads to slots that eliminates collisions. When we
56	<	* reach capacity, we search for this mapping by randomly varying
57	<	* the hash codes of colliding threads.  Because search is random,
58	<	* and failures only become known via CAS failures, convergence
59	<	* will be slow, and because threads are typically not bound to
60	<	* CPUS forever, may not occur at all. However, despite these
61	<	* limitations, observed contention is typically very low in these
62	<	* cases.
63	<	*
64	<	* Table entries are of class Adder; a form of AtomicLong padded
65	<	* to reduce cache contention on most processors. Padding is
66	<	* overkill for most Atomics because they are most often
48	>	* 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 without this precaution.  Except for
55	<	* slot adders[0], Adders are constructed upon first use, which
56	<	* further improves per-thread locality and helps reduce (an
57	<	* already large) footprint.
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	>	*
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	>	* By default, the table is lazily initialized.  Upon first use,
72	>	* the table is set to size 2 (the minimum non-empty size), but
73	>	* containing only a single Adder. The maximum table size is
74	>	* bounded by nearest power of two >= the number of CPUS.  The
75	>	* table size is capped because, when there are more threads than
76	>	* CPUs, supposing that each thread were bound to a CPU, there
77	>	* would exist a perfect hash function mapping threads to slots
78	>	* that eliminates collisions. When we reach capacity, we search
79	>	* for this mapping by randomly varying the hash codes of
80	>	* colliding threads.  Because search is random, and failures only
81	>	* become known via CAS failures, convergence will be slow, and
82	>	* because threads are typically not bound to CPUS forever, may
83	>	* not occur at all. However, despite these limitations, observed
84	>	* contention is typically low in these cases.
85		*
86		* A single spinlock is used for resizing the table as well as
87	<	* populating slots with new Adders. Upon lock contention, threads
88	<	* try other slots rather than blocking. We guarantee that at
89	<	* least one slot (0) exists, so retries will eventually find a
90	<	* candidate Adder. During these retries, there is increased
87	>	* populating slots with new Adders. After initialization, there
88	>	* is no need for a blocking lock: Upon lock contention, threads
89	>	* try other slots rather than blocking. After initialization, at
90	>	* least one slot exists, so retries will eventually find a
91	>	* candidate Adder.  During these retries, there is increased
92		* contention and reduced locality, which is still better than
93		* alternatives.
94		*/
95
96	<	/**
85	<	* Number of processors, to place a cap on table growth.
86	<	*/
87	<	static final int NCPU = Runtime.getRuntime().availableProcessors();
96	>	private static final int NCPU = Runtime.getRuntime().availableProcessors();
97
98		/**
99	<	* Padded version of AtomicLong
99	>	* 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 extends AtomicLong {
104	<	long p0, p1, p2, p3, p4, p5, p6, p7, p8, p9, pa, pb, pc, pd;
105	<	Adder(long x) { super(x); }
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
110		/**
111	<	* Holder for the thread-local hash code. The code starts off with
112	<	* a given random value, but may be set to a different
100	<	* pseudo-random value (using a cheaper but adequate xorshift
101	<	* generator) upon collisions.
111	>	* Holder for the thread-local hash code. The code is initially
112	>	* random, but may be set to a different value upon collisions.
113		*/
114		static final class HashCode {
115	+	static final Random rng = new Random();
116		int code;
117	<	HashCode(int h) { code = h; }
117	>	HashCode() {
118	>	int h = rng.nextInt(); // Avoid zero, because of xorShift rehash
119	>	code = (h == 0) ? 1 : h;
120	>	}
121		}
122
123		/**
124		* The corresponding ThreadLocal class
125		*/
126		static final class ThreadHashCode extends ThreadLocal<HashCode> {
127	<	static final Random rng = new Random();
113	<	public HashCode initialValue() {
114	<	int h = rng.nextInt();
115	<	return new HashCode((h == 0) ? 1 : h); // ensure nonzero
116	<	}
127	>	public HashCode initialValue() { return new HashCode(); }
128		}
129
130		/**
131		* Static per-thread hash codes. Shared across all StripedAdders
132	<	* because adjustments due to collisions in one table are likely
133	<	* to be appropriate for others.
132	>	* to reduce ThreadLocal pollution and because adjustments due to
133	>	* collisions in one table are likely to be appropriate for
134	>	* others.
135		*/
136		static final ThreadHashCode threadHashCode = new ThreadHashCode();
137
138		/**
139	<	* Table of adders. Minimum size 2. Size grows to be at most NCPU.
139	>	* Table of adders. When non-null, size is a power of 2, at least 2.
140		*/
141		private transient volatile Adder[] adders;
142
143		/**
144	<	* Serves as a lock when resizing and/or creating Adders.  There
133	<	* is no need for a blocking lock: When busy, other threads try
134	<	* other slots.
144	>	* Spinlock (locked via CAS) used when resizing and/or creating Adders.
145		*/
146	<	private final AtomicInteger mutex;
137	<
138	<	/**
139	<	* Marsaglia XorShift random generator for rehashing on collisions
140	<	*/
141	<	private static int xorShift(int r) {
142	<	r ^= r << 13;
143	<	r ^= r >>> 17;
144	<	return r ^ (r << 5);
145	<	}
146	>	private volatile int busy;
147
148		/**
149		* Creates a new adder with zero sum.
150		*/
151		public StripedAdder() {
151	–	this(2);
152		}
153
154		/**
#	Line 164 | Line 164 | public class StripedAdder implements Ser
164		while (size < cap)
165		size <<= 1;
166		Adder[] as = new Adder[size];
167	<	as[0] = new Adder(0); // ensure at least one available adder
167	>	for (int i = 0; i < size; ++i)
168	>	as[i] = new Adder(0);
169		this.adders = as;
169	–	this.mutex = new AtomicInteger();
170		}
171
172		/**
#	Line 175 | Line 175 | public class StripedAdder implements Ser
175		* @param x the value to add
176		*/
177		public void add(long x) {
178	+	Adder[] as; Adder a; int n;  // locals to hold volatile reads
179		HashCode hc = threadHashCode.get();
180	<	for (int h = hc.code;;) {
181	<	Adder[] as = adders;
182	<	int n = as.length;
183	<	Adder a = as[h & (n - 1)];
184	<	if (a != null) {
185	<	long v = a.get();
186	<	if (a.compareAndSet(v, v + x))
187	<	break;
188	<	if (n >= NCPU) {                 // Collision when table at max
189	<	h = hc.code = xorShift(h);   // change code
190	<	continue;
180	>	int h = hc.code;
181	>	boolean collide;
182	>	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	>	collide = true;
188	>	}
189	>	else
190	>	collide = false;
191	>	retryAdd(x, hc, collide);
192	>	}
193	>
194	>	/**
195	>	* Handle cases of add involving initialization, resizing,
196	>	* creating new Adders, and/or contention. See above for
197	>	* explanation. This method suffers the usual non-modularity
198	>	* problems of optimistic retry code, relying on rechecked sets of
199	>	* reads.
200	>	*/
201	>	private void retryAdd(long x, HashCode hc, boolean collide) {
202	>	int h = hc.code;
203	>	for (;;) {
204	>	Adder[] as; Adder a; int n;
205	>	if ((as = adders) != null && (n = as.length) > 0) {
206	>	if ((a = as[(n - 1) & h]) != null) {
207	>	boolean shared = true;      // Slot exists
208	>	if (collide && n < NCPU && busy == 0 &&
209	>	UNSAFE.compareAndSwapInt(this, busyOffset, 0, 1)) {
210	>	try {
211	>	if (adders == as) { // Expand table
212	>	Adder[] rs = new Adder[n << 1];
213	>	for (int i = 0; i < n; ++i)
214	>	rs[i] = as[i];
215	>	adders = rs;
216	>	shared = false;
217	>	}
218	>	} finally {
219	>	busy = 0;
220	>	}
221	>	if (shared || (h & n) != 0) {
222	>	collide = false;
223	>	continue;           // Array or index changed
224	>	}
225	>	}
226	>	long v = a.value;
227	>	if (UNSAFE.compareAndSwapLong(a, valueOffset, v, v + x))
228	>	break;
229	>	collide = shared;
230		}
231	<	}
232	<	final AtomicInteger mutex = this.mutex;
233	<	if (mutex.get() != 0)
234	<	h = xorShift(h);                 // Try elsewhere
235	<	else if (mutex.compareAndSet(0, 1)) {
236	<	boolean created = false;
237	<	try {
238	<	Adder[] rs = adders;
239	<	if (a != null && rs == as)   // Resize table
240	<	rs = adders = Arrays.copyOf(as, as.length << 1);
241	<	int j = h & (rs.length - 1);
242	<	if (rs[j] == null) {         // Create adder
243	<	rs[j] = new Adder(x);
244	<	created = true;
231	>	else {                          // Try to attach new Adder
232	>	if (busy == 0 &&
233	>	UNSAFE.compareAndSwapInt(this, busyOffset, 0, 1)) {
234	>	boolean created = false;
235	>	try {                   // Recheck under lock
236	>	Adder[] rs; int m, j;
237	>	if ((rs = adders) != null && (m = rs.length) > 0 &&
238	>	rs[j = (m - 1) & h] == null) {
239	>	rs[j] = new Adder(x);
240	>	created = true;
241	>	}
242	>	} finally {
243	>	busy = 0;
244	>	}
245	>	if (created)
246	>	break;
247	>	continue;               // Slot is now non-empty
248		}
249	<	} finally {
207	<	mutex.set(0);
249	>	collide = false;
250		}
251	<	if (created) {
252	<	hc.code = h;                 // Use this adder next time
253	<	break;
251	>	h ^= h << 13;                   // Rehash
252	>	h ^= h >>> 17;
253	>	h ^= h << 5;
254	>	}
255	>	else if (busy == 0) {               // Default-initialize
256	>	Adder r = new Adder(x);
257	>	Adder[] rs = new Adder[2];
258	>	rs[h & 1] = r;
259	>	if (adders == as && busy == 0 &&
260	>	UNSAFE.compareAndSwapInt(this, busyOffset, 0, 1)) {
261	>	boolean init = false;
262	>	try {
263	>	if (adders == as) {
264	>	adders = rs;
265	>	init = true;
266	>	}
267	>	} finally {
268	>	busy = 0;
269	>	}
270	>	if (init)
271	>	break;
272		}
273		}
274	+	else if (adders == as)              // Lost initialization race
275	+	Thread.yield();
276		}
277	+	hc.code = h;                            // Record index for next time
278		}
279
280		/**
#	Line 222 | Line 285 | public class StripedAdder implements Ser
285		* @return the estimated sum
286		*/
287		public long sum() {
288	<	long sum = 0;
288	>	long sum = 0L;
289		Adder[] as = adders;
290	<	int n = as.length;
291	<	for (int i = 0; i < n; ++i) {
292	<	Adder a = as[i];
293	<	if (a != null)
294	<	sum += a.get();
290	>	if (as != null) {
291	>	int n = as.length;
292	>	for (int i = 0; i < n; ++i) {
293	>	Adder a = as[i];
294	>	if (a != null)
295	>	sum += a.value;
296	>	}
297		}
298		return sum;
299		}
#	Line 240 | Line 305 | public class StripedAdder implements Ser
305		*/
306		public void reset() {
307		Adder[] as = adders;
308	<	int n = as.length;
309	<	for (int i = 0; i < n; ++i) {
310	<	Adder a = as[i];
311	<	if (a != null)
312	<	a.set(0L);
308	>	if (as != null) {
309	>	int n = as.length;
310	>	for (int i = 0; i < n; ++i) {
311	>	Adder a = as[i];
312	>	if (a != null)
313	>	a.value = 0L;
314	>	}
315		}
316		}
317
#	Line 268 | Line 335 | public class StripedAdder implements Ser
335		* @return the estimated sum
336		*/
337		public long sumAndReset() {
338	<	long sum = 0;
338	>	long sum = 0L;
339		Adder[] as = adders;
340	<	int n = as.length;
341	<	for (int i = 0; i < n; ++i) {
342	<	Adder a = as[i];
343	<	if (a != null) {
344	<	sum += a.get();
345	<	a.set(0L);
340	>	if (as != null) {
341	>	int n = as.length;
342	>	for (int i = 0; i < n; ++i) {
343	>	Adder a = as[i];
344	>	if (a != null) {
345	>	sum += a.value;
346	>	a.value = 0L;
347	>	}
348		}
349		}
350		return sum;
#	Line 290 | Line 359 | public class StripedAdder implements Ser
359		private void readObject(ObjectInputStream s)
360		throws IOException, ClassNotFoundException {
361		s.defaultReadObject();
362	<	long c = s.readLong();
363	<	Adder[] as = new Adder[2];
295	<	as[0] = new Adder(c);
296	<	this.adders = as;
297	<	mutex.set(0);
362	>	busy = 0;
363	>	add(s.readLong());
364		}
365
366	<	}
366	>	// Unsafe mechanics
367	>	private static final sun.misc.Unsafe UNSAFE;
368	>	private static final long busyOffset;
369	>	private static final long valueOffset;
370	>	static {
371	>	try {
372	>	UNSAFE = getUnsafe();
373	>	Class<?> sk = StripedAdder.class;
374	>	busyOffset = UNSAFE.objectFieldOffset
375	>	(sk.getDeclaredField("busy"));
376	>	Class<?> ak = Adder.class;
377	>	valueOffset = UNSAFE.objectFieldOffset
378	>	(ak.getDeclaredField("value"));
379	>	} catch (Exception e) {
380	>	throw new Error(e);
381	>	}
382	>	}
383
384	+	/**
385	+	* Returns a sun.misc.Unsafe.  Suitable for use in a 3rd party package.
386	+	* Replace with a simple call to Unsafe.getUnsafe when integrating
387	+	* into a jdk.
388	+	*
389	+	* @return a sun.misc.Unsafe
390	+	*/
391	+	private static sun.misc.Unsafe getUnsafe() {
392	+	try {
393	+	return sun.misc.Unsafe.getUnsafe();
394	+	} catch (SecurityException se) {
395	+	try {
396	+	return java.security.AccessController.doPrivileged
397	+	(new java.security
398	+	.PrivilegedExceptionAction<sun.misc.Unsafe>() {
399	+	public sun.misc.Unsafe run() throws Exception {
400	+	java.lang.reflect.Field f = sun.misc
401	+	.Unsafe.class.getDeclaredField("theUnsafe");
402	+	f.setAccessible(true);
403	+	return (sun.misc.Unsafe) f.get(null);
404	+	}});
405	+	} catch (java.security.PrivilegedActionException e) {
406	+	throw new RuntimeException("Could not initialize intrinsics",
407	+	e.getCause());
408	+	}
409	+	}
410	+	}
411
412	+	}

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