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<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>
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