source: trunk/doc/src/platforms/emb-performance.qdoc@ 846

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/*!
    \page qt-performance.html
    \title Qt Performance Tuning
    \ingroup qtce
    \ingroup qt-embedded-linux
    \brief Ways to improve performance on embedded platforms.

    When building embedded applications on low-powered devices,
    \l{Qt for Windows CE} and \l{Qt for Embedded Linux} provide
    a number of options that reduce the memory and/or CPU requirements
    by making various trade-offs. These options range from variations
    in programming style, to linking and memory allocation.

    Note that the most direct way of saving resources, is to avoid compiling
    in features that are not required. See the \l{Fine-Tuning Features in Qt}
    {fine tuning features} documentation for details.

    \tableofcontents

    \section1 Programming Style

    Rather than creating dialogs and widgets every time they are
    needed, and delete them when they are no longer required, create
    them once and use the QWidget::hide() and QWidget::show()
    functions whenever appropriate. To avoid a slow startup of the
    application, delay the creation of dialogs and widgets until they
    are requested. All this will improve the CPU performance, it
    requires a little more memory, but will be much faster.

    \section1 Static vs. Dynamic Linking

    A lot of CPU and memory is used by the ELF (Executable and Linking
    Format) linking process. Significant savings can be achieved by
    using a static build of the application suite; rather than having
    a collection of executables which link dynamically to Qt's
    libraries, all the applications is built into into a single
    executable which is statically linked to Qt's libraries.

    This improves the 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).

    \table 100%
    \row
    \o \bold {Creating a Static Build}

    To compile Qt as a static library, use the \c -static option when
    running configure:

    \snippet doc/src/snippets/code/doc_src_emb-performance.qdoc 0

    To build the 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 \c main() function. Then,
    write an application that provides a means of switching between
    the applications.  The \l Qt Extended platform is an example using this
    approach: It can be built either as a set of dynamically linked
    executables, or as a single static application.

    Note that the application still should link dynamically against
    the standard C library and any other libraries which might be used
    by other applications on the target device.

    \endtable

    When installing end-user applications, this approach may not be an
    option, but when building a single application suite for a device
    with limited CPU power and memory, this option could be very
    beneficial.

    \section1 Alternative Memory Allocation

    The libraries shipped with some C++ compilers on some platforms
    have poor performance in the built-in "new" and "delete"
    operators. Improved memory allocation and performance may be
    gained by re-implementing these functions:

    \snippet doc/src/snippets/code/doc_src_emb-performance.qdoc 1

    The example above shows the necessary code to switch to the plain
    C memory allocators.

    \section1 Bypassing the Backing Store

    When rendering, Qt uses the concept of a backing store; i.e., a
    paint buffer, to reduce flicker and to support graphics operations
    such as blending.

    The default behavior is for each client to render
    its widgets into memory while the server is responsible for
    putting the contents of the memory onto the screen. But when the
    hardware is known and well defined, as is often the case with
    software for embedded devices, it might be useful to bypass the
    backing store, allowing the clients to manipulate the underlying
    hardware directly.
    \if defined(qtce)
    This is achieved by setting the Qt::WA_PaintOnScreen window attribute
    for each widget.
    \else

    There are two approaches to direct painting: The first approach is
    to set the Qt::WA_PaintOnScreen window attribute for each widget,
    the other is to use the QDirectPainter class to reserve a region
    of the framebuffer.
      For more information, see the
      \l{Qt for Embedded Linux Architecture#Direct Painting}{direct painting}
      section of the \l{Qt for Embedded Linux Architecture}{architecture}
      documentation.
    \endif
*/