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/*
* Copyright 2015 Ben Manes. All Rights Reserved.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
/*
* 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 com.github.benmanes.caffeine.cache;
import static com.github.benmanes.caffeine.cache.Caffeine.ceilingPowerOfTwo;
import java.util.Arrays;
import java.util.concurrent.ThreadLocalRandom;
import java.util.function.Consumer;
import org.checkerframework.checker.nullness.qual.Nullable;
/**
* A base class providing the mechanics for supporting dynamic striping of bounded buffers. This
* implementation is an adaption of the numeric 64-bit {@link java.util.concurrent.atomic.Striped64}
* class, which is used by atomic counters. The approach was modified to lazily grow an array of
* buffers in order to minimize memory usage for caches that are not heavily contended on.
*
* @author [email protected] (Doug Lea)
* @author [email protected] (Ben Manes)
*/
abstract class StripedBuffer implements Buffer {
/*
* This class maintains a lazily-initialized table of atomically updated buffers. 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 Buffer and should be padded 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 Buffers are relatively large, we avoid creating them until they are needed.
* When there is no contention, all updates are made to a single buffer. Upon contention (a failed
* CAS inserting into the buffer), the table is expanded 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 ("tableBusy") is used for initializing and resizing the table, as well as
* populating slots with new Buffers. There is no need for a blocking lock; when the lock is not
* available, threads try other slots. 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 Buffer 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 Buffer 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 buffers, under the assumption that for long-running
* instances, observed contention levels will recur, so the buffers will eventually be needed
* again; and for short-lived ones, it does not matter.
*/
static final long TABLE_BUSY = UnsafeAccess.objectFieldOffset(StripedBuffer.class, "tableBusy");
static final long PROBE = UnsafeAccess.objectFieldOffset(Thread.class, "threadLocalRandomProbe");
/** Number of CPUS. */
static final int NCPU = Runtime.getRuntime().availableProcessors();
/** The bound on the table size. */
static final int MAXIMUM_TABLE_SIZE = 4 * ceilingPowerOfTwo(NCPU);
/** The maximum number of attempts when trying to expand the table. */
static final int ATTEMPTS = 3;
/** Table of buffers. When non-null, size is a power of 2. */
transient volatile Buffer @Nullable[] table;
/** Spinlock (locked via CAS) used when resizing and/or creating Buffers. */
transient volatile int tableBusy;
/** CASes the tableBusy field from 0 to 1 to acquire lock. */
final boolean casTableBusy() {
return UnsafeAccess.UNSAFE.compareAndSwapInt(this, TABLE_BUSY, 0, 1);
}
/**
* Returns the probe value for the current thread. Duplicated from ThreadLocalRandom because of
* packaging restrictions.
*/
static final int getProbe() {
return UnsafeAccess.UNSAFE.getInt(Thread.currentThread(), PROBE);
}
/**
* 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;
UnsafeAccess.UNSAFE.putInt(Thread.currentThread(), PROBE, probe);
return probe;
}
/**
* Creates a new buffer instance after resizing to accommodate a producer.
*
* @param e the producer's element
* @return a newly created buffer populated with a single element
*/
protected abstract Buffer create(E e);
@Override
public int offer(E e) {
int mask;
int result = 0;
Buffer buffer;
boolean uncontended = true;
Buffer[] buffers = table;
if ((buffers == null)
|| (mask = buffers.length - 1) < 0
|| (buffer = buffers[getProbe() & mask]) == null
|| !(uncontended = ((result = buffer.offer(e)) != Buffer.FAILED))) {
expandOrRetry(e, uncontended);
}
return result;
}
@Override
public void drainTo(Consumer consumer) {
Buffer[] buffers = table;
if (buffers == null) {
return;
}
for (Buffer buffer : buffers) {
if (buffer != null) {
buffer.drainTo(consumer);
}
}
}
@Override
public int reads() {
Buffer[] buffers = table;
if (buffers == null) {
return 0;
}
int reads = 0;
for (Buffer buffer : buffers) {
if (buffer != null) {
reads += buffer.reads();
}
}
return reads;
}
@Override
public int writes() {
Buffer[] buffers = table;
if (buffers == null) {
return 0;
}
int writes = 0;
for (Buffer buffer : buffers) {
if (buffer != null) {
writes += buffer.writes();
}
}
return writes;
}
/**
* Handles cases of updates involving initialization, resizing, creating new Buffers, 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 e the element to add
* @param wasUncontended false if CAS failed before call
*/
@SuppressWarnings("PMD.ConfusingTernary")
final void expandOrRetry(E e, boolean wasUncontended) {
int h;
if ((h = getProbe()) == 0) {
ThreadLocalRandom.current(); // force initialization
h = getProbe();
wasUncontended = true;
}
boolean collide = false; // True if last slot nonempty
for (int attempt = 0; attempt < ATTEMPTS; attempt++) {
Buffer[] buffers;
Buffer buffer;
int n;
if (((buffers = table) != null) && ((n = buffers.length) > 0)) {
if ((buffer = buffers[(n - 1) & h]) == null) {
if ((tableBusy == 0) && casTableBusy()) { // Try to attach new Buffer
boolean created = false;
try { // Recheck under lock
Buffer[] rs;
int mask, j;
if (((rs = table) != null) && ((mask = rs.length) > 0)
&& (rs[j = (mask - 1) & h] == null)) {
rs[j] = create(e);
created = true;
}
} finally {
tableBusy = 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 (buffer.offer(e) != Buffer.FAILED) {
break;
} else if (n >= MAXIMUM_TABLE_SIZE || table != buffers) {
collide = false; // At max size or stale
} else if (!collide) {
collide = true;
} else if (tableBusy == 0 && casTableBusy()) {
try {
if (table == buffers) { // Expand table unless stale
table = Arrays.copyOf(buffers, n << 1);
}
} finally {
tableBusy = 0;
}
collide = false;
continue; // Retry with expanded table
}
h = advanceProbe(h);
} else if ((tableBusy == 0) && (table == buffers) && casTableBusy()) {
boolean init = false;
try { // Initialize table
if (table == buffers) {
@SuppressWarnings({"unchecked", "rawtypes"})
Buffer[] rs = new Buffer[1];
rs[0] = create(e);
table = rs;
init = true;
}
} finally {
tableBusy = 0;
}
if (init) {
break;
}
}
}
}
}
