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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.
*/
package com.github.benmanes.caffeine.cache;
import java.util.concurrent.atomic.AtomicReferenceArray;
import java.util.function.Consumer;
/**
* A striped, non-blocking, bounded buffer.
*
* @author [email protected] (Ben Manes)
* @param the type of elements maintained by this buffer
*/
final class BoundedBuffer extends StripedBuffer {
/*
* A circular ring buffer stores the elements being transferred by the producers to the consumer.
* The monotonically increasing count of reads and writes allow indexing sequentially to the next
* element location based upon a power-of-two sizing.
*
* The producers race to read the counts, check if there is available capacity, and if so then try
* once to CAS to the next write count. If the increment is successful then the producer lazily
* publishes the element. The producer does not retry or block when unsuccessful due to a failed
* CAS or the buffer being full.
*
* The consumer reads the counts and takes the available elements. The clearing of the elements
* and the next read count are lazily set.
*
* This implementation is striped to further increase concurrency by rehashing and dynamically
* adding new buffers when contention is detected, up to an internal maximum. When rehashing in
* order to discover an available buffer, the producer may retry adding its element to determine
* whether it found a satisfactory buffer or if resizing is necessary.
*/
/** The maximum number of elements per buffer. */
static final int BUFFER_SIZE = 16;
static final int MASK = BUFFER_SIZE - 1;
@Override
protected Buffer create(E e) {
return new RingBuffer<>(e);
}
static final class RingBuffer extends BBHeader.ReadAndWriteCounterRef implements Buffer {
final AtomicReferenceArray buffer;
@SuppressWarnings({"unchecked", "cast", "rawtypes"})
public RingBuffer(E e) {
buffer = new AtomicReferenceArray<>(BUFFER_SIZE);
buffer.lazySet(0, e);
}
@Override
public int offer(E e) {
long head = readCounter;
long tail = relaxedWriteCounter();
long size = (tail - head);
if (size >= BUFFER_SIZE) {
return Buffer.FULL;
}
if (casWriteCounter(tail, tail + 1)) {
int index = (int) (tail & MASK);
buffer.lazySet(index, e);
return Buffer.SUCCESS;
}
return Buffer.FAILED;
}
@Override
public void drainTo(Consumer consumer) {
long head = readCounter;
long tail = relaxedWriteCounter();
long size = (tail - head);
if (size == 0) {
return;
}
do {
int index = (int) (head & MASK);
E e = buffer.get(index);
if (e == null) {
// not published yet
break;
}
buffer.lazySet(index, null);
consumer.accept(e);
head++;
} while (head != tail);
lazySetReadCounter(head);
}
@Override
public int reads() {
return (int) readCounter;
}
@Override
public int writes() {
return (int) writeCounter;
}
}
}
/** The namespace for field padding through inheritance. */
final class BBHeader {
@SuppressWarnings("PMD.AbstractClassWithoutAbstractMethod")
abstract static class PadReadCounter {
byte p000, p001, p002, p003, p004, p005, p006, p007;
byte p008, p009, p010, p011, p012, p013, p014, p015;
byte p016, p017, p018, p019, p020, p021, p022, p023;
byte p024, p025, p026, p027, p028, p029, p030, p031;
byte p032, p033, p034, p035, p036, p037, p038, p039;
byte p040, p041, p042, p043, p044, p045, p046, p047;
byte p048, p049, p050, p051, p052, p053, p054, p055;
byte p056, p057, p058, p059, p060, p061, p062, p063;
byte p064, p065, p066, p067, p068, p069, p070, p071;
byte p072, p073, p074, p075, p076, p077, p078, p079;
byte p080, p081, p082, p083, p084, p085, p086, p087;
byte p088, p089, p090, p091, p092, p093, p094, p095;
byte p096, p097, p098, p099, p100, p101, p102, p103;
byte p104, p105, p106, p107, p108, p109, p110, p111;
byte p112, p113, p114, p115, p116, p117, p118, p119;
}
/** Enforces a memory layout to avoid false sharing by padding the read count. */
abstract static class ReadCounterRef extends PadReadCounter {
static final long READ_OFFSET =
UnsafeAccess.objectFieldOffset(ReadCounterRef.class, "readCounter");
volatile long readCounter;
void lazySetReadCounter(long count) {
UnsafeAccess.UNSAFE.putOrderedLong(this, READ_OFFSET, count);
}
}
abstract static class PadWriteCounter extends ReadCounterRef {
byte p120, p121, p122, p123, p124, p125, p126, p127;
byte p128, p129, p130, p131, p132, p133, p134, p135;
byte p136, p137, p138, p139, p140, p141, p142, p143;
byte p144, p145, p146, p147, p148, p149, p150, p151;
byte p152, p153, p154, p155, p156, p157, p158, p159;
byte p160, p161, p162, p163, p164, p165, p166, p167;
byte p168, p169, p170, p171, p172, p173, p174, p175;
byte p176, p177, p178, p179, p180, p181, p182, p183;
byte p184, p185, p186, p187, p188, p189, p190, p191;
byte p192, p193, p194, p195, p196, p197, p198, p199;
byte p200, p201, p202, p203, p204, p205, p206, p207;
byte p208, p209, p210, p211, p212, p213, p214, p215;
byte p216, p217, p218, p219, p220, p221, p222, p223;
byte p224, p225, p226, p227, p228, p229, p230, p231;
byte p232, p233, p234, p235, p236, p237, p238, p239;
}
/** Enforces a memory layout to avoid false sharing by padding the write count. */
abstract static class ReadAndWriteCounterRef extends PadWriteCounter {
static final long WRITE_OFFSET =
UnsafeAccess.objectFieldOffset(ReadAndWriteCounterRef.class, "writeCounter");
volatile long writeCounter;
ReadAndWriteCounterRef() {
UnsafeAccess.UNSAFE.putOrderedLong(this, WRITE_OFFSET, 1);
}
long relaxedWriteCounter() {
return UnsafeAccess.UNSAFE.getLong(this, WRITE_OFFSET);
}
boolean casWriteCounter(long expect, long update) {
return UnsafeAccess.UNSAFE.compareAndSwapLong(this, WRITE_OFFSET, expect, update);
}
}
}
