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Diffstat (limited to 'guava/src/com/google/common/cache/Striped64.java')
| -rw-r--r-- | guava/src/com/google/common/cache/Striped64.java | 347 |
1 files changed, 0 insertions, 347 deletions
diff --git a/guava/src/com/google/common/cache/Striped64.java b/guava/src/com/google/common/cache/Striped64.java deleted file mode 100644 index e045453..0000000 --- a/guava/src/com/google/common/cache/Striped64.java +++ /dev/null @@ -1,347 +0,0 @@ -/* - * Written by Doug Lea with assistance from members of JCP JSR-166 - * Expert Group and released to the public domain, as explained at - * http://creativecommons.org/publicdomain/zero/1.0/ - */ - -/* - * Source: - * http://gee.cs.oswego.edu/cgi-bin/viewcvs.cgi/jsr166/src/jsr166e/Striped64.java?revision=1.7 - */ - -package com.google.common.cache; - -import java.util.Random; - -/** - * A package-local class holding common representation and mechanics - * for classes supporting dynamic striping on 64bit values. The class - * extends Number so that concrete subclasses must publicly do so. - */ -abstract class Striped64 extends Number { - /* - * This class maintains a lazily-initialized table of atomically - * updated variables, plus an extra "base" field. The table size - * is a power of two. Indexing uses masked per-thread hash codes. - * Nearly all declarations in this class are package-private, - * accessed directly by subclasses. - * - * Table entries are of class Cell; a variant of AtomicLong padded - * to reduce cache contention on most processors. Padding is - * overkill for most Atomics because they are usually irregularly - * scattered in memory and thus don't interfere much with each - * other. But Atomic objects residing in arrays will tend to be - * placed adjacent to each other, and so will most often share - * cache lines (with a huge negative performance impact) without - * this precaution. - * - * In part because Cells are relatively large, we avoid creating - * them until they are needed. When there is no contention, all - * updates are made to the base field. Upon first contention (a - * failed CAS on base update), the table is initialized to size 2. - * The table size is doubled upon further contention until - * reaching the nearest power of two greater than or equal to the - * number of CPUS. Table slots remain empty (null) until they are - * needed. - * - * A single spinlock ("busy") is used for initializing and - * resizing the table, as well as populating slots with new Cells. - * There is no need for a blocking lock: When the lock is not - * available, threads try other slots (or the base). During these - * retries, there is increased contention and reduced locality, - * which is still better than alternatives. - * - * Per-thread hash codes are initialized to random values. - * Contention and/or table collisions are indicated by failed - * CASes when performing an update operation (see method - * retryUpdate). Upon a collision, if the table size is less than - * the capacity, it is doubled in size unless some other thread - * holds the lock. If a hashed slot is empty, and lock is - * available, a new Cell is created. Otherwise, if the slot - * exists, a CAS is tried. Retries proceed by "double hashing", - * using a secondary hash (Marsaglia XorShift) to try to find a - * free slot. - * - * The table size is capped because, when there are more threads - * than CPUs, supposing that each thread were bound to a CPU, - * there would exist a perfect hash function mapping threads to - * slots that eliminates collisions. When we reach capacity, we - * search for this mapping by randomly varying the hash codes of - * colliding threads. Because search is random, and collisions - * only become known via CAS failures, convergence can be slow, - * and because threads are typically not bound to CPUS forever, - * may not occur at all. However, despite these limitations, - * observed contention rates are typically low in these cases. - * - * It is possible for a Cell to become unused when threads that - * once hashed to it terminate, as well as in the case where - * doubling the table causes no thread to hash to it under - * expanded mask. We do not try to detect or remove such cells, - * under the assumption that for long-running instances, observed - * contention levels will recur, so the cells will eventually be - * needed again; and for short-lived ones, it does not matter. - */ - - /** - * Padded variant of AtomicLong supporting only raw accesses plus CAS. - * The value field is placed between pads, hoping that the JVM doesn't - * reorder them. - * - * JVM intrinsics note: It would be possible to use a release-only - * form of CAS here, if it were provided. - */ - static final class Cell { - volatile long p0, p1, p2, p3, p4, p5, p6; - volatile long value; - volatile long q0, q1, q2, q3, q4, q5, q6; - Cell(long x) { value = x; } - - final boolean cas(long cmp, long val) { - return UNSAFE.compareAndSwapLong(this, valueOffset, cmp, val); - } - - // Unsafe mechanics - private static final sun.misc.Unsafe UNSAFE; - private static final long valueOffset; - static { - try { - UNSAFE = getUnsafe(); - Class<?> ak = Cell.class; - valueOffset = UNSAFE.objectFieldOffset - (ak.getDeclaredField("value")); - } catch (Exception e) { - throw new Error(e); - } - } - - } - - /** - * Holder for the thread-local hash code. The code is initially - * random, but may be set to a different value upon collisions. - */ - static final class HashCode { - static final Random rng = new Random(); - int code; - HashCode() { - int h = rng.nextInt(); // Avoid zero to allow xorShift rehash - code = (h == 0) ? 1 : h; - } - } - - /** - * The corresponding ThreadLocal class - */ - static final class ThreadHashCode extends ThreadLocal<HashCode> { - public HashCode initialValue() { return new HashCode(); } - } - - /** - * Static per-thread hash codes. Shared across all instances to - * reduce ThreadLocal pollution and because adjustments due to - * collisions in one table are likely to be appropriate for - * others. - */ - static final ThreadHashCode threadHashCode = new ThreadHashCode(); - - /** Number of CPUS, to place bound on table size */ - static final int NCPU = Runtime.getRuntime().availableProcessors(); - - /** - * Table of cells. When non-null, size is a power of 2. - */ - transient volatile Cell[] cells; - - /** - * Base value, used mainly when there is no contention, but also as - * a fallback during table initialization races. Updated via CAS. - */ - transient volatile long base; - - /** - * Spinlock (locked via CAS) used when resizing and/or creating Cells. - */ - transient volatile int busy; - - /** - * Package-private default constructor - */ - Striped64() { - } - - /** - * CASes the base field. - */ - final boolean casBase(long cmp, long val) { - return UNSAFE.compareAndSwapLong(this, baseOffset, cmp, val); - } - - /** - * CASes the busy field from 0 to 1 to acquire lock. - */ - final boolean casBusy() { - return UNSAFE.compareAndSwapInt(this, busyOffset, 0, 1); - } - - /** - * Computes the function of current and new value. Subclasses - * should open-code this update function for most uses, but the - * virtualized form is needed within retryUpdate. - * - * @param currentValue the current value (of either base or a cell) - * @param newValue the argument from a user update call - * @return result of the update function - */ - abstract long fn(long currentValue, long newValue); - - /** - * Handles cases of updates involving initialization, resizing, - * creating new Cells, and/or contention. See above for - * explanation. This method suffers the usual non-modularity - * problems of optimistic retry code, relying on rechecked sets of - * reads. - * - * @param x the value - * @param hc the hash code holder - * @param wasUncontended false if CAS failed before call - */ - final void retryUpdate(long x, HashCode hc, boolean wasUncontended) { - int h = hc.code; - boolean collide = false; // True if last slot nonempty - for (;;) { - Cell[] as; Cell a; int n; long v; - if ((as = cells) != null && (n = as.length) > 0) { - if ((a = as[(n - 1) & h]) == null) { - if (busy == 0) { // Try to attach new Cell - Cell r = new Cell(x); // Optimistically create - if (busy == 0 && casBusy()) { - boolean created = false; - try { // Recheck under lock - Cell[] rs; int m, j; - if ((rs = cells) != null && - (m = rs.length) > 0 && - rs[j = (m - 1) & h] == null) { - rs[j] = r; - created = true; - } - } finally { - busy = 0; - } - if (created) - break; - continue; // Slot is now non-empty - } - } - collide = false; - } - else if (!wasUncontended) // CAS already known to fail - wasUncontended = true; // Continue after rehash - else if (a.cas(v = a.value, fn(v, x))) - break; - else if (n >= NCPU || cells != as) - collide = false; // At max size or stale - else if (!collide) - collide = true; - else if (busy == 0 && casBusy()) { - try { - if (cells == as) { // Expand table unless stale - Cell[] rs = new Cell[n << 1]; - for (int i = 0; i < n; ++i) - rs[i] = as[i]; - cells = rs; - } - } finally { - busy = 0; - } - collide = false; - continue; // Retry with expanded table - } - h ^= h << 13; // Rehash - h ^= h >>> 17; - h ^= h << 5; - } - else if (busy == 0 && cells == as && casBusy()) { - boolean init = false; - try { // Initialize table - if (cells == as) { - Cell[] rs = new Cell[2]; - rs[h & 1] = new Cell(x); - cells = rs; - init = true; - } - } finally { - busy = 0; - } - if (init) - break; - } - else if (casBase(v = base, fn(v, x))) - break; // Fall back on using base - } - hc.code = h; // Record index for next time - } - - /** - * Sets base and all cells to the given value. - */ - final void internalReset(long initialValue) { - Cell[] as = cells; - base = initialValue; - if (as != null) { - int n = as.length; - for (int i = 0; i < n; ++i) { - Cell a = as[i]; - if (a != null) - a.value = initialValue; - } - } - } - - // Unsafe mechanics - private static final sun.misc.Unsafe UNSAFE; - private static final long baseOffset; - private static final long busyOffset; - static { - try { - UNSAFE = getUnsafe(); - Class<?> sk = Striped64.class; - baseOffset = UNSAFE.objectFieldOffset - (sk.getDeclaredField("base")); - busyOffset = UNSAFE.objectFieldOffset - (sk.getDeclaredField("busy")); - } catch (Exception e) { - throw new Error(e); - } - } - - /** - * Returns a sun.misc.Unsafe. Suitable for use in a 3rd party package. - * Replace with a simple call to Unsafe.getUnsafe when integrating - * into a jdk. - * - * @return a sun.misc.Unsafe - */ - private static sun.misc.Unsafe getUnsafe() { - try { - return sun.misc.Unsafe.getUnsafe(); - } catch (SecurityException tryReflectionInstead) {} - try { - return java.security.AccessController.doPrivileged - (new java.security.PrivilegedExceptionAction<sun.misc.Unsafe>() { - public sun.misc.Unsafe run() throws Exception { - Class<sun.misc.Unsafe> k = sun.misc.Unsafe.class; - for (java.lang.reflect.Field f : k.getDeclaredFields()) { - f.setAccessible(true); - Object x = f.get(null); - if (k.isInstance(x)) - return k.cast(x); - } - throw new NoSuchFieldError("the Unsafe"); - }}); - } catch (java.security.PrivilegedActionException e) { - throw new RuntimeException("Could not initialize intrinsics", - e.getCause()); - } - } - -} |