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| 32 | <td align="right" valign="center"><img src="logo32.png" align="right" width="64" height="32" border="0"></td></tr></table><h1 align=center>Qt/Embedded Performance Tuning</h1>
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| 33 |
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| 34 |
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| 35 | When building embedded applications on low-powered devices, a number
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| 36 | of options are available that would not be considered in a desktop
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| 37 | application environment. These options reduce the memory and/or CPU
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| 38 | requirements at the cost of other factors.
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| 39 | <p> <ul>
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| 40 | <li> <a href="emb-features.html"><b>Tuning the functionality of Qt</a>
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| 41 | <li> <a href="#general">General programming style</a>
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| 42 | <li> <a href="#static">Static vs. Dynamic linking</a>
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| 43 | <li> <a href="#alloc">Alternative memory allocation</a>
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| 44 | </ul>
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| 45 | <p> <a name="general"></a>
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| 46 | <h2> General programming style
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| 47 | </h2>
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| 48 | <a name="1"></a><p> The following guidelines will improve CPU performance:
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| 49 | <ul>
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| 50 | <li> Create dialogs and widgets once, then <a href="qwidget.html#hide">QWidget::hide</a>() and
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| 51 | <a href="qwidget.html#show">QWidget::show</a>() them, rather than creating them and deleting
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| 52 | them every time they are needed.
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| 53 | This will use a little more memory, but will be much faster.
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| 54 | Try to create them the first time "lazily" to avoid slow
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| 55 | startup (e.g. only create a Find dialog the first time the
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| 56 | user invokes it).
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| 57 | </ul>
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| 58 | <p> <a name="static"></a>
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| 59 | <h2> Static vs. Dynamic linking
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| 60 | </h2>
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| 61 | <a name="2"></a><p> A lot of CPU and memory is used by the ELF linking process. You can
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| 62 | make significant savings by using a static build of your application
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| 63 | suite. This means that rather than having a dynamic library (<tt>libqte.so</tt>) and a collection of executables which link dynamically to
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| 64 | that library, you build all the applications into a single executable
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| 65 | and statically link that with a static library (<tt>libqt.a</tt>). This
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| 66 | improves start-up time, and reduces memory usage, at the expense of
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| 67 | flexibility (to add a new application, you must recompile the single
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| 68 | executable) and robustness (if one application has a bug, it might
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| 69 | harm other applications). If you need to install end-user
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| 70 | applications, this may not be an option, but if you are building a
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| 71 | single application suite for a device with limited CPU power and
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| 72 | memory, this option could be very beneficial.
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| 73 | <p> To compile Qt as a static library, add the <tt>-static</tt> options when
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| 74 | you run configure.
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| 75 | <p> To build your application suite as an all-in-one application, design each
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| 76 | application as a stand-alone widget or set of widgets, with only minimal
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| 77 | code in the main() function. Then, write an application that gives
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| 78 | some way to switch between the applications (e.g. a <a href="qiconview.html">QIconView</a>).
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| 79 | <a href="http://www.trolltech.com/products/qtopia/index.html">Qtopia</a> is an example of this. It can be built either as a set of
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| 80 | dynamically linked executables, or as a single static application.
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| 81 | <p> Note that you should generally still link dynamically against the
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| 82 | standard C library and any other libraries which might be used by
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| 83 | other applications on your device.
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| 84 | <p> <a name="alloc"></a>
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| 85 | <h2> Alternative memory allocation
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| 86 | </h2>
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| 87 | <a name="3"></a><p> We have found that the libraries shipped with some C++ compilers on
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| 88 | some platforms have poor performance in the built-in "new" and "delete"
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| 89 | operators. You might gain performance by re-implementing these
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| 90 | functions. For example, you can switch to the plain C allocators
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| 91 | by adding the following to your code:
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| 92 | <p> <pre>
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| 93 | void* operator new[]( size_t size )
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| 94 | {
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| 95 | return malloc( size );
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| 96 | }
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| 97 |
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| 98 | void* operator new( size_t size )
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| 99 | {
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| 100 | return malloc( size );
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| 101 | }
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| 102 |
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| 103 | void operator delete[]( void *p )
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| 104 | {
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| 105 | free( p );
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| 106 | }
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| 107 |
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| 108 | void operator delete[]( void *p, size_t size )
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| 109 | {
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| 110 | free( p );
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| 111 | }
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| 112 |
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| 113 | void operator delete( void *p )
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| 114 | {
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| 115 | free( p );
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| 116 | }
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| 117 |
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| 118 | void operator delete( void *p, size_t size )
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| 119 | {
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| 120 | free( p );
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| 121 | }
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| 122 | </pre>
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| 123 |
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| 124 |
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| 125 | <!-- eof -->
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| 126 | <p><address><hr><div align=center>
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| 128 | <td>Copyright © 2007
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| 129 | <a href="troll.html">Trolltech</a><td align=center><a href="trademarks.html">Trademarks</a>
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| 130 | <td align=right><div align=right>Qt 3.3.8</div>
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| 131 | </table></div></address></body>
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