[QGIS-Developer] QGIS 3.0 Sluggish on MacOS 10.13 with Retina Display

[Nyall Dawson]

On 21 February 2018 at 06:34, Jeremy Palmer <palmerjnz at gmail.com> wrote:
> Hi All,
>
> For some time I've been noticing that QGIS 3.0 on MacOSX has sluggish canvas
> redraw speeds. When comparing between QGIS 2.18 and 3.0 I see ~4x more CPU
> activity in 3.0 when performing the same set of operations on the same
> dataset.
>
> My test case was using a single raster VRT with overviews, and zooming in
> and panning around the map cavnas for 20 sec. On average each time I tested
> this case the total used CPU time for 2.18 was about 6 secs, but in
> 3.0dev-64369f8099 (complied against QT 5.10.0 with homebrew) I get total CPU
> time of 25 secs. In QGIS 3.0 most of the time the application is using 100%
> CPU (one core) and spikes to about 180-200% CPU when the canvas redraws are
> performed. When using a profiler it seems much of the time is taken up in
> QImage::copy(QRect const&) within two seperate, but equivalent, call
> subtrees. See [1] for more info.

There's not much to go on in that stack - it's all Qt internals.

Are you sure this isn't solely due to the fact that QGIS is painting
so many more pixels? I'd like to see a test where you force a low dpi
resolution for both 2.18 and 3.0.

Nyall