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 /*
8 * Source:
9 * http://gee.cs.oswego.edu/cgi-bin/viewcvs.cgi/jsr166/src/jsr166e/Striped64.java?revision=1.9
10 */
11
12 package com.google.common.cache;
13
14 import com.google.common.annotations.GwtIncompatible;
15 import java.util.Random;
16 import org.checkerframework.checker.nullness.qual.Nullable;
17
18 /**
19 * A package-local class holding common representation and mechanics for classes supporting dynamic
20 * striping on 64bit values. The class extends Number so that concrete subclasses must publicly do
21 * so.
22 */
23 @GwtIncompatible
24 abstract class Striped64 extends Number {
25 /*
26 * This class maintains a lazily-initialized table of atomically
27 * updated variables, plus an extra "base" field. The table size
28 * is a power of two. Indexing uses masked per-thread hash codes.
29 * Nearly all declarations in this class are package-private,
30 * accessed directly by subclasses.
31 *
32 * Table entries are of class Cell; a variant of AtomicLong padded
33 * to reduce cache contention on most processors. Padding is
34 * overkill for most Atomics because they are usually irregularly
35 * scattered in memory and thus don't interfere much with each
36 * other. But Atomic objects residing in arrays will tend to be
37 * placed adjacent to each other, and so will most often share
38 * cache lines (with a huge negative performance impact) without
39 * this precaution.
40 *
41 * In part because Cells are relatively large, we avoid creating
42 * them until they are needed. When there is no contention, all
43 * updates are made to the base field. Upon first contention (a
44 * failed CAS on base update), the table is initialized to size 2.
45 * The table size is doubled upon further contention until
46 * reaching the nearest power of two greater than or equal to the
47 * number of CPUS. Table slots remain empty (null) until they are
48 * needed.
49 *
50 * A single spinlock ("busy") is used for initializing and
51 * resizing the table, as well as populating slots with new Cells.
52 * There is no need for a blocking lock; when the lock is not
53 * available, threads try other slots (or the base). During these
54 * retries, there is increased contention and reduced locality,
55 * which is still better than alternatives.
56 *
57 * Per-thread hash codes are initialized to random values.
58 * Contention and/or table collisions are indicated by failed
59 * CASes when performing an update operation (see method
60 * retryUpdate). Upon a collision, if the table size is less than
61 * the capacity, it is doubled in size unless some other thread
62 * holds the lock. If a hashed slot is empty, and lock is
63 * available, a new Cell is created. Otherwise, if the slot
64 * exists, a CAS is tried. Retries proceed by "double hashing",
65 * using a secondary hash (Marsaglia XorShift) to try to find a
66 * free slot.
67 *
68 * The table size is capped because, when there are more threads
69 * than CPUs, supposing that each thread were bound to a CPU,
70 * there would exist a perfect hash function mapping threads to
71 * slots that eliminates collisions. When we reach capacity, we
72 * search for this mapping by randomly varying the hash codes of
73 * colliding threads. Because search is random, and collisions
74 * only become known via CAS failures, convergence can be slow,
75 * and because threads are typically not bound to CPUS forever,
76 * may not occur at all. However, despite these limitations,
77 * observed contention rates are typically low in these cases.
78 *
79 * It is possible for a Cell to become unused when threads that
80 * once hashed to it terminate, as well as in the case where
81 * doubling the table causes no thread to hash to it under
82 * expanded mask. We do not try to detect or remove such cells,
83 * under the assumption that for long-running instances, observed
84 * contention levels will recur, so the cells will eventually be
85 * needed again; and for short-lived ones, it does not matter.
86 */
87
88 /**
89 * Padded variant of AtomicLong supporting only raw accesses plus CAS. The value field is placed
90 * between pads, hoping that the JVM doesn't reorder them.
91 *
92 * <p>JVM intrinsics note: It would be possible to use a release-only form of CAS here, if it were
93 * provided.
94 */
95 static final class Cell {
96 volatile long p0, p1, p2, p3, p4, p5, p6;
97 volatile long value;
98 volatile long q0, q1, q2, q3, q4, q5, q6;
99
100 Cell(long x) {
101 value = x;
102 }
103
104 final boolean cas(long cmp, long val) {
105 return UNSAFE.compareAndSwapLong(this, valueOffset, cmp, val);
106 }
107
108 // Unsafe mechanics
109 private static final sun.misc.Unsafe UNSAFE;
110 private static final long valueOffset;
111
112 static {
113 try {
114 UNSAFE = getUnsafe();
115 Class<?> ak = Cell.class;
116 valueOffset = UNSAFE.objectFieldOffset(ak.getDeclaredField("value"));
117 } catch (Exception e) {
118 throw new Error(e);
119 }
120 }
121 }
122
123 /**
124 * ThreadLocal holding a single-slot int array holding hash code. Unlike the JDK8 version of this
125 * class, we use a suboptimal int[] representation to avoid introducing a new type that can impede
126 * class-unloading when ThreadLocals are not removed.
127 */
128 static final ThreadLocal<int[]> threadHashCode = new ThreadLocal<>();
129
130 /** Generator of new random hash codes */
131 static final Random rng = new Random();
132
133 /** Number of CPUS, to place bound on table size */
134 static final int NCPU = Runtime.getRuntime().availableProcessors();
135
136 /** Table of cells. When non-null, size is a power of 2. */
137 transient volatile Cell @Nullable [] cells;
138
139 /**
140 * Base value, used mainly when there is no contention, but also as a fallback during table
141 * initialization races. Updated via CAS.
142 */
143 transient volatile long base;
144
145 /** Spinlock (locked via CAS) used when resizing and/or creating Cells. */
146 transient volatile int busy;
147
148 /** Package-private default constructor */
149 Striped64() {}
150
151 /** CASes the base field. */
152 final boolean casBase(long cmp, long val) {
153 return UNSAFE.compareAndSwapLong(this, baseOffset, cmp, val);
154 }
155
156 /** CASes the busy field from 0 to 1 to acquire lock. */
157 final boolean casBusy() {
158 return UNSAFE.compareAndSwapInt(this, busyOffset, 0, 1);
159 }
160
161 /**
162 * Computes the function of current and new value. Subclasses should open-code this update
163 * function for most uses, but the virtualized form is needed within retryUpdate.
164 *
165 * @param currentValue the current value (of either base or a cell)
166 * @param newValue the argument from a user update call
167 * @return result of the update function
168 */
169 abstract long fn(long currentValue, long newValue);
170
171 /**
172 * Handles cases of updates involving initialization, resizing, creating new Cells, and/or
173 * contention. See above for explanation. This method suffers the usual non-modularity problems of
174 * optimistic retry code, relying on rechecked sets of reads.
175 *
176 * @param x the value
177 * @param hc the hash code holder
178 * @param wasUncontended false if CAS failed before call
179 */
180 final void retryUpdate(long x, int[] hc, boolean wasUncontended) {
181 int h;
182 if (hc == null) {
183 threadHashCode.set(hc = new int[1]); // Initialize randomly
184 int r = rng.nextInt(); // Avoid zero to allow xorShift rehash
185 h = hc[0] = (r == 0) ? 1 : r;
186 } else h = hc[0];
187 boolean collide = false; // True if last slot nonempty
188 for (; ; ) {
189 Cell[] as;
190 Cell a;
191 int n;
192 long v;
193 if ((as = cells) != null && (n = as.length) > 0) {
194 if ((a = as[(n - 1) & h]) == null) {
195 if (busy == 0) { // Try to attach new Cell
196 Cell r = new Cell(x); // Optimistically create
197 if (busy == 0 && casBusy()) {
198 boolean created = false;
199 try { // Recheck under lock
200 Cell[] rs;
201 int m, j;
202 if ((rs = cells) != null && (m = rs.length) > 0 && rs[j = (m - 1) & h] == null) {
203 rs[j] = r;
204 created = true;
205 }
206 } finally {
207 busy = 0;
208 }
209 if (created) break;
210 continue; // Slot is now non-empty
211 }
212 }
213 collide = false;
214 } else if (!wasUncontended) // CAS already known to fail
215 wasUncontended = true; // Continue after rehash
216 else if (a.cas(v = a.value, fn(v, x))) break;
217 else if (n >= NCPU || cells != as) collide = false; // At max size or stale
218 else if (!collide) collide = true;
219 else if (busy == 0 && casBusy()) {
220 try {
221 if (cells == as) { // Expand table unless stale
222 Cell[] rs = new Cell[n << 1];
223 for (int i = 0; i < n; ++i) rs[i] = as[i];
224 cells = rs;
225 }
226 } finally {
227 busy = 0;
228 }
229 collide = false;
230 continue; // Retry with expanded table
231 }
232 h ^= h << 13; // Rehash
233 h ^= h >>> 17;
234 h ^= h << 5;
235 hc[0] = h; // Record index for next time
236 } else if (busy == 0 && cells == as && casBusy()) {
237 boolean init = false;
238 try { // Initialize table
239 if (cells == as) {
240 Cell[] rs = new Cell[2];
241 rs[h & 1] = new Cell(x);
242 cells = rs;
243 init = true;
244 }
245 } finally {
246 busy = 0;
247 }
248 if (init) break;
249 } else if (casBase(v = base, fn(v, x))) break; // Fall back on using base
250 }
251 }
252
253 /** Sets base and all cells to the given value. */
254 final void internalReset(long initialValue) {
255 Cell[] as = cells;
256 base = initialValue;
257 if (as != null) {
258 int n = as.length;
259 for (int i = 0; i < n; ++i) {
260 Cell a = as[i];
261 if (a != null) a.value = initialValue;
262 }
263 }
264 }
265
266 // Unsafe mechanics
267 private static final sun.misc.Unsafe UNSAFE;
268 private static final long baseOffset;
269 private static final long busyOffset;
270
271 static {
272 try {
273 UNSAFE = getUnsafe();
274 Class<?> sk = Striped64.class;
275 baseOffset = UNSAFE.objectFieldOffset(sk.getDeclaredField("base"));
276 busyOffset = UNSAFE.objectFieldOffset(sk.getDeclaredField("busy"));
277 } catch (Exception e) {
278 throw new Error(e);
279 }
280 }
281
282 /**
283 * Returns a sun.misc.Unsafe. Suitable for use in a 3rd party package. Replace with a simple call
284 * to Unsafe.getUnsafe when integrating into a jdk.
285 *
286 * @return a sun.misc.Unsafe
287 */
288 private static sun.misc.Unsafe getUnsafe() {
289 try {
290 return sun.misc.Unsafe.getUnsafe();
291 } catch (SecurityException tryReflectionInstead) {
292 }
293 try {
294 return java.security.AccessController.doPrivileged(
295 new java.security.PrivilegedExceptionAction<sun.misc.Unsafe>() {
296 @Override
297 public sun.misc.Unsafe run() throws Exception {
298 Class<sun.misc.Unsafe> k = sun.misc.Unsafe.class;
299 for (java.lang.reflect.Field f : k.getDeclaredFields()) {
300 f.setAccessible(true);
301 Object x = f.get(null);
302 if (k.isInstance(x)) return k.cast(x);
303 }
304 throw new NoSuchFieldError("the Unsafe");
305 }
306 });
307 } catch (java.security.PrivilegedActionException e) {
308 throw new RuntimeException("Could not initialize intrinsics", e.getCause());
309 }
310 }
311 }