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[Qtopia Core]

# Performance Tuning

When building embedded applications on low-powered devices, a number of options are available 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.

But note that the most direct way of saving resources, is to avoid compiling in features that are not required. See Fine-Tuning Features for details.

- Programming Style
- Static vs. Dynamic Linking
- Alternative Memory Allocation
- Bypassing the Backing Store

## 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 appropiate. To avoid a a slow startup of the application, delay the creation of dialogs and widgets until they are requested.

This will improve CPU performance: The approach requires a little more memory, but will be much faster.

## 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.

This means that rather than having dynamic Qt libraries and a collection of executables which link dynamically to these libraries, all the applications is built into into a single executable which is statically linked to static Qt 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).

Creating a Static Build To compile Qt as a static library, use the `-static` option when running configure:
    ./configure -static
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 `main()` function. Then, write an application that provides a means of switching between the applications. 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.

The Qtopia platform is an example using this apporach: It can be built either as a set of dynamically linked executables, or as a single static application.

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.

## 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:

    void *operator new[](size_t size)
    {
        return malloc(size);
    }

    void *operator new(size_t size)
    {
        return malloc(size);
    }

    void operator delete[](void *ptr)
    {
        free(ptr);
    }

    void operator delete[](void *ptr, size_t)
    {
        free(ptr);
    }

    void operator delete(void *ptr)
    {
        free(ptr);
    }

    void operator delete(void *ptr, size_t)
    {
        free(ptr);
    }

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

## Bypassing the Backing Store

The backing store is a paint buffer that is used to reduce flicker, and to support graphics operations such as blending. On some devices, the memory requirements of the backing store are

To bypass the backing store for any given application, it must be executed with the following value set in the running environment:

    QT_ONSCREEN_PAINT = 1

This option only has an effect on opaque top-level windows. Enabling this option also has the side effect that software-based mouse cursors will be invisible when positioned any of the application's opaque windows since the cursor image cannot be superimposed on them.

[Qtopia Core]