<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <!-- /home/espenr/tmp/qt-3.3.8-espenr-2499/qt-x11-free-3.3.8/doc/qws.doc:532 --> <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"> <title>Qt/Embedded Performance Tuning</title> <style type="text/css"><!-- fn { margin-left: 1cm; text-indent: -1cm; } a:link { color: #004faf; text-decoration: none } a:visited { color: #672967; text-decoration: none } body { background: #ffffff; color: black; } --></style> </head> <body> <table border="0" cellpadding="0" cellspacing="0" width="100%"> <tr bgcolor="#E5E5E5"> <td valign=center> <a href="index.html"> <font color="#004faf">Home</font></a> | <a href="classes.html"> <font color="#004faf">All Classes</font></a> | <a href="mainclasses.html"> <font color="#004faf">Main Classes</font></a> | <a href="annotated.html"> <font color="#004faf">Annotated</font></a> | <a href="groups.html"> <font color="#004faf">Grouped Classes</font></a> | <a href="functions.html"> <font color="#004faf">Functions</font></a> </td> <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> When building embedded applications on low-powered devices, a number of options are available that would not be considered in a desktop application environment. These options reduce the memory and/or CPU requirements at the cost of other factors. <p> <ul> <li> <a href="emb-features.html"><b>Tuning the functionality of Qt</a> <li> <a href="#general">General programming style</a> <li> <a href="#static">Static vs. Dynamic linking</a> <li> <a href="#alloc">Alternative memory allocation</a> </ul> <p> <a name="general"></a> <h2> General programming style </h2> <a name="1"></a><p> The following guidelines will improve CPU performance: <ul> <li> Create dialogs and widgets once, then <a href="qwidget.html#hide">QWidget::hide</a>() and <a href="qwidget.html#show">QWidget::show</a>() them, rather than creating them and deleting them every time they are needed. This will use a little more memory, but will be much faster. Try to create them the first time "lazily" to avoid slow startup (e.g. only create a Find dialog the first time the user invokes it). </ul> <p> <a name="static"></a> <h2> Static vs. Dynamic linking </h2> <a name="2"></a><p> A lot of CPU and memory is used by the ELF linking process. You can make significant savings by using a static build of your application suite. This means that rather than having a dynamic library (<tt>libqte.so</tt>) and a collection of executables which link dynamically to that library, you build all the applications into a single executable and statically link that with a static library (<tt>libqt.a</tt>). This improves start-up time, and reduces memory usage, at the expense of flexibility (to add a new application, you must recompile the single executable) and robustness (if one application has a bug, it might harm other applications). If you need to install end-user applications, this may not be an option, but if you are building a single application suite for a device with limited CPU power and memory, this option could be very beneficial. <p> To compile Qt as a static library, add the <tt>-static</tt> options when you run configure. <p> To build your application suite as an all-in-one application, design each application as a stand-alone widget or set of widgets, with only minimal code in the main() function. Then, write an application that gives some way to switch between the applications (e.g. a <a href="qiconview.html">QIconView</a>). <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 dynamically linked executables, or as a single static application. <p> Note that you should generally still link dynamically against the standard C library and any other libraries which might be used by other applications on your device. <p> <a name="alloc"></a> <h2> Alternative memory allocation </h2> <a name="3"></a><p> We have found that the libraries shipped with some C++ compilers on some platforms have poor performance in the built-in "new" and "delete" operators. You might gain performance by re-implementing these functions. For example, you can switch to the plain C allocators by adding the following to your code: <p> <pre> void* operator new[]( size_t size ) { return malloc( size ); } void* operator new( size_t size ) { return malloc( size ); } void operator delete[]( void *p ) { free( p ); } void operator delete[]( void *p, size_t size ) { free( p ); } void operator delete( void *p ) { free( p ); } void operator delete( void *p, size_t size ) { free( p ); } </pre> <!-- eof --> <p><address><hr><div align=center> <table width=100% cellspacing=0 border=0><tr> <td>Copyright © 2007 <a href="troll.html">Trolltech</a><td align=center><a href="trademarks.html">Trademarks</a> <td align=right><div align=right>Qt 3.3.8</div> </table></div></address></body> </html>