| 1 | /****************************************************************************
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| 2 | **
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| 3 | ** Copyright (C) 2011 Nokia Corporation and/or its subsidiary(-ies).
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| 4 | ** All rights reserved.
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| 5 | ** Contact: Nokia Corporation ([email protected])
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| 6 | **
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| 7 | ** This file is part of the documentation of the Qt Toolkit.
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| 8 | **
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| 9 | ** $QT_BEGIN_LICENSE:FDL$
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| 10 | ** Commercial Usage
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| 11 | ** Licensees holding valid Qt Commercial licenses may use this file in
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| 12 | ** accordance with the Qt Commercial License Agreement provided with the
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| 13 | ** Software or, alternatively, in accordance with the terms contained in a
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| 14 | ** written agreement between you and Nokia.
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| 15 | **
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| 16 | ** GNU Free Documentation License
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| 17 | ** Alternatively, this file may be used under the terms of the GNU Free
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| 18 | ** Documentation License version 1.3 as published by the Free Software
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| 19 | ** Foundation and appearing in the file included in the packaging of this
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| 20 | ** file.
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| 21 | **
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| 22 | ** If you have questions regarding the use of this file, please contact
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| 23 | ** Nokia at [email protected].
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| 24 | ** $QT_END_LICENSE$
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| 25 | **
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| 26 | ****************************************************************************/
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| 27 |
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| 28 | /*!
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| 29 | \page qt-performance.html
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| 30 | \title Qt Performance Tuning
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| 31 | \ingroup qtce
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| 32 | \ingroup qt-embedded-linux
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| 33 | \brief Ways to improve performance on embedded platforms.
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| 34 |
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| 35 | When building embedded applications on low-powered devices,
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| 36 | \l{Qt for Windows CE} and \l{Qt for Embedded Linux} provide
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| 37 | a number of options that reduce the memory and/or CPU requirements
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| 38 | by making various trade-offs. These options range from variations
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| 39 | in programming style, to linking and memory allocation.
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| 40 |
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| 41 | Note that the most direct way of saving resources, is to avoid compiling
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| 42 | in features that are not required. See the \l{Fine-Tuning Features in Qt}
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| 43 | {fine tuning features} documentation for details.
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| 44 |
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| 45 | \tableofcontents
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| 46 |
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| 47 | \section1 Programming Style
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| 48 |
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| 49 | Rather than creating dialogs and widgets every time they are
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| 50 | needed, and delete them when they are no longer required, create
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| 51 | them once and use the QWidget::hide() and QWidget::show()
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| 52 | functions whenever appropriate. To avoid a slow startup of the
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| 53 | application, delay the creation of dialogs and widgets until they
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| 54 | are requested. All this will improve the CPU performance, it
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| 55 | requires a little more memory, but will be much faster.
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| 56 |
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| 57 | \section1 Static vs. Dynamic Linking
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| 58 |
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| 59 | A lot of CPU and memory is used by the ELF (Executable and Linking
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| 60 | Format) linking process. Significant savings can be achieved by
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| 61 | using a static build of the application suite; rather than having
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| 62 | a collection of executables which link dynamically to Qt's
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| 63 | libraries, all the applications is built into into a single
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| 64 | executable which is statically linked to Qt's libraries.
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| 65 |
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| 66 | This improves the start-up time and reduces memory usage at the
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| 67 | expense of flexibility (to add a new application, you must
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| 68 | recompile the single executable) and robustness (if one
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| 69 | application has a bug, it might harm other applications).
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| 70 |
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| 71 | \table 100%
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| 72 | \row
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| 73 | \o \bold {Creating a Static Build}
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| 74 |
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| 75 | To compile Qt as a static library, use the \c -static option when
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| 76 | running configure:
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| 77 |
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| 78 | \snippet doc/src/snippets/code/doc_src_emb-performance.qdoc 0
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| 79 |
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| 80 | To build the application suite as an all-in-one application,
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| 81 | design each application as a stand-alone widget (or set of
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| 82 | widgets) with only minimal code in the \c main() function. Then,
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| 83 | write an application that provides a means of switching between
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| 84 | the applications. The \l Qt Extended platform is an example using this
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| 85 | approach: It can be built either as a set of dynamically linked
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| 86 | executables, or as a single static application.
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| 87 |
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| 88 | Note that the application still should link dynamically against
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| 89 | the standard C library and any other libraries which might be used
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| 90 | by other applications on the target device.
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| 91 |
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| 92 | \endtable
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| 93 |
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| 94 | When installing end-user applications, this approach may not be an
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| 95 | option, but when building a single application suite for a device
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| 96 | with limited CPU power and memory, this option could be very
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| 97 | beneficial.
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| 98 |
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| 99 | \section1 Alternative Memory Allocation
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| 100 |
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| 101 | The libraries shipped with some C++ compilers on some platforms
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| 102 | have poor performance in the built-in "new" and "delete"
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| 103 | operators. Improved memory allocation and performance may be
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| 104 | gained by re-implementing these functions:
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