ViewVC Help

View File | Revision Log | Show Annotations | Download File | Root Listing

root/jsr166/jsr166/src/jsr166e/StripedAdder.java

Comparing jsr166/src/jsr166e/StripedAdder.java (file contents):
Revision 1.6 by dl, Tue Jul 26 17:16:36 2011 UTC vs.
Revision 1.12 by dl, Sat Jul 30 16:26:34 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 15 | Line 14 | import java.io.ObjectInputStream;
14		import java.io.ObjectOutputStream;
15
16		/**
17	<	* A set of variables that together maintain a sum.  When updates
18	<	* (method {@link #add}) are contended across threads, this set of
19	<	* adder variables may grow dynamically to reduce contention. Method
21	<	* {@link #sum} returns the current combined total across these
22	<	* adders. This value is <em>NOT</em> an atomic snapshot (concurrent
23	<	* updates may occur while the sum is being calculated), and so cannot
24	<	* be used alone for fine-grained synchronization control.
17	>	* One or more variables that together maintain an initially zero sum.
18	>	* When updates (method {@link #add}) are contended across threads,
19	>	* the set of variables may grow dynamically to reduce contention.
20		*
21	<	* <p> This class may be applicable when many threads frequently
22	<	* update a common sum that is used for purposes such as collecting
23	<	* statistics. In this case, performance may be significantly faster
24	<	* than using a shared {@link AtomicLong}, at the expense of using
25	<	* much more space.  On the other hand, if it is known that only one
26	<	* thread can ever update the sum, performance may be significantly
27	<	* slower than just updating a local variable.
21	>	* <p> This class is usually preferable to {@link AtomicLong} when
22	>	* multiple threads update a common sum that is used for purposes such
23	>	* as collecting statistics, not for fine-grained synchronization
24	>	* control.  Under high update contention, throughput of this class is
25	>	* expected to be significantly higher, at the expense of higher space
26	>	* consumption.  Under low contention, this class imposes very little
27	>	* time and space overhead compared to AtomicLong.  On the other hand,
28	>	* in contexts where it is statically known that only one thread can
29	>	* ever update a sum, time and space overhead is noticeably greater
30	>	* than just updating a local variable.
31		*
32	<	* <p>A StripedAdder may optionally be constructed with a given
33	<	* expected contention level; i.e., the number of threads that are
34	<	* expected to concurrently update the sum. Supplying an accurate
35	<	* value may improve performance by reducing the need for dynamic
36	<	* adjustment.
32	>	* <p> Method {@link #sum} returns the current combined total across
33	>	* the variables maintaining the sum.  This value is <em>NOT</em> an
34	>	* atomic snapshot: Concurrent updates may occur while the sum is
35	>	* being calculated.  However, updates cannot be "lost", so invocation
36	>	* of <code>sum</code> in the absence of concurrent updates always
37	>	* returns an accurate result.  The sum may also be <code>reset</code>
38	>	* to zero, as an alternative to creating a new adder.  However,
39	>	* method {@link #reset} is intrinsically racy, so should only be used
40	>	* when it is known that no threads are concurrently updating the sum.
41	>	*
42	>	* <p><em>jsr166e note: This class is targeted to be placed in
43	>	* java.util.concurrent.atomic<em>
44		*
45		* @author Doug Lea
46		*/
#	Line 43 | Line 48 | public class StripedAdder implements Ser
48		private static final long serialVersionUID = 7249069246863182397L;
49
50		/*
51	<	* A StripedAdder maintains a table of Atomic long variables. The
52	<	* table is indexed by per-thread hash codes.
51	>	* A StripedAdder maintains a lazily-initialized table of
52	>	* atomically updated variables, plus an extra "base" field. The
53	>	* table size is a power of two. Indexing uses masked per-thread
54	>	* hash codes
55		*
56	<	* By default, the table is lazily initialized, to minimize
57	<	* footprint until adders are used. On first use, the table is set
58	<	* to size DEFAULT_INITIAL_SIZE (currently 8). Table size is
59	<	* bounded by the number of CPUS (if larger than the default
60	<	* size).
56	>	* Table entries are of class Cell; a variant of AtomicLong padded
57	>	* to reduce cache contention on most processors. Padding is
58	>	* overkill for most Atomics because they are usually irregularly
59	>	* scattered in memory and thus don't interfere much with each
60	>	* other. But Atomic objects residing in arrays will tend to be
61	>	* placed adjacent to each other, and so will most often share
62	>	* cache lines (with a huge negative performance impact) without
63	>	* this precaution.
64	>	*
65	>	* In part because Cells are relatively large, we avoid creating
66	>	* them until they are needed.  When there is no contention, all
67	>	* updates are made to the base field.  Upon first contention (a
68	>	* failed CAS on base update), the table is initialized to size 2.
69	>	* The table size is doubled upon further contention until
70	>	* reaching the nearest power of two greater than or equal to the
71	>	* number of CPUS.
72		*
73		* Per-thread hash codes are initialized to random values.
74	<	* Collisions are indicated by failed CASes when performing an add
75	<	* operation (see method retryAdd). Upon a collision, if the table
76	<	* size is less than the capacity, it is doubled in size unless
77	<	* some other thread holds lock. If a hashed slot is empty, and
78	<	* lock is available, a new Adder is created. Otherwise, if the
79	<	* slot exists, a CAS is tried.  Retries proceed by "double
80	<	* hashing", using a secondary hash (Marsaglia XorShift) to try to
81	<	* find a free slot.
74	>	* Contention and/or table collisions are indicated by failed
75	>	* CASes when performing an add operation (see method
76	>	* retryAdd). Upon a collision, if the table size is less than the
77	>	* capacity, it is doubled in size unless some other thread holds
78	>	* the lock. If a hashed slot is empty, and lock is available, a
79	>	* new Cell is created. Otherwise, if the slot exists, a CAS is
80	>	* tried.  Retries proceed by "double hashing", using a secondary
81	>	* hash (Marsaglia XorShift) to try to find a free slot.
82		*
83		* The table size is capped because, when there are more threads
84		* than CPUs, supposing that each thread were bound to a CPU,
85		* there would exist a perfect hash function mapping threads to
86		* slots that eliminates collisions. When we reach capacity, we
87		* search for this mapping by randomly varying the hash codes of
88	<	* colliding threads.  Because search is random, and failures only
89	<	* become known via CAS failures, convergence will be slow, and
90	<	* because threads are typically not bound to CPUS forever, may
91	<	* not occur at all. However, despite these limitations, observed
92	<	* contention is typically low in these cases.
88	>	* colliding threads.  Because search is random, and collisions
89	>	* only become known via CAS failures, convergence can be slow,
90	>	* and because threads are typically not bound to CPUS forever,
91	>	* may not occur at all. However, despite these limitations,
92	>	* observed contention rates are typically low in these cases.
93		*
94	<	* Table entries are of class Adder; a form of AtomicLong padded
95	<	* to reduce cache contention on most processors. Padding is
96	<	* overkill for most Atomics because they are usually irregularly
97	<	* scattered in memory and thus don't interfere much with each
98	<	* other. But Atomic objects residing in arrays will tend to be
99	<	* placed adjacent to each other, and so will most often share
100	<	* cache lines without this precaution.  Adders are by default
101	<	* constructed upon first use, which further improves per-thread
102	<	* locality and helps reduce footprint.
94	>	* A single spinlock is used for initializing and resizing the
95	>	* table, as well as populating slots with new Cells.  There is no
96	>	* need for a blocking lock: Upon lock contention, threads try
97	>	* other slots (or the base) rather than blocking.  During these
98	>	* retries, there is increased contention and reduced locality,
99	>	* which is still better than alternatives.
100	>	*
101	>	* It is possible for a Cell to become unused when threads that
102	>	* once hashed to it terminate, as well as in the case where
103	>	* doubling the table causes no thread to hash to it under
104	>	* expanded mask.  We do not try to detect or remove such cells,
105	>	* under the assumption that for long-running adders, observed
106	>	* contention levels will recur, so the cells will eventually be
107	>	* needed again; and for short-lived ones, it does not matter.
108		*
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. After initialization, at
89	–	* least one slot exists, so retries will eventually find a
90	–	* candidate Adder.  During these retries, there is increased
91	–	* contention and reduced locality, which is still better than
92	–	* alternatives.
93	–	*/
94	–
95	–	/**
96	–	* Padded version of AtomicLong
109		*/
98	–	static final class Adder extends AtomicLong {
99	–	long p0, p1, p2, p3, p4, p5, p6, p7, p8, p9, pa, pb, pc, pd, pe;
100	–	Adder(long x) { super(x); }
101	–	}
110
111		private static final int NCPU = Runtime.getRuntime().availableProcessors();
112
113		/**
114	<	* Table bounds. DEFAULT_INITIAL_SIZE is the table size set upon
115	<	* first use under default constructor, and must be a power of
116	<	* two. There is not much point in making size a lot smaller than
117	<	* that of Adders though.  CAP is the maximum allowed table size.
118	<	*/
119	<	private static final int DEFAULT_INITIAL_SIZE = 8;
120	<	private static final int CAP = Math.max(NCPU, DEFAULT_INITIAL_SIZE);
114	>	* Padded variant of AtomicLong.  The value field is placed
115	>	* between pads, hoping that the JVM doesn't reorder them.
116	>	* Updates are via inlined CAS in methods add and retryAdd.
117	>	*/
118	>	static final class Cell {
119	>	volatile long p0, p1, p2, p3, p4, p5, p6;
120	>	volatile long value;
121	>	volatile long q0, q1, q2, q3, q4, q5, q6;
122	>	Cell(long x) { value = x; }
123	>	}
124
125		/**
126		* Holder for the thread-local hash code. The code is initially
#	Line 119 | Line 130 | public class StripedAdder implements Ser
130		static final Random rng = new Random();
131		int code;
132		HashCode() {
133	<	int h = rng.nextInt();
134	<	code = (h == 0) ? 1 : h; // ensure nonzero
133	>	int h = rng.nextInt(); // Avoid zero to allow xorShift rehash
134	>	code = (h == 0) ? 1 : h;
135		}
136		}
137
#	Line 140 | Line 151 | public class StripedAdder implements Ser
151		static final ThreadHashCode threadHashCode = new ThreadHashCode();
152
153		/**
154	<	* Table of adders. Size is power of two, grows to be at most CAP.
154	>	* Table of cells. When non-null, size is a power of 2.
155		*/
156	<	private transient volatile Adder[] adders;
156	>	private transient volatile Cell[] cells;
157
158		/**
159	<	* Serves as a lock when resizing and/or creating Adders.  There
160	<	* is no need for a blocking lock: Except during initialization
150	<	* races, when busy, other threads try other slots. However,
151	<	* during (double-checked) initializations, we use the
152	<	* "synchronized" lock on this object.
159	>	* Base sum, used mainly when there is no contention, but also as
160	>	* a fallback during table initializion races. Updated via CAS.
161		*/
162	<	private final AtomicInteger mutex;
162	>	private transient volatile long base;
163
164		/**
165	<	* Creates a new adder with zero sum.
165	>	* Spinlock (locked via CAS) used when resizing and/or creating Cells.
166		*/
167	<	public StripedAdder() {
160	<	this.mutex = new AtomicInteger();
161	<	// remaining initialization on first call to add.
162	<	}
167	>	private transient volatile int busy;
168
169		/**
170	<	* Creates a new adder with zero sum, and with stripes presized
171	<	* for the given expected contention level.
172	<	*
168	<	* @param expectedContention the expected number of threads that
169	<	* will concurrently update the sum.
170	<	*/
171	<	public StripedAdder(int expectedContention) {
172	<	int cap = (expectedContention < CAP) ? expectedContention : CAP;
173	<	int size = 1;
174	<	while (size < cap)
175	<	size <<= 1;
176	<	Adder[] as = new Adder[size];
177	<	for (int i = 0; i < size; ++i)
178	<	as[i] = new Adder(0);
179	<	this.adders = as;
180	<	this.mutex = new AtomicInteger();
170	>	* Creates a new adder with initial sum of zero.
171	>	*/
172	>	public StripedAdder() {
173		}
174
175		/**
#	Line 186 | Line 178 | public class StripedAdder implements Ser
178		* @param x the value to add
179		*/
180		public void add(long x) {
181	<	Adder[] as; Adder a; int n; long v; // locals to hold volatile reads
182	<	HashCode hc = threadHashCode.get();
183	<	int h = hc.code;
184	<	if ((as = adders) == null || (n = as.length) < 1 ||
185	<	(a = as[(n - 1) & h]) == null ||
186	<	!a.compareAndSet(v = a.get(), v + x))
187	<	retryAdd(x, hc);
181	>	Cell[] as; long v; HashCode hc; Cell a; int n; boolean contended;
182	>	if ((as = cells) != null ||
183	>	!UNSAFE.compareAndSwapLong(this, baseOffset, v = base, v + x)) {
184	>	int h = (hc = threadHashCode.get()).code;
185	>	if (as != null && (n = as.length) > 0 &&
186	>	(a = as[(n - 1) & h]) != null) {
187	>	if (UNSAFE.compareAndSwapLong(a, valueOffset,
188	>	v = a.value, v + x))
189	>	return;
190	>	contended = true;
191	>	}
192	>	else
193	>	contended = false;
194	>	retryAdd(x, hc, contended);
195	>	}
196		}
197
198		/**
199		* Handle cases of add involving initialization, resizing,
200	<	* creating new Adders, and/or contention. See above for
201	<	* explanation.
200	>	* creating new Cells, and/or contention. See above for
201	>	* explanation. This method suffers the usual non-modularity
202	>	* problems of optimistic retry code, relying on rechecked sets of
203	>	* reads.
204	>	*
205	>	* @param x the value to add
206	>	* @param hc the hash code holder
207	>	* @param precontended true if CAS failed before call
208		*/
209	<	private void retryAdd(long x, HashCode hc) {
209	>	private void retryAdd(long x, HashCode hc, boolean precontended) {
210		int h = hc.code;
211	<	final AtomicInteger mutex = this.mutex;
206	<	int collisions = 1 - mutex.get(); // first guess: collides if not locked
211	>	boolean collide = false; // true if last slot nonempty
212		for (;;) {
213	<	Adder[] as; Adder a; long v; int k, n;
214	<	while ((as = adders) == null || (n = as.length) < 1) {
215	<	synchronized(mutex) {                // Try to initialize
216	<	if (adders == null) {
217	<	Adder[] rs = new Adder[DEFAULT_INITIAL_SIZE];
218	<	rs[h & (DEFAULT_INITIAL_SIZE - 1)] = new Adder(0);
219	<	adders = rs;
213	>	Cell[] as; Cell a; int n;
214	>	if ((as = cells) != null && (n = as.length) > 0) {
215	>	if ((a = as[(n - 1) & h]) == null) {
216	>	if (busy == 0) {            // Try to attach new Cell
217	>	Cell r = new Cell(x);   // Optimistically create
218	>	if (busy == 0 &&
219	>	UNSAFE.compareAndSwapInt(this, busyOffset, 0, 1)) {
220	>	boolean created = false;
221	>	try {               // Recheck under lock
222	>	Cell[] rs; int m, j;
223	>	if ((rs = cells) != null &&
224	>	(m = rs.length) > 0 &&
225	>	rs[j = (m - 1) & h] == null) {
226	>	rs[j] = r;
227	>	created = true;
228	>	}
229	>	} finally {
230	>	busy = 0;
231	>	}
232	>	if (created)
233	>	break;
234	>	continue;           // Slot is now non-empty
235	>	}
236		}
237	+	collide = false;
238		}
239	<	collisions = 0;
240	<	}
241	<
242	<	if ((a = as[k = (n - 1) & h]) == null) { // Try to add slot
243	<	if (mutex.get() == 0 && mutex.compareAndSet(0, 1)) {
222	<	try {
223	<	if (adders == as && as[k] == null)
224	<	a = as[k] = new Adder(x);
225	<	} finally {
226	<	mutex.set(0);
227	<	}
228	<	if (a != null)
239	>	else if (precontended)          // CAS already known to fail
240	>	precontended = false;       // Continue after rehash
241	>	else {
242	>	long v = a.value;
243	>	if (UNSAFE.compareAndSwapLong(a, valueOffset, v, v + x))
244		break;
245	+	if (!collide)
246	+	collide = true;
247	+	else if (n >= NCPU || cells != as)
248	+	collide = false;        // Can't expand
249	+	else if (busy == 0 &&
250	+	UNSAFE.compareAndSwapInt(this, busyOffset, 0, 1)) {
251	+	collide = false;
252	+	try {
253	+	if (cells == as) { // Expand table
254	+	Cell[] rs = new Cell[n << 1];
255	+	for (int i = 0; i < n; ++i)
256	+	rs[i] = as[i];
257	+	cells = rs;
258	+	}
259	+	} finally {
260	+	busy = 0;
261	+	}
262	+	continue;
263	+	}
264		}
265	<	collisions = 0;
265	>	h ^= h << 13;                   // Rehash
266	>	h ^= h >>> 17;
267	>	h ^= h << 5;
268		}
269	<	else if (collisions != 0 && n < CAP &&   // Try to expand table
270	<	mutex.get() == 0 && mutex.compareAndSet(0, 1)) {
271	<	try {
272	<	if (adders == as) {
273	<	Adder[] rs = new Adder[n << 1];
274	<	for (int i = 0; i < n; ++i)
275	<	rs[i] = as[i];
276	<	adders = rs;
269	>	else if (busy == 0 && cells == as &&
270	>	UNSAFE.compareAndSwapInt(this, busyOffset, 0, 1)) {
271	>	boolean init = false;
272	>	try {                           // Initialize
273	>	if (cells == as) {
274	>	Cell r = new Cell(x);
275	>	Cell[] rs = new Cell[2];
276	>	rs[h & 1] = r;
277	>	cells = rs;
278	>	init = true;
279		}
280		} finally {
281	<	mutex.set(0);
281	>	busy = 0;
282		}
283	<	collisions = 0;
283	>	if (init)
284	>	break;
285	>	}
286	>	else {                              // Lost initialization race
287	>	long b = base;                  // Fall back on using base
288	>	if (UNSAFE.compareAndSwapLong(this, baseOffset, b, b + x))
289	>	break;
290		}
247	–	else if (a.compareAndSet(v = a.get(), v + x))
248	–	break;
249	–	else
250	–	collisions = 1;
251	–	h ^= h << 13;                            // Rehash
252	–	h ^= h >>> 17;
253	–	h ^= h << 5;
291		}
292	<	hc.code = h;
292	>	hc.code = h;                            // Record index for next time
293		}
294
295		/**
296	<	* Returns an estimate of the current sum.  The result is
297	<	* calculated by summing multiple variables, so may not be
298	<	* accurate if updates occur concurrently with this method.
296	>	* Equivalent to {@code add(1)}.
297	>	*/
298	>	public void increment() {
299	>	add(1L);
300	>	}
301	>
302	>	/**
303	>	* Equivalent to {@code add(-1)}.
304	>	*/
305	>	public void decrement() {
306	>	add(-1L);
307	>	}
308	>
309	>	/**
310	>	* Returns the current sum.  The result is only guaranteed to be
311	>	* accurate in the absence of concurrent updates. Otherwise, it
312	>	* may fail to reflect one or more updates occuring while
313	>	* calculating the result.
314		*
315	<	* @return the estimated sum
315	>	* @return the sum
316		*/
317		public long sum() {
318	<	long sum = 0L;
319	<	Adder[] as = adders;
318	>	Cell[] as = cells;
319	>	long sum = base;
320		if (as != null) {
321		int n = as.length;
322		for (int i = 0; i < n; ++i) {
323	<	Adder a = as[i];
323	>	Cell a = as[i];
324		if (a != null)
325	<	sum += a.get();
325	>	sum += a.value;
326		}
327		}
328		return sum;
329		}
330
331		/**
332	<	* Resets each of the variables to zero. This is effective in
333	<	* fully resetting the sum only if there are no concurrent
334	<	* updates.
332	>	* Resets variables maintaining the sum to zero.  This is
333	>	* effective in setting the sum to zero only if there are no
334	>	* concurrent updates.
335		*/
336		public void reset() {
337	<	Adder[] as = adders;
337	>	Cell[] as = cells;
338	>	base = 0L;
339		if (as != null) {
340		int n = as.length;
341		for (int i = 0; i < n; ++i) {
342	<	Adder a = as[i];
342	>	Cell a = as[i];
343		if (a != null)
344	<	a.set(0L);
344	>	a.value = 0L;
345		}
346		}
347		}
348
349		/**
350	<	* Equivalent to {@code add(1)}.
351	<	*/
352	<	public void increment() {
353	<	add(1L);
354	<	}
355	<
303	<	/**
304	<	* Equivalent to {@code add(-1)}.
305	<	*/
306	<	public void decrement() {
307	<	add(-1L);
308	<	}
309	<
310	<	/**
311	<	* Equivalent to {@link #sum} followed by {@link #reset}.
350	>	* Equivalent in effect to {@link #sum} followed by {@link
351	>	* #reset}. This method may apply for example during quiescent
352	>	* points between multithreaded computations.  If there are
353	>	* updates concurrent with this method, the returned value is
354	>	* <em>not</em> guaranteed to be the final sum occurring before
355	>	* the reset.
356		*
357	<	* @return the estimated sum
357	>	* @return the sum
358		*/
359	<	public long sumAndReset() {
360	<	long sum = 0L;
361	<	Adder[] as = adders;
359	>	public long sumThenReset() {
360	>	Cell[] as = cells;
361	>	long sum = base;
362	>	base = 0L;
363		if (as != null) {
364		int n = as.length;
365		for (int i = 0; i < n; ++i) {
366	<	Adder a = as[i];
366	>	Cell a = as[i];
367		if (a != null) {
368	<	sum += a.get();
369	<	a.set(0L);
368	>	sum += a.value;
369	>	a.value = 0L;
370		}
371		}
372		}
#	Line 337 | Line 382 | public class StripedAdder implements Ser
382		private void readObject(ObjectInputStream s)
383		throws IOException, ClassNotFoundException {
384		s.defaultReadObject();
385	<	mutex.set(0);
386	<	add(s.readLong());
385	>	busy = 0;
386	>	cells = null;
387	>	base = s.readLong();
388	>	}
389	>
390	>	// Unsafe mechanics
391	>	private static final sun.misc.Unsafe UNSAFE;
392	>	private static final long baseOffset;
393	>	private static final long busyOffset;
394	>	private static final long valueOffset;
395	>	static {
396	>	try {
397	>	UNSAFE = getUnsafe();
398	>	Class<?> sk = StripedAdder.class;
399	>	baseOffset = UNSAFE.objectFieldOffset
400	>	(sk.getDeclaredField("base"));
401	>	busyOffset = UNSAFE.objectFieldOffset
402	>	(sk.getDeclaredField("busy"));
403	>	Class<?> ak = Cell.class;
404	>	valueOffset = UNSAFE.objectFieldOffset
405	>	(ak.getDeclaredField("value"));
406	>	} catch (Exception e) {
407	>	throw new Error(e);
408	>	}
409	>	}
410	>
411	>	/**
412	>	* Returns a sun.misc.Unsafe.  Suitable for use in a 3rd party package.
413	>	* Replace with a simple call to Unsafe.getUnsafe when integrating
414	>	* into a jdk.
415	>	*
416	>	* @return a sun.misc.Unsafe
417	>	*/
418	>	private static sun.misc.Unsafe getUnsafe() {
419	>	try {
420	>	return sun.misc.Unsafe.getUnsafe();
421	>	} catch (SecurityException se) {
422	>	try {
423	>	return java.security.AccessController.doPrivileged
424	>	(new java.security
425	>	.PrivilegedExceptionAction<sun.misc.Unsafe>() {
426	>	public sun.misc.Unsafe run() throws Exception {
427	>	java.lang.reflect.Field f = sun.misc
428	>	.Unsafe.class.getDeclaredField("theUnsafe");
429	>	f.setAccessible(true);
430	>	return (sun.misc.Unsafe) f.get(null);
431	>	}});
432	>	} catch (java.security.PrivilegedActionException e) {
433	>	throw new RuntimeException("Could not initialize intrinsics",
434	>	e.getCause());
435	>	}
436	>	}
437		}
438
439		}

Diff Legend

–	Removed lines
+	Added lines
<	Changed lines
>	Changed lines