| /* |
| * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. |
| * |
| * This code is free software; you can redistribute it and/or modify it |
| * under the terms of the GNU General Public License version 2 only, as |
| * published by the Free Software Foundation. Oracle designates this |
| * particular file as subject to the "Classpath" exception as provided |
| * by Oracle in the LICENSE file that accompanied this code. |
| * |
| * This code is distributed in the hope that it will be useful, but WITHOUT |
| * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or |
| * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License |
| * version 2 for more details (a copy is included in the LICENSE file that |
| * accompanied this code). |
| * |
| * You should have received a copy of the GNU General Public License version |
| * 2 along with this work; if not, write to the Free Software Foundation, |
| * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. |
| * |
| * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA |
| * or visit www.oracle.com if you need additional information or have any |
| * questions. |
| */ |
| |
| /* |
| * This file is available under and governed by the GNU General Public |
| * License version 2 only, as published by the Free Software Foundation. |
| * However, the following notice accompanied the original version of this |
| * file: |
| * |
| * 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/ |
| */ |
| |
| package java.util.concurrent.atomic; |
| |
| import java.lang.invoke.MethodHandles; |
| import java.lang.invoke.VarHandle; |
| import java.util.Arrays; |
| import java.util.concurrent.ThreadLocalRandom; |
| import java.util.function.DoubleBinaryOperator; |
| import java.util.function.LongBinaryOperator; |
| |
| /** |
| * 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. |
| */ |
| @SuppressWarnings("serial") |
| 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 |
| * (via @Contended) to reduce cache contention. 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 ("cellsBusy") 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. |
| * |
| * The Thread probe fields maintained via ThreadLocalRandom serve |
| * as per-thread hash codes. We let them remain uninitialized as |
| * zero (if they come in this way) until they contend at slot |
| * 0. They are then initialized to values that typically do not |
| * often conflict with others. Contention and/or table collisions |
| * are indicated by failed CASes when performing an update |
| * operation. 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. |
| * |
| * JVM intrinsics note: It would be possible to use a release-only |
| * form of CAS here, if it were provided. |
| */ |
| @jdk.internal.vm.annotation.Contended static final class Cell { |
| volatile long value; |
| Cell(long x) { value = x; } |
| final boolean cas(long cmp, long val) { |
| return VALUE.compareAndSet(this, cmp, val); |
| } |
| final void reset() { |
| VALUE.setVolatile(this, 0L); |
| } |
| final void reset(long identity) { |
| VALUE.setVolatile(this, identity); |
| } |
| |
| // VarHandle mechanics |
| private static final VarHandle VALUE; |
| static { |
| try { |
| MethodHandles.Lookup l = MethodHandles.lookup(); |
| VALUE = l.findVarHandle(Cell.class, "value", long.class); |
| } catch (ReflectiveOperationException e) { |
| throw new Error(e); |
| } |
| } |
| } |
| |
| /** 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 cellsBusy; |
| |
| /** |
| * Package-private default constructor. |
| */ |
| Striped64() { |
| } |
| |
| /** |
| * CASes the base field. |
| */ |
| final boolean casBase(long cmp, long val) { |
| return BASE.compareAndSet(this, cmp, val); |
| } |
| |
| /** |
| * CASes the cellsBusy field from 0 to 1 to acquire lock. |
| */ |
| final boolean casCellsBusy() { |
| return CELLSBUSY.compareAndSet(this, 0, 1); |
| } |
| |
| /** |
| * Returns the probe value for the current thread. |
| * Duplicated from ThreadLocalRandom because of packaging restrictions. |
| */ |
| static final int getProbe() { |
| return (int) THREAD_PROBE.get(Thread.currentThread()); |
| } |
| |
| /** |
| * Pseudo-randomly advances and records the given probe value for the |
| * given thread. |
| * Duplicated from ThreadLocalRandom because of packaging restrictions. |
| */ |
| static final int advanceProbe(int probe) { |
| probe ^= probe << 13; // xorshift |
| probe ^= probe >>> 17; |
| probe ^= probe << 5; |
| THREAD_PROBE.set(Thread.currentThread(), probe); |
| return probe; |
| } |
| |
| /** |
| * 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 fn the update function, or null for add (this convention |
| * avoids the need for an extra field or function in LongAdder). |
| * @param wasUncontended false if CAS failed before call |
| */ |
| final void longAccumulate(long x, LongBinaryOperator fn, |
| boolean wasUncontended) { |
| int h; |
| if ((h = getProbe()) == 0) { |
| ThreadLocalRandom.current(); // force initialization |
| h = getProbe(); |
| wasUncontended = true; |
| } |
| boolean collide = false; // True if last slot nonempty |
| done: 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 (cellsBusy == 0) { // Try to attach new Cell |
| Cell r = new Cell(x); // Optimistically create |
| if (cellsBusy == 0 && casCellsBusy()) { |
| 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; |
| break done; |
| } |
| } finally { |
| cellsBusy = 0; |
| } |
| 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 == null) ? v + x : fn.applyAsLong(v, x))) |
| break; |
| else if (n >= NCPU || cells != as) |
| collide = false; // At max size or stale |
| else if (!collide) |
| collide = true; |
| else if (cellsBusy == 0 && casCellsBusy()) { |
| try { |
| if (cells == as) // Expand table unless stale |
| cells = Arrays.copyOf(as, n << 1); |
| } finally { |
| cellsBusy = 0; |
| } |
| collide = false; |
| continue; // Retry with expanded table |
| } |
| h = advanceProbe(h); |
| } |
| else if (cellsBusy == 0 && cells == as && casCellsBusy()) { |
| try { // Initialize table |
| if (cells == as) { |
| Cell[] rs = new Cell[2]; |
| rs[h & 1] = new Cell(x); |
| cells = rs; |
| break done; |
| } |
| } finally { |
| cellsBusy = 0; |
| } |
| } |
| // Fall back on using base |
| else if (casBase(v = base, |
| (fn == null) ? v + x : fn.applyAsLong(v, x))) |
| break done; |
| } |
| } |
| |
| private static long apply(DoubleBinaryOperator fn, long v, double x) { |
| double d = Double.longBitsToDouble(v); |
| d = (fn == null) ? d + x : fn.applyAsDouble(d, x); |
| return Double.doubleToRawLongBits(d); |
| } |
| |
| /** |
| * Same as longAccumulate, but injecting long/double conversions |
| * in too many places to sensibly merge with long version, given |
| * the low-overhead requirements of this class. So must instead be |
| * maintained by copy/paste/adapt. |
| */ |
| final void doubleAccumulate(double x, DoubleBinaryOperator fn, |
| boolean wasUncontended) { |
| int h; |
| if ((h = getProbe()) == 0) { |
| ThreadLocalRandom.current(); // force initialization |
| h = getProbe(); |
| wasUncontended = true; |
| } |
| boolean collide = false; // True if last slot nonempty |
| done: 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 (cellsBusy == 0) { // Try to attach new Cell |
| Cell r = new Cell(Double.doubleToRawLongBits(x)); |
| if (cellsBusy == 0 && casCellsBusy()) { |
| 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; |
| break done; |
| } |
| } finally { |
| cellsBusy = 0; |
| } |
| 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, apply(fn, v, x))) |
| break; |
| else if (n >= NCPU || cells != as) |
| collide = false; // At max size or stale |
| else if (!collide) |
| collide = true; |
| else if (cellsBusy == 0 && casCellsBusy()) { |
| try { |
| if (cells == as) // Expand table unless stale |
| cells = Arrays.copyOf(as, n << 1); |
| } finally { |
| cellsBusy = 0; |
| } |
| collide = false; |
| continue; // Retry with expanded table |
| } |
| h = advanceProbe(h); |
| } |
| else if (cellsBusy == 0 && cells == as && casCellsBusy()) { |
| try { // Initialize table |
| if (cells == as) { |
| Cell[] rs = new Cell[2]; |
| rs[h & 1] = new Cell(Double.doubleToRawLongBits(x)); |
| cells = rs; |
| break done; |
| } |
| } finally { |
| cellsBusy = 0; |
| } |
| } |
| // Fall back on using base |
| else if (casBase(v = base, apply(fn, v, x))) |
| break done; |
| } |
| } |
| |
| // VarHandle mechanics |
| private static final VarHandle BASE; |
| private static final VarHandle CELLSBUSY; |
| private static final VarHandle THREAD_PROBE; |
| static { |
| try { |
| MethodHandles.Lookup l = MethodHandles.lookup(); |
| BASE = l.findVarHandle(Striped64.class, |
| "base", long.class); |
| CELLSBUSY = l.findVarHandle(Striped64.class, |
| "cellsBusy", int.class); |
| l = java.security.AccessController.doPrivileged( |
| new java.security.PrivilegedAction<>() { |
| public MethodHandles.Lookup run() { |
| try { |
| return MethodHandles.privateLookupIn(Thread.class, MethodHandles.lookup()); |
| } catch (ReflectiveOperationException e) { |
| throw new Error(e); |
| } |
| }}); |
| THREAD_PROBE = l.findVarHandle(Thread.class, |
| "threadLocalRandomProbe", int.class); |
| } catch (ReflectiveOperationException e) { |
| throw new Error(e); |
| } |
| } |
| |
| } |