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 |
1.4 |
* 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 |
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 |
|
|
* 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 |
dl |
1.6 |
* 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 |
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 |
1.8 |
* Table of adders. When non-null, size is a power of 2, at least 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 |
|
|
int size = 2; |
164 |
|
|
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 |
1.8 |
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 |
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 |
1.4 |
*/ |
201 |
dl |
1.8 |
private void retryAdd(long x, HashCode hc, boolean collide) { |
202 |
dl |
1.4 |
int h = hc.code; |
203 |
dl |
1.6 |
for (;;) { |
204 |
dl |
1.8 |
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 |
|
|
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 |
dl |
1.6 |
} |
249 |
dl |
1.8 |
collide = false; |
250 |
dl |
1.6 |
} |
251 |
dl |
1.8 |
h ^= h << 13; // Rehash |
252 |
|
|
h ^= h >>> 17; |
253 |
|
|
h ^= h << 5; |
254 |
dl |
1.6 |
} |
255 |
dl |
1.8 |
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 |
dl |
1.5 |
try { |
263 |
dl |
1.8 |
if (adders == as) { |
264 |
|
|
adders = rs; |
265 |
|
|
init = true; |
266 |
|
|
} |
267 |
dl |
1.5 |
} finally { |
268 |
dl |
1.8 |
busy = 0; |
269 |
dl |
1.5 |
} |
270 |
dl |
1.8 |
if (init) |
271 |
dl |
1.6 |
break; |
272 |
dl |
1.1 |
} |
273 |
dl |
1.5 |
} |
274 |
dl |
1.8 |
else if (adders == as) // Lost initialization race |
275 |
|
|
Thread.yield(); |
276 |
dl |
1.1 |
} |
277 |
dl |
1.8 |
hc.code = h; // Record index for next time |
278 |
dl |
1.1 |
} |
279 |
|
|
|
280 |
|
|
/** |
281 |
|
|
* Returns an estimate of the current sum. The result is |
282 |
|
|
* calculated by summing multiple variables, so may not be |
283 |
|
|
* accurate if updates occur concurrently with this method. |
284 |
jsr166 |
1.2 |
* |
285 |
dl |
1.1 |
* @return the estimated sum |
286 |
|
|
*/ |
287 |
|
|
public long sum() { |
288 |
dl |
1.4 |
long sum = 0L; |
289 |
dl |
1.1 |
Adder[] as = adders; |
290 |
dl |
1.4 |
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 |
dl |
1.8 |
sum += a.value; |
296 |
dl |
1.4 |
} |
297 |
dl |
1.1 |
} |
298 |
|
|
return sum; |
299 |
|
|
} |
300 |
|
|
|
301 |
|
|
/** |
302 |
|
|
* Resets each of the variables to zero. This is effective in |
303 |
|
|
* fully resetting the sum only if there are no concurrent |
304 |
|
|
* updates. |
305 |
|
|
*/ |
306 |
|
|
public void reset() { |
307 |
|
|
Adder[] as = adders; |
308 |
dl |
1.4 |
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 |
dl |
1.8 |
a.value = 0L; |
314 |
dl |
1.4 |
} |
315 |
dl |
1.1 |
} |
316 |
|
|
} |
317 |
|
|
|
318 |
|
|
/** |
319 |
|
|
* Equivalent to {@code add(1)}. |
320 |
|
|
*/ |
321 |
|
|
public void increment() { |
322 |
|
|
add(1L); |
323 |
|
|
} |
324 |
|
|
|
325 |
|
|
/** |
326 |
|
|
* Equivalent to {@code add(-1)}. |
327 |
|
|
*/ |
328 |
|
|
public void decrement() { |
329 |
|
|
add(-1L); |
330 |
|
|
} |
331 |
|
|
|
332 |
|
|
/** |
333 |
|
|
* Equivalent to {@link #sum} followed by {@link #reset}. |
334 |
|
|
* |
335 |
|
|
* @return the estimated sum |
336 |
|
|
*/ |
337 |
|
|
public long sumAndReset() { |
338 |
dl |
1.4 |
long sum = 0L; |
339 |
dl |
1.1 |
Adder[] as = adders; |
340 |
dl |
1.4 |
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 |
dl |
1.8 |
sum += a.value; |
346 |
|
|
a.value = 0L; |
347 |
dl |
1.4 |
} |
348 |
dl |
1.1 |
} |
349 |
|
|
} |
350 |
|
|
return sum; |
351 |
|
|
} |
352 |
|
|
|
353 |
|
|
private void writeObject(java.io.ObjectOutputStream s) |
354 |
|
|
throws java.io.IOException { |
355 |
|
|
s.defaultWriteObject(); |
356 |
|
|
s.writeLong(sum()); |
357 |
|
|
} |
358 |
|
|
|
359 |
|
|
private void readObject(ObjectInputStream s) |
360 |
|
|
throws IOException, ClassNotFoundException { |
361 |
|
|
s.defaultReadObject(); |
362 |
dl |
1.8 |
busy = 0; |
363 |
dl |
1.4 |
add(s.readLong()); |
364 |
dl |
1.1 |
} |
365 |
|
|
|
366 |
dl |
1.8 |
// 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 |
dl |
1.1 |
} |