1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
// Licensed to the Apache Software Foundation (ASF) under one
// or more contributor license agreements. See the NOTICE file
// distributed with this work for additional information
// regarding copyright ownership. The ASF licenses this file
// to you 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.
// bthread - A M:N threading library to make applications more concurrent.
// Author: Ge,Jun (gejun@baidu.com)
// Date: Tue Jul 10 17:40:58 CST 2012
#ifndef BTHREAD_WORK_STEALING_QUEUE_H
#define BTHREAD_WORK_STEALING_QUEUE_H
#include "butil/macros.h"
#include "butil/atomicops.h"
#include "butil/logging.h"
namespace bthread {
template <typename T>
class WorkStealingQueue {
public:
WorkStealingQueue()
: _bottom(1)
, _capacity(0)
, _buffer(NULL)
, _top(1) {
}
~WorkStealingQueue() {
delete [] _buffer;
_buffer = NULL;
}
int init(size_t capacity) {
if (_capacity != 0) {
LOG(ERROR) << "Already initialized";
return -1;
}
if (capacity == 0) {
LOG(ERROR) << "Invalid capacity=" << capacity;
return -1;
}
if (capacity & (capacity - 1)) {
LOG(ERROR) << "Invalid capacity=" << capacity
<< " which must be power of 2";
return -1;
}
_buffer = new(std::nothrow) T[capacity];
if (NULL == _buffer) {
return -1;
}
_capacity = capacity;
return 0;
}
// Push an item into the queue.
// Returns true on pushed.
// May run in parallel with steal().
// Never run in parallel with pop() or another push().
bool push(const T& x) {
const size_t b = _bottom.load(butil::memory_order_relaxed);
const size_t t = _top.load(butil::memory_order_acquire);
if (b >= t + _capacity) { // Full queue.
return false;
}
_buffer[b & (_capacity - 1)] = x;
_bottom.store(b + 1, butil::memory_order_release);
return true;
}
// Pop an item from the queue.
// Returns true on popped and the item is written to `val'.
// May run in parallel with steal().
// Never run in parallel with push() or another pop().
bool pop(T* val) {
const size_t b = _bottom.load(butil::memory_order_relaxed);
size_t t = _top.load(butil::memory_order_relaxed);
if (t >= b) {
// fast check since we call pop() in each sched.
// Stale _top which is smaller should not enter this branch.
return false;
}
const size_t newb = b - 1;
_bottom.store(newb, butil::memory_order_relaxed);
butil::atomic_thread_fence(butil::memory_order_seq_cst);
t = _top.load(butil::memory_order_relaxed);
if (t > newb) {
_bottom.store(b, butil::memory_order_relaxed);
return false;
}
*val = _buffer[newb & (_capacity - 1)];
if (t != newb) {
return true;
}
// Single last element, compete with steal()
const bool popped = _top.compare_exchange_strong(
t, t + 1, butil::memory_order_seq_cst, butil::memory_order_relaxed);
_bottom.store(b, butil::memory_order_relaxed);
return popped;
}
// Steal one item from the queue.
// Returns true on stolen.
// May run in parallel with push() pop() or another steal().
bool steal(T* val) {
size_t t = _top.load(butil::memory_order_acquire);
size_t b = _bottom.load(butil::memory_order_acquire);
if (t >= b) {
// Permit false negative for performance considerations.
return false;
}
do {
butil::atomic_thread_fence(butil::memory_order_seq_cst);
b = _bottom.load(butil::memory_order_acquire);
if (t >= b) {
return false;
}
*val = _buffer[t & (_capacity - 1)];
} while (!_top.compare_exchange_strong(t, t + 1,
butil::memory_order_seq_cst,
butil::memory_order_relaxed));
return true;
}
size_t volatile_size() const {
const size_t b = _bottom.load(butil::memory_order_relaxed);
const size_t t = _top.load(butil::memory_order_relaxed);
return (b <= t ? 0 : (b - t));
}
size_t capacity() const { return _capacity; }
private:
// Copying a concurrent structure makes no sense.
DISALLOW_COPY_AND_ASSIGN(WorkStealingQueue);
butil::atomic<size_t> _bottom;
size_t _capacity;
T* _buffer;
butil::atomic<size_t> BAIDU_CACHELINE_ALIGNMENT _top;
};
} // namespace bthread
#endif // BTHREAD_WORK_STEALING_QUEUE_H