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
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
|
/**
* Copyright (C) 2016 MongoDB Inc.
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU Affero General Public License, version 3,
* as published by the Free Software Foundation.
*
* This program 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 Affero General Public License for more details.
*
* You should have received a copy of the GNU Affero General Public License
* along with this program. If not, see <http://www.gnu.org/licenses/>.
*
* As a special exception, the copyright holders give permission to link the
* code of portions of this program with the OpenSSL library under certain
* conditions as described in each individual source file and distribute
* linked combinations including the program with the OpenSSL library. You
* must comply with the GNU Affero General Public License in all respects for
* all of the code used other than as permitted herein. If you modify file(s)
* with this exception, you may extend this exception to your version of the
* file(s), but you are not obligated to do so. If you do not wish to do so,
* delete this exception statement from your version. If you delete this
* exception statement from all source files in the program, then also delete
* it in the license file.
*/
#pragma once
#include "mongo/platform/basic.h"
#include <boost/intrusive_ptr.hpp>
#include <boost/multi_index/hashed_index.hpp>
#include <boost/multi_index/member.hpp>
#include <boost/multi_index/sequenced_index.hpp>
#include <boost/multi_index_container.hpp>
#include <boost/optional.hpp>
#include <iostream>
#include <unordered_map>
#include <unordered_set>
#include "mongo/bson/bsonobj.h"
#include "mongo/db/pipeline/value.h"
#include "mongo/stdx/functional.h"
namespace mongo {
using boost::multi_index_container;
using boost::multi_index::sequenced;
using boost::multi_index::hashed_unique;
using boost::multi_index::member;
using boost::multi_index::indexed_by;
/**
* A least-recently-used cache from key to a set of values. It does not implement any default size
* limit, but includes the ability to evict down to both a specific number of elements, and down to
* a specific amount of memory. Memory usage includes only the size of the elements in the cache at
* the time of insertion, not the overhead incurred by the data structures in use.
*/
class LookupSetCache {
public:
/**
* Insert "value" into the set with key "key". If "key" is already present in the cache, move it
* to the middle of the cache. Otherwise, insert a new key in the middle of the cache.
*
* Note: In this case, "middle" refers to the sequence of the cache, where "first" is the item
* most recently used, and "last" is the item least recently used.
*
* We insert and update in the middle because when a key is updated, we can't assume that it's
* important to keep in the cache (i.e., that we should put it at the front), but it's also
* likely we don't want to evict it (i.e., we want to make sure it isn't at the back).
*/
void insert(Value key, BSONObj value) {
// Get an iterator to the middle of the container.
size_t middle = size() / 2;
auto it = _container.begin();
std::advance(it, middle);
auto result = _container.insert(it, {key, {value}});
if (!result.second) {
// We did not insert due to a duplicate key.
auto cached = *result.first;
// Update the cached value, moving it to the middle of the cache.
cached.second.insert(value);
_container.replace(result.first, cached);
_container.relocate(it, result.first);
} else {
_memoryUsage += key.getApproximateSize();
}
_memoryUsage += static_cast<size_t>(value.objsize());
}
/**
* Evict the least-recently-used item.
*/
void evictOne() {
if (_container.empty()) {
return;
}
const Cached& pair = _container.back();
size_t keySize = pair.first.getApproximateSize();
invariant(keySize <= _memoryUsage);
_memoryUsage -= keySize;
for (auto&& elem : pair.second) {
size_t valueSize = static_cast<size_t>(elem.objsize());
invariant(valueSize <= _memoryUsage);
_memoryUsage -= valueSize;
}
_container.erase(std::prev(_container.end()));
}
/**
* Evicts from the cache until there are 'num' items remaining.
*/
void evictUntilSize(size_t num) {
while (size() > num) {
evictOne();
}
}
/**
* Returns the number of elements in the cache.
*/
size_t size() const {
return _container.size();
}
/**
* Evict items in LRU order until the cache's size is less than or equal to "maximum".
*/
void evictDownTo(size_t maximum) {
while (_memoryUsage > maximum && !_container.empty()) {
evictOne();
}
}
/**
* Clear the cache, resetting the memory usage.
*/
void clear() {
_container.clear();
_memoryUsage = 0;
}
/**
* Retrieve the set of values with key "key". If not found, returns boost::none.
*/
boost::optional<std::unordered_set<BSONObj, BSONObj::Hasher>> operator[](Value key) {
auto it = boost::multi_index::get<1>(_container).find(key);
if (it != boost::multi_index::get<1>(_container).end()) {
boost::multi_index::get<0>(_container)
.relocate(boost::multi_index::get<0>(_container).begin(),
boost::multi_index::project<0>(_container, it));
return (*it).second;
}
return boost::none;
}
private:
using Cached = std::pair<Value, std::unordered_set<BSONObj, BSONObj::Hasher>>;
// boost::multi_index_container provides a system for implementing a cache. Here, we create
// a container of std::pair<Value, BSONObjSet>, that is both sequenced, and has a unique
// index on the Value. From this, we are able to evict the least-recently-used member, and
// maintain key uniqueness.
using IndexedContainer = multi_index_container<
Cached,
indexed_by<sequenced<>, hashed_unique<member<Cached, Value, &Cached::first>, Value::Hash>>>;
IndexedContainer _container;
size_t _memoryUsage = 0;
};
} // namespace mongo
|
