| 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; |
| 43 |
|
|
| 44 |
|
/* |
| 45 |
|
* A StripedAdder maintains a table of Atomic long variables. The |
| 46 |
< |
* table is indexed by per-thread hash codes that are initialized |
| 48 |
< |
* to random values. |
| 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 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. Adders are |
| 55 |
< |
* constructed upon first use, which further improves per-thread |
| 56 |
< |
* locality and helps reduce (an 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 |
| 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 |
| 91 |
> |
* candidate Adder. During these retries, there is increased |
| 92 |
|
* contention and reduced locality, which is still better than |
| 93 |
|
* alternatives. |
| 94 |
|
*/ |
| 95 |
|
|
| 96 |
< |
/** |
| 84 |
< |
* Number of processors, to place a cap on table growth. |
| 85 |
< |
*/ |
| 86 |
< |
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 value upon |
| 99 |
< |
* collisions in retryAdd. |
| 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(); |
| 111 |
< |
public HashCode initialValue() { |
| 112 |
< |
int h = rng.nextInt(); |
| 113 |
< |
return new HashCode((h == 0) ? 1 : h); // ensure nonzero |
| 114 |
< |
} |
| 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 |
| 131 |
< |
* is no need for a blocking lock: When busy, other threads try |
| 132 |
< |
* other slots. |
| 144 |
> |
* Spinlock (locked via CAS) used when resizing and/or creating Adders. |
| 145 |
|
*/ |
| 146 |
< |
private final AtomicInteger mutex; |
| 146 |
> |
private volatile int busy; |
| 147 |
|
|
| 148 |
|
/** |
| 149 |
|
* Creates a new adder with zero sum. |
| 150 |
|
*/ |
| 151 |
|
public StripedAdder() { |
| 140 |
– |
this.mutex = new AtomicInteger(); |
| 141 |
– |
// remaining initialization on first call to add. |
| 152 |
|
} |
| 153 |
|
|
| 154 |
|
/** |
| 167 |
|
for (int i = 0; i < size; ++i) |
| 168 |
|
as[i] = new Adder(0); |
| 169 |
|
this.adders = as; |
| 160 |
– |
this.mutex = new AtomicInteger(); |
| 170 |
|
} |
| 171 |
|
|
| 172 |
|
/** |
| 175 |
|
* @param x the value to add |
| 176 |
|
*/ |
| 177 |
|
public void add(long x) { |
| 178 |
< |
Adder[] as; Adder a; int n; long v; // locals to hold volatile reads |
| 178 |
> |
Adder[] as; Adder a; int n; // locals to hold volatile reads |
| 179 |
|
HashCode hc = threadHashCode.get(); |
| 180 |
< |
if ((as = adders) == null || (n = as.length) < 1 || |
| 181 |
< |
(a = as[hc.code & (n - 1)]) == null || |
| 182 |
< |
!a.compareAndSet(v = a.get(), v + x)) |
| 183 |
< |
retryAdd(x, hc); |
| 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. |
| 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) { |
| 201 |
> |
private void retryAdd(long x, HashCode hc, boolean collide) { |
| 202 |
|
int h = hc.code; |
| 203 |
< |
final AtomicInteger mutex = this.mutex; |
| 204 |
< |
AtomicInteger lock = null; // nonnull when held |
| 205 |
< |
try { |
| 206 |
< |
for (;;) { |
| 207 |
< |
Adder[] as; Adder a; long v; int n, k; // locals for volatiles |
| 208 |
< |
boolean needLock = true; |
| 209 |
< |
if ((as = adders) == null || (n = as.length) < 1) { |
| 210 |
< |
if (lock != null) // default-initialize |
| 211 |
< |
adders = new Adder[2]; |
| 212 |
< |
} |
| 213 |
< |
else if ((a = as[k = h & (n - 1)]) == null) { |
| 214 |
< |
if (lock != null) { // attach new adder |
| 215 |
< |
as[k] = new Adder(x); |
| 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 |
+ |
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 |
+ |
collide = false; |
| 250 |
|
} |
| 251 |
< |
else if (a.compareAndSet(v = a.get(), v + x)) |
| 252 |
< |
break; |
| 253 |
< |
else if (n >= NCPU) // cannot expand |
| 254 |
< |
needLock = false; |
| 255 |
< |
else if (lock != null) // expand table |
| 256 |
< |
adders = Arrays.copyOf(as, n << 1); |
| 257 |
< |
|
| 258 |
< |
if (lock == null) { |
| 259 |
< |
if (needLock && mutex.get() == 0 && |
| 260 |
< |
mutex.compareAndSet(0, 1)) |
| 261 |
< |
lock = mutex; |
| 262 |
< |
else { // try elsewhere |
| 263 |
< |
h ^= h << 13; // Marsaglia XorShift |
| 264 |
< |
h ^= h >>> 17; |
| 265 |
< |
h ^= h << 5; |
| 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 |
< |
} finally { |
| 275 |
< |
if (lock != null) |
| 219 |
< |
lock.set(0); |
| 274 |
> |
else if (adders == as) // Lost initialization race |
| 275 |
> |
Thread.yield(); |
| 276 |
|
} |
| 277 |
< |
if (hc.code != h) // avoid unneeded writes |
| 222 |
< |
hc.code = h; |
| 277 |
> |
hc.code = h; // Record index for next time |
| 278 |
|
} |
| 279 |
|
|
| 280 |
|
/** |
| 292 |
|
for (int i = 0; i < n; ++i) { |
| 293 |
|
Adder a = as[i]; |
| 294 |
|
if (a != null) |
| 295 |
< |
sum += a.get(); |
| 295 |
> |
sum += a.value; |
| 296 |
|
} |
| 297 |
|
} |
| 298 |
|
return sum; |
| 310 |
|
for (int i = 0; i < n; ++i) { |
| 311 |
|
Adder a = as[i]; |
| 312 |
|
if (a != null) |
| 313 |
< |
a.set(0L); |
| 313 |
> |
a.value = 0L; |
| 314 |
|
} |
| 315 |
|
} |
| 316 |
|
} |
| 342 |
|
for (int i = 0; i < n; ++i) { |
| 343 |
|
Adder a = as[i]; |
| 344 |
|
if (a != null) { |
| 345 |
< |
sum += a.get(); |
| 346 |
< |
a.set(0L); |
| 345 |
> |
sum += a.value; |
| 346 |
> |
a.value = 0L; |
| 347 |
|
} |
| 348 |
|
} |
| 349 |
|
} |
| 359 |
|
private void readObject(ObjectInputStream s) |
| 360 |
|
throws IOException, ClassNotFoundException { |
| 361 |
|
s.defaultReadObject(); |
| 362 |
< |
mutex.set(0); |
| 362 |
> |
busy = 0; |
| 363 |
|
add(s.readLong()); |
| 364 |
|
} |
| 365 |
|
|
| 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 |
|
} |
