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4 | <title>The Overhead in the Number Class Wrapper</title> | |
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24 | </div> | |
25 | <div class="section"> | |
26 | <div class="titlepage"><div><div><h3 class="title"> | |
27 | <a name="boost_multiprecision.perf.overhead"></a><a class="link" href="overhead.html" title="The Overhead in the Number Class Wrapper">The Overhead in the | |
28 | Number Class Wrapper</a> | |
29 | </h3></div></div></div> | |
30 | <p> | |
31 | Using a simple <a href="../../../../performance/arithmetic_backend.hpp" target="_top">backend | |
32 | class</a> that wraps any built in arithmetic type we can measure the | |
33 | overhead involved in wrapping a type inside the <code class="computeroutput"><span class="identifier">number</span></code> | |
34 | frontend, and the effect that turning on expression templates has. The following | |
35 | table compares the performance between <code class="computeroutput"><span class="keyword">double</span></code> | |
36 | and a <code class="computeroutput"><span class="keyword">double</span></code> wrapped inside | |
37 | class <code class="computeroutput"><span class="identifier">number</span></code>: | |
38 | </p> | |
39 | <div class="informaltable"><table class="table"> | |
40 | <colgroup> | |
41 | <col> | |
42 | <col> | |
43 | <col> | |
44 | </colgroup> | |
45 | <thead><tr> | |
46 | <th> | |
47 | <p> | |
48 | Type | |
49 | </p> | |
50 | </th> | |
51 | <th> | |
52 | <p> | |
53 | Bessel Function Evaluation | |
54 | </p> | |
55 | </th> | |
56 | <th> | |
57 | <p> | |
58 | Non-Central T Evaluation | |
59 | </p> | |
60 | </th> | |
61 | </tr></thead> | |
62 | <tbody> | |
63 | <tr> | |
64 | <td> | |
65 | <p> | |
66 | <code class="computeroutput"><span class="keyword">double</span></code> | |
67 | </p> | |
68 | </td> | |
69 | <td> | |
70 | <p> | |
71 | <span class="bold"><strong>1.0 (0.016s)</strong></span> | |
72 | </p> | |
73 | </td> | |
74 | <td> | |
75 | <p> | |
76 | <span class="bold"><strong>1.0</strong></span> (0.46s) | |
77 | </p> | |
78 | </td> | |
79 | </tr> | |
80 | <tr> | |
81 | <td> | |
82 | <p> | |
83 | <code class="computeroutput"><span class="identifier">number</span><span class="special"><</span><span class="identifier">arithmetic_backend</span><span class="special"><</span><span class="keyword">double</span><span class="special">>,</span> | |
84 | <span class="identifier">et_off</span><span class="special">></span></code> | |
85 | </p> | |
86 | </td> | |
87 | <td> | |
88 | <p> | |
89 | 1.2 (0.019s) | |
90 | </p> | |
91 | </td> | |
92 | <td> | |
93 | <p> | |
94 | <span class="bold"><strong>1.0</strong></span>(0.46s) | |
95 | </p> | |
96 | </td> | |
97 | </tr> | |
98 | <tr> | |
99 | <td> | |
100 | <p> | |
101 | <code class="computeroutput"><span class="identifier">number</span><span class="special"><</span><span class="identifier">arithmetic_backend</span><span class="special"><</span><span class="keyword">double</span><span class="special">>,</span> | |
102 | <span class="identifier">et_on</span><span class="special">></span></code> | |
103 | </p> | |
104 | </td> | |
105 | <td> | |
106 | <p> | |
107 | 1.2 (0.019s) | |
108 | </p> | |
109 | </td> | |
110 | <td> | |
111 | <p> | |
112 | 1.7 (0.79s) | |
113 | </p> | |
114 | </td> | |
115 | </tr> | |
116 | </tbody> | |
117 | </table></div> | |
118 | <p> | |
119 | As you can see whether or not there is an overhead, and how large it is depends | |
120 | on the actual situation, but the overhead is in any cases small. Expression | |
121 | templates generally add a greater overhead the more complex the expression | |
122 | becomes due to the logic of figuring out how to best unpack and evaluate | |
123 | the expression, but of course this is also the situation where you save more | |
124 | temporaries. For a "trivial" backend like this, saving temporaries | |
125 | has no benefit, but for larger types it becomes a bigger win. | |
126 | </p> | |
127 | <p> | |
128 | The following table compares arithmetic using either <code class="computeroutput"><span class="keyword">long</span> | |
129 | <span class="keyword">long</span></code> or <code class="computeroutput"><span class="identifier">number</span><span class="special"><</span><span class="identifier">arithmetic_backend</span><span class="special"><</span><span class="keyword">long</span> <span class="keyword">long</span><span class="special">></span> <span class="special">></span></code> for the <a href="../../../../performance/voronoi_performance.cpp" target="_top">voronoi-diagram | |
130 | builder test</a>: | |
131 | </p> | |
132 | <div class="informaltable"><table class="table"> | |
133 | <colgroup> | |
134 | <col> | |
135 | <col> | |
136 | </colgroup> | |
137 | <thead><tr> | |
138 | <th> | |
139 | <p> | |
140 | Type | |
141 | </p> | |
142 | </th> | |
143 | <th> | |
144 | <p> | |
145 | Relative time | |
146 | </p> | |
147 | </th> | |
148 | </tr></thead> | |
149 | <tbody> | |
150 | <tr> | |
151 | <td> | |
152 | <p> | |
153 | <code class="computeroutput"><span class="keyword">long</span> <span class="keyword">long</span></code> | |
154 | </p> | |
155 | </td> | |
156 | <td> | |
157 | <p> | |
158 | <span class="bold"><strong>1.0</strong></span>(0.0823s) | |
159 | </p> | |
160 | </td> | |
161 | </tr> | |
162 | <tr> | |
163 | <td> | |
164 | <p> | |
165 | <code class="computeroutput"><span class="identifier">number</span><span class="special"><</span><span class="identifier">arithmetic_backend</span><span class="special"><</span><span class="keyword">long</span> <span class="keyword">long</span><span class="special">>,</span> <span class="identifier">et_off</span><span class="special">></span></code> | |
166 | </p> | |
167 | </td> | |
168 | <td> | |
169 | <p> | |
170 | 1.05 (0.0875s) | |
171 | </p> | |
172 | </td> | |
173 | </tr> | |
174 | </tbody> | |
175 | </table></div> | |
176 | <p> | |
177 | This test involves mainly creating a lot of temporaries and performing a | |
178 | small amount of arithmetic on them, with very little difference in performance | |
179 | between the native and "wrapped" types. | |
180 | </p> | |
181 | <p> | |
182 | The test code was compiled with Microsoft Visual Studio 2010 with all optimisations | |
183 | turned on (/Ox), and used MPIR-2.3.0 and <a href="http://libtom.org/?page=features&newsitems=5&whatfile=ltm" target="_top">libtommath</a>-0.42.0. | |
184 | The tests were run on 32-bit Windows Vista machine. | |
185 | </p> | |
186 | </div> | |
187 | <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> | |
188 | <td align="left"></td> | |
189 | <td align="right"><div class="copyright-footer">Copyright © 2002-2013 John Maddock and Christopher Kormanyos<p> | |
190 | Distributed under the Boost Software License, Version 1.0. (See accompanying | |
191 | 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>) | |
192 | </p> | |
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