source: trunk/doc/src/emb-performance.qdoc@ 321

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41
42/*!
43 \page qt-performance.html
44 \title Qt Performance Tuning
45 \ingroup qtce
46 \ingroup qt-embedded-linux
47 \brief Ways to improve performance on embedded platforms.
48
49 When building embedded applications on low-powered devices,
50 \l{Qt for Windows CE} and \l{Qt for Embedded Linux} provide
51 a number of options that reduce the memory and/or CPU requirements
52 by making various trade-offs. These options range from variations
53 in programming style, to linking and memory allocation.
54
55 Note that the most direct way of saving resources, is to avoid compiling
56 in features that are not required. See the \l{Fine-Tuning Features in Qt}
57 {fine tuning features} documentation for details.
58
59 \tableofcontents
60
61 \section1 Programming Style
62
63 Rather than creating dialogs and widgets every time they are
64 needed, and delete them when they are no longer required, create
65 them once and use the QWidget::hide() and QWidget::show()
66 functions whenever appropriate. To avoid a slow startup of the
67 application, delay the creation of dialogs and widgets until they
68 are requested. All this will improve the CPU performance, it
69 requires a little more memory, but will be much faster.
70
71 \section1 Static vs. Dynamic Linking
72
73 A lot of CPU and memory is used by the ELF (Executable and Linking
74 Format) linking process. Significant savings can be achieved by
75 using a static build of the application suite; rather than having
76 a collection of executables which link dynamically to Qt's
77 libraries, all the applications is built into into a single
78 executable which is statically linked to Qt's libraries.
79
80 This improves the start-up time and reduces memory usage at the
81 expense of flexibility (to add a new application, you must
82 recompile the single executable) and robustness (if one
83 application has a bug, it might harm other applications).
84
85 \table 100%
86 \row
87 \o \bold {Creating a Static Build}
88
89 To compile Qt as a static library, use the \c -static option when
90 running configure:
91
92 \snippet doc/src/snippets/code/doc_src_emb-performance.qdoc 0
93
94 To build the application suite as an all-in-one application,
95 design each application as a stand-alone widget (or set of
96 widgets) with only minimal code in the \c main() function. Then,
97 write an application that provides a means of switching between
98 the applications. The \l Qt Extended platform is an example using this
99 approach: It can be built either as a set of dynamically linked
100 executables, or as a single static application.
101
102 Note that the application still should link dynamically against
103 the standard C library and any other libraries which might be used
104 by other applications on the target device.
105
106 \endtable
107
108 When installing end-user applications, this approach may not be an
109 option, but when building a single application suite for a device
110 with limited CPU power and memory, this option could be very
111 beneficial.
112
113 \section1 Alternative Memory Allocation
114
115 The libraries shipped with some C++ compilers on some platforms
116 have poor performance in the built-in "new" and "delete"
117 operators. Improved memory allocation and performance may be
118 gained by re-implementing these functions:
119
120 \snippet doc/src/snippets/code/doc_src_emb-performance.qdoc 1
121
122 The example above shows the necessary code to switch to the plain
123 C memory allocators.
124
125 \section1 Bypassing the Backing Store
126
127 When rendering, Qt uses the concept of a backing store; i.e., a
128 paint buffer, to reduce flicker and to support graphics operations
129 such as blending.
130
131 The default behavior is for each client to render
132 its widgets into memory while the server is responsible for
133 putting the contents of the memory onto the screen. But when the
134 hardware is known and well defined, as is often the case with
135 software for embedded devices, it might be useful to bypass the
136 backing store, allowing the clients to manipulate the underlying
137 hardware directly.
138 \if defined(qtce)
139 This is achieved by setting the Qt::WA_PaintOnScreen window attribute
140 for each widget.
141 \else
142
143 There are two approaches to direct painting: The first approach is
144 to set the Qt::WA_PaintOnScreen window attribute for each widget,
145 the other is to use the QDirectPainter class to reserve a region
146 of the framebuffer.
147 For more information, see the
148 \l{Qt for Embedded Linux Architecture#Direct Painting}{direct painting}
149 section of the \l{Qt for Embedded Linux Architecture}{architecture}
150 documentation.
151 \endif
152*/
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