tuttimer5.html
9.38 KB
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
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Timer.5 - Synchronising handlers in multithreaded programs</title>
<link rel="stylesheet" href="../../boostbook.css" type="text/css">
<meta name="generator" content="DocBook XSL Stylesheets V1.75.2">
<link rel="home" href="../../index.html" title="Asio">
<link rel="up" href="../tutorial.html" title="Tutorial">
<link rel="prev" href="tuttimer4/src.html" title="Source listing for Timer.4">
<link rel="next" href="tuttimer5/src.html" title="Source listing for Timer.5">
</head>
<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
<table cellpadding="2" width="100%"><tr><td valign="top"><img alt="asio C++ library" width="250" height="60" src="../../asio.png"></td></tr></table>
<hr>
<div class="spirit-nav">
<a accesskey="p" href="tuttimer4/src.html"><img src="../../prev.png" alt="Prev"></a><a accesskey="u" href="../tutorial.html"><img src="../../up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../home.png" alt="Home"></a><a accesskey="n" href="tuttimer5/src.html"><img src="../../next.png" alt="Next"></a>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="asio.tutorial.tuttimer5"></a><a class="link" href="tuttimer5.html" title="Timer.5 - Synchronising handlers in multithreaded programs">Timer.5 - Synchronising handlers
in multithreaded programs</a>
</h3></div></div></div>
<p>
This tutorial demonstrates the use of the <a class="link" href="../reference/strand.html" title="strand">strand</a>
class template to synchronise callback handlers in a multithreaded program.
</p>
<p>
The previous four tutorials avoided the issue of handler synchronisation
by calling the <a class="link" href="../reference/io_context/run.html" title="io_context::run">io_context::run()</a>
function from one thread only. As you already know, the asio library provides
a guarantee that callback handlers will only be called from threads that
are currently calling <a class="link" href="../reference/io_context/run.html" title="io_context::run">io_context::run()</a>.
Consequently, calling <a class="link" href="../reference/io_context/run.html" title="io_context::run">io_context::run()</a>
from only one thread ensures that callback handlers cannot run concurrently.
</p>
<p>
The single threaded approach is usually the best place to start when developing
applications using asio. The downside is the limitations it places on programs,
particularly servers, including:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
Poor responsiveness when handlers can take a long time to complete.
</li>
<li class="listitem">
An inability to scale on multiprocessor systems.
</li>
</ul></div>
<p>
If you find yourself running into these limitations, an alternative approach
is to have a pool of threads calling <a class="link" href="../reference/io_context/run.html" title="io_context::run">io_context::run()</a>.
However, as this allows handlers to execute concurrently, we need a method
of synchronisation when handlers might be accessing a shared, thread-unsafe
resource.
</p>
<pre class="programlisting">#include <iostream>
#include <asio.hpp>
#include <boost/bind/bind.hpp>
</pre>
<p>
We start by defining a class called <code class="computeroutput">printer</code>, similar to the
class in the previous tutorial. This class will extend the previous tutorial
by running two timers in parallel.
</p>
<pre class="programlisting">class printer
{
public:
</pre>
<p>
In addition to initialising a pair of asio::steady_timer members, the constructor
initialises the <code class="computeroutput">strand_</code> member, an object of type asio::strand<asio::io_context::executor_type>.
</p>
<p>
The <a class="link" href="../reference/strand.html" title="strand">strand</a> class template is
an executor adapter that guarantees that, for those handlers that are dispatched
through it, an executing handler will be allowed to complete before the next
one is started. This is guaranteed irrespective of the number of threads
that are calling <a class="link" href="../reference/io_context/run.html" title="io_context::run">io_context::run()</a>.
Of course, the handlers may still execute concurrently with other handlers
that were not dispatched through an <a class="link" href="../reference/strand.html" title="strand">strand</a>,
or were dispatched through a different <a class="link" href="../reference/strand.html" title="strand">strand</a>
object.
</p>
<pre class="programlisting"> printer(asio::io_context& io)
: strand_(asio::make_strand(io)),
timer1_(io, asio::chrono::seconds(1)),
timer2_(io, asio::chrono::seconds(1)),
count_(0)
{
</pre>
<p>
When initiating the asynchronous operations, each callback handler is "bound"
to an asio::strand<asio::io_context::executor_type> object. The asio::bind_executor()
function returns a new handler that automatically dispatches its contained
handler through the <a class="link" href="../reference/strand.html" title="strand">strand</a> object.
By binding the handlers to the same <a class="link" href="../reference/strand.html" title="strand">strand</a>,
we are ensuring that they cannot execute concurrently.
</p>
<pre class="programlisting"> timer1_.async_wait(asio::bind_executor(strand_,
boost::bind(&printer::print1, this)));
timer2_.async_wait(asio::bind_executor(strand_,
boost::bind(&printer::print2, this)));
}
~printer()
{
std::cout << "Final count is " << count_ << std::endl;
}
</pre>
<p>
In a multithreaded program, the handlers for asynchronous operations should
be synchronised if they access shared resources. In this tutorial, the shared
resources used by the handlers (<code class="computeroutput">print1</code> and <code class="computeroutput">print2</code>)
are <code class="computeroutput">std::cout</code> and the <code class="computeroutput">count_</code> data member.
</p>
<pre class="programlisting"> void print1()
{
if (count_ < 10)
{
std::cout << "Timer 1: " << count_ << std::endl;
++count_;
timer1_.expires_at(timer1_.expiry() + asio::chrono::seconds(1));
timer1_.async_wait(asio::bind_executor(strand_,
boost::bind(&printer::print1, this)));
}
}
void print2()
{
if (count_ < 10)
{
std::cout << "Timer 2: " << count_ << std::endl;
++count_;
timer2_.expires_at(timer2_.expiry() + asio::chrono::seconds(1));
timer2_.async_wait(asio::bind_executor(strand_,
boost::bind(&printer::print2, this)));
}
}
private:
asio::strand<asio::io_context::executor_type> strand_;
asio::steady_timer timer1_;
asio::steady_timer timer2_;
int count_;
};
</pre>
<p>
The <code class="computeroutput">main</code> function now causes <a class="link" href="../reference/io_context/run.html" title="io_context::run">io_context::run()</a>
to be called from two threads: the main thread and one additional thread.
This is accomplished using an <a class="link" href="../reference/thread.html" title="thread">thread</a>
object.
</p>
<p>
Just as it would with a call from a single thread, concurrent calls to <a class="link" href="../reference/io_context/run.html" title="io_context::run">io_context::run()</a> will continue
to execute while there is "work" left to do. The background thread
will not exit until all asynchronous operations have completed.
</p>
<pre class="programlisting">int main()
{
asio::io_context io;
printer p(io);
asio::thread t(boost::bind(&asio::io_context::run, &io));
io.run();
t.join();
return 0;
}
</pre>
<p>
See the <a class="link" href="tuttimer5/src.html" title="Source listing for Timer.5">full source listing</a>
</p>
<p>
Return to the <a class="link" href="../tutorial.html" title="Tutorial">tutorial index</a>
</p>
<p>
Previous: <a class="link" href="tuttimer4.html" title="Timer.4 - Using a member function as a handler">Timer.4 - Using a member
function as a handler</a>
</p>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright © 2003-2020 Christopher M.
Kohlhoff<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
</p>
</div></td>
</tr></table>
<hr>
<div class="spirit-nav">
<a accesskey="p" href="tuttimer4/src.html"><img src="../../prev.png" alt="Prev"></a><a accesskey="u" href="../tutorial.html"><img src="../../up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../home.png" alt="Home"></a><a accesskey="n" href="tuttimer5/src.html"><img src="../../next.png" alt="Next"></a>
</div>
</body>
</html>