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- <title>The Overhead in the Number Class Wrapper</title>
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- <div class="titlepage"><div><div><h3 class="title">
- <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
- Number Class Wrapper</a>
- </h3></div></div></div>
- <p>
- Using a simple <a href="../../../../performance/arithmetic_backend.hpp" target="_top">backend
- class</a> that wraps any built in arithmetic type we can measure the
- overhead involved in wrapping a type inside the <code class="computeroutput"><span class="identifier">number</span></code>
- frontend, and the effect that turning on expression templates has. The following
- table compares the performance between <code class="computeroutput"><span class="keyword">double</span></code>
- and a <code class="computeroutput"><span class="keyword">double</span></code> wrapped inside
- class <code class="computeroutput"><span class="identifier">number</span></code>:
- </p>
- <div class="informaltable"><table class="table">
- <colgroup>
- <col>
- <col>
- <col>
- </colgroup>
- <thead><tr>
- <th>
- <p>
- Type
- </p>
- </th>
- <th>
- <p>
- Bessel Function Evaluation
- </p>
- </th>
- <th>
- <p>
- Non-Central T Evaluation
- </p>
- </th>
- </tr></thead>
- <tbody>
- <tr>
- <td>
- <p>
- <code class="computeroutput"><span class="keyword">double</span></code>
- </p>
- </td>
- <td>
- <p>
- <span class="bold"><strong>1.0 (0.016s)</strong></span>
- </p>
- </td>
- <td>
- <p>
- <span class="bold"><strong>1.0</strong></span> (0.46s)
- </p>
- </td>
- </tr>
- <tr>
- <td>
- <p>
- <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>
- <span class="identifier">et_off</span><span class="special">></span></code>
- </p>
- </td>
- <td>
- <p>
- 1.2 (0.019s)
- </p>
- </td>
- <td>
- <p>
- <span class="bold"><strong>1.0</strong></span>(0.46s)
- </p>
- </td>
- </tr>
- <tr>
- <td>
- <p>
- <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>
- <span class="identifier">et_on</span><span class="special">></span></code>
- </p>
- </td>
- <td>
- <p>
- 1.2 (0.019s)
- </p>
- </td>
- <td>
- <p>
- 1.7 (0.79s)
- </p>
- </td>
- </tr>
- </tbody>
- </table></div>
- <p>
- As you can see whether or not there is an overhead, and how large it is depends
- on the actual situation, but the overhead is in any cases small. Expression
- templates generally add a greater overhead the more complex the expression
- becomes due to the logic of figuring out how to best unpack and evaluate
- the expression, but of course this is also the situation where you save more
- temporaries. For a "trivial" backend like this, saving temporaries
- has no benefit, but for larger types it becomes a bigger win.
- </p>
- <p>
- The following table compares arithmetic using either <code class="computeroutput"><span class="keyword">long</span>
- <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
- builder test</a>:
- </p>
- <div class="informaltable"><table class="table">
- <colgroup>
- <col>
- <col>
- </colgroup>
- <thead><tr>
- <th>
- <p>
- Type
- </p>
- </th>
- <th>
- <p>
- Relative time
- </p>
- </th>
- </tr></thead>
- <tbody>
- <tr>
- <td>
- <p>
- <code class="computeroutput"><span class="keyword">long</span> <span class="keyword">long</span></code>
- </p>
- </td>
- <td>
- <p>
- <span class="bold"><strong>1.0</strong></span>(0.0823s)
- </p>
- </td>
- </tr>
- <tr>
- <td>
- <p>
- <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>
- </p>
- </td>
- <td>
- <p>
- 1.05 (0.0875s)
- </p>
- </td>
- </tr>
- </tbody>
- </table></div>
- <p>
- This test involves mainly creating a lot of temporaries and performing a
- small amount of arithmetic on them, with very little difference in performance
- between the native and "wrapped" types.
- </p>
- <p>
- The test code was compiled with Microsoft Visual Studio 2010 with all optimisations
- turned on (/Ox), and used MPIR-2.3.0 and <a href="http://libtom.net" target="_top">libtommath</a>-0.42.0.
- The tests were run on 32-bit Windows Vista machine.
- </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 © 2002-2019 John Maddock
- and Christopher Kormanyos<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>
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