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
|
<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE erlref SYSTEM "erlref.dtd">
<erlref>
<header>
<copyright>
<year>2018</year>
<holder>Ericsson AB. All Rights Reserved.</holder>
</copyright>
<legalnotice>
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.
</legalnotice>
<title>counters</title>
</header>
<module>counters</module>
<modulesummary>Counter Functions</modulesummary>
<description>
<p>This module provides a set of functions to do operations towards
shared mutable counter variables. The implementation does not utilize any
software level locking, which makes it very efficient for concurrent
access. The counters are organized into arrays with the follwing
semantics:</p>
<list type="bulleted">
<item>
<p>Counters are 64 bit signed integers.</p>
</item>
<item>
<p>Counters wrap around at overflow and underflow operations.</p>
</item>
<item><p>Counters are initialized to zero and can then only be written to
by adding or subtracting.</p>
</item>
<item>
<p>Write operations guarantee atomicity. No intermediate results can be
seen from a single write operation.</p>
</item>
<item>
<p>Two types of counter arrays can be created with options <c>atomics</c> or
<c>write_concurrency</c>. The <c>atomics</c> counters have good allround
performance with nice consistent semantics while
<c>write_concurrency</c> counters offers even better concurrent
write performance at the expense of some potential read
inconsistencies. See <seealso marker="#new/2"><c>new/2</c></seealso>.</p>
</item>
<item>
<p>Indexes into counter arrays are one-based. A counter array of
size N contains N counters with index from 1 to N.</p>
</item>
</list>
</description>
<datatypes>
<datatype>
<name name="counters_ref"/>
<desc><p>Identifies a counter array returned from
<seealso marker="#new/2"><c>new/2</c></seealso>.</p>
</desc>
</datatype>
</datatypes>
<funcs>
<func>
<name name="new" arity="2"/>
<fsummary>Create counter array</fsummary>
<desc>
<p>Create a new counter array of <c><anno>Size</anno></c> counters.</p>
<p>Argument <c><anno>Opts</anno></c> is a list of the following possible
options:</p>
<taglist>
<tag><c>atomics</c> (Default)</tag>
<item><p>Counters will be sequentially consistent. If write
operation A is done sequencially before write operation B, then a concurrent reader
may see none of them, only A, or both A and B. It cannot see only B.</p>
</item>
<tag><c>write_concurrency</c></tag>
<item><p>This is an optimization to achieve very efficient concurrent
<seealso marker="#add/3"><c>add</c></seealso> and <seealso
marker="#sub/3"><c>sub</c></seealso> operations at the expense of potential read
inconsistency and memory consumption per counter.</p>
<p>Read operations may see sequentially inconsistent results with
regard to concurrent write operations. Even if write operation A is done
sequencially before write operation B, a concurrent reader may see any
combination of A and B, including only B. A read operation is only
guaranteed to see all writes done sequentially before the read. No writes
are ever lost, but will eventually all be seen.</p>
<p>The typical use case for <c>write_concurrency</c> is when
concurrent calls to <seealso marker="#add/3"><c>add</c></seealso> and
<seealso marker="#sub/3"><c>sub</c></seealso> toward the same counters
are very frequent, while calls to <seealso marker="#get/2"><c>get</c>
</seealso> and <seealso marker="#put/3"><c>put</c></seealso> are much
less frequent. The lack of absolute read consistency must also be
acceptable.</p>
</item>
</taglist>
</desc>
</func>
<func>
<name name="get" arity="2"/>
<fsummary>Read counter value</fsummary>
<desc>
<p>Read counter value.</p>
</desc>
</func>
<func>
<name name="add" arity="3"/>
<fsummary>Add to counter</fsummary>
<desc>
<p>Add <c><anno>Incr</anno></c> to counter at index
<c><anno>Ix</anno></c>.</p>
</desc>
</func>
<func>
<name name="sub" arity="3"/>
<fsummary>Subtract from counter</fsummary>
<desc>
<p>Subtract <c><anno>Decr</anno></c> from counter at index
<c><anno>Ix</anno></c>.</p>
</desc>
</func>
<func>
<name name="put" arity="3"/>
<fsummary>Set counter to value</fsummary>
<desc>
<p>Write <c><anno>Value</anno></c> to counter at index
<c><anno>Ix</anno></c>.</p>
<note>
<p>Despite its name, the <c>write_concurrency</c> optimization does not
improve <c>put</c>. A call to <c>put</c> is a relative heavy
operation compared to the very lightweight and scalable <seealso
marker="#add/3"><c>add</c></seealso> and <seealso marker="#sub/3">
<c>sub</c></seealso>. The cost for a <c>put</c> with
<c>write_concurrency</c> is lika a <seealso marker="#get/2"><c>get</c>
</seealso> plus a <c>put</c> without <c>write_concurrency</c>.</p>
</note>
</desc>
</func>
<func>
<name name="info" arity="1"/>
<fsummary>Get information about counter array.</fsummary>
<desc>
<p>Return information about a counter array in a map. The map
has the following keys (at least):</p>
<taglist>
<tag><c>size</c></tag>
<item><p>The number of counters in the array.</p></item>
<tag><c>memory</c></tag>
<item><p>Approximate memory consumption for the array in
bytes.</p></item>
</taglist>
</desc>
</func>
</funcs>
</erlref>
|
