blob: bb95eb964ecb10680022540a368197e0b8bc06ca [file] [log] [blame]
/*
* 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;
import java.util.concurrent.atomic.AtomicIntegerFieldUpdater;
import java.util.concurrent.atomic.AtomicLongFieldUpdater;
/**
* 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 {
@SuppressWarnings("UnusedDeclaration")
volatile long p0, p1, p2, p3, p4, p5, p6;
volatile long value;
@SuppressWarnings("UnusedDeclaration")
volatile long q0, q1, q2, q3, q4, q5, q6;
Cell(long x) { value = x; }
final boolean cas(long cmp, long val) {
return valueUpdater.compareAndSet(this, cmp, val);
}
private static final AtomicLongFieldUpdater<Cell> valueUpdater =
AtomicLongFieldUpdater.newUpdater(Cell.class, "value");
}
/**
* 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 baseUpdater.compareAndSet(this, cmp, val);
}
/**
* CASes the busy field from 0 to 1 to acquire lock.
*/
final boolean casBusy() {
return busyUpdater.compareAndSet(this, 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];
System.arraycopy(as, 0, rs, 0, n);
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) {
for (Cell a : as) {
if (a != null)
a.value = initialValue;
}
}
}
private static final AtomicLongFieldUpdater<Striped64> baseUpdater =
AtomicLongFieldUpdater.newUpdater(Striped64.class, "base");
private static final AtomicIntegerFieldUpdater<Striped64> busyUpdater =
AtomicIntegerFieldUpdater.newUpdater(Striped64.class, "busy");
}