[Interest] Qt5 performance on imx6 with full hd

Jacob Kroon jacob.kroon at gmail.com
Tue May 27 10:28:17 CEST 2014

Hi Gunnar,

On Fri, May 23, 2014 at 11:39 AM, Gunnar Sletta <gunnar.sletta at jolla.com>wrote:

> On 23 May 2014, at 10:32, Jacob Kroon <jacob.kroon at gmail.com> wrote:
> > Hi,
> >
> > I'm experimenting with a Qt application running on the Wandboards,
> > at full hd resolution, 1920x1080x32. I have a static background image,
> and some small animated Qml-elements on the screen. I'm not entirely
> satisfied with the resulting performance, and I think it is because of the
> background image being constantly fully redrawn in each frame.
> If it cannot render a single image at that resolution, then it is not a
> suitable hardware for that resolution :)
> If this is the case your options are to reduce the bit depth, reduce the
> resolution or reduce the framerate (such as going for 30FPS instead of
> 60FPS)
> Is the image alpha blended? If it is a JPEG it should be opaque, but if it
> is a PNG it will most likely have an alpha channel (even thought it really
> doesn't. I know, stupid, but that how it is). For some GPUs blending can
> add a bit of overhead, and doing it fullscreen can be what tips the balance.
For the background image, what I did was that I used a tiled noise image,
and did a radial blend on it. The resulting image was cached though. But
yes, the resulting image showed up as "alpha" in the renderer debug output,
I've switched to having a static prepared png image, and now the background
image shows up as "opaque".

> I've experimented with
> >
> > * setClearBeforeRendering(false), since the background will be drawn
> anyway, there is no point in clearing before rendering. This seemed to have
> little impact on performance though.
> The effect of turning off clearing is highly hardware dependent. Some
> drivers/GPUs will benefit from not clearing as the clear is just yet
> another pass over all pixels. Others will use the clear as an indication of
> "new frame" and will have to do all manner of nasty stuff, like storing the
> depth buffer into system memory because you didn't clear it before the
> frame began.
> See the performance guidelines of your GPU for the actual recommendation
> for your chip.
It seems that setting "QSG_ANTIALIASING_METHOD=msaa" gives a huge
improvement on the cpu usage.
allows me to lower the cpu usage even further. Still, the background image
is taking a considerable time it seems. With bg image I get ~27 fps,
without it I get ~85 fps (with vsync turned off)

> > * Letting Qt5 render into /dev/fb1 overlay on the imx6, with no
> background image, but instead write the background image manually into
> /dev/fb0. In this way, the IPU will blend the result onto the display. This
> seemed to be even worse than letting the GPU render the background.
> >
> > Can the scenegraph be smart enough in such a way that it will only
> "clear" dirty rectangles with a user supplied background image ? Or are
> there any other tricks I am not aware of ?
> The default scene graph renderer renders the full screen. Doing partial
> updates requires a lot of support from the underlying drivers and hardware.
> If you really want to, you can give it a shot of course. You can copy the
> existing renderer, adapt it to expose dirty areas and make use of partial
> swap buffers extension if this is available (and the underlying display
> stack actually does propagate the sub regions all the way to the display).
Writing a custom renderer would have to be a final resort ..

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