[QGIS-Developer] Preview job and slow datasources

[Alessandro Pasotti]

Maybe a crazy idea, but if this proves to be an issue, what about a
monitoring process that prompts the user to disable this feature when
memory goes over a certain threshold?

On Fri, Nov 17, 2017 at 8:21 AM, Patrick Dunford <enzedrailmaps at gmail.com>
wrote:

> Most of them are geoJpeg, there are a smaller number of geoTiff
>
> On 17/11/17 20:10, Matthias Kuhn wrote:
>
> Thanks, that's an interesting information.
>
> What datatype are your rasters?
>
> On 11/17/2017 03:53 AM, Patrick Dunford wrote:
>
> With the second command
>
> iface.mapCanvas().setPreviewJobsEnabled(False)
>
> when I run that command it drops back to 16 GB
>
> On 16/11/17 23:15, Matthias Kuhn wrote:
>
> Hi Patrick,
>
> Would be interesting to do this test with previews enabled and disabled.
> Then we'll see if it's actually the previews or some other mechanism that
> is causing this.
>
> IIRC only a composite image is saved in the preview jobs and not each
> layer separately, but that's just what I remember and not evidence based.
>
> You can enable them in the python console with
>
>     iface.mapCanvas().setPreviewJobsEnabled(True)
>
> and disable with
>
>     iface.mapCanvas().setPreviewJobsEnabled(False)
>
> Thanks for a feedback
>
> Matthias
>
> On 11/16/2017 08:42 AM, Patrick Dunford wrote:
>
> In my current project using Qgis 2.99, turning off the rasters uses 16 GB,
> turning them on uses 52 GB
>
> I do not believe it is independent from the number of layers that are
> being displayed.
>
> Previous versions of the software did not cache every single raster (the
> number of rasters actually being displayed on the canvas at any one time is
> a small fraction of the total number in the project)
> On 16/11/17 20:11, Matthias Kuhn wrote:
>
> Hi Patrick,
>
> This uses some memory (~ canvas width pixels * canvas height pixels * 8
> preview images * 32 bit RGBA), so let's assume 50 MB to 100 MB.
>
> This consumption is independent from the number or type of layers.
>
> Matthias
>
> On 11/16/2017 08:02 AM, Patrick Dunford wrote:
>
> So to put it another way this is the reason why Qgis wants to use a huge
> amount of memory (40 GB) when I have a lot of raster images loaded in the
> background.
>
> On 16/11/17 18:56, Tim Sutton wrote:
>
> Hi
>
> On 16 Nov 2017, at 04:35, Patrick Dunford <enzedrailmaps at gmail.com> wrote:
>
> What is this "preview job" function?
>
>
> Its application logic to prefetch / pre-render offscreen content in
> anticipation of user panning the map.
>
> Regards
>
> Tim
>
>
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>
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>
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-- 
Alessandro Pasotti
w3:   www.itopen.it
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