[QGIS-Developer] Preview job and slow datasources

[Patrick Dunford]

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 <mailto: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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>>>>>
>>>>> —
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> *Tim Sutton*
>>>>>
>>>>> *Co-founder:*Kartoza
>>>>> *Project chair:*QGIS.org <http://QGIS.org>
>>>>>
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>>>>>
>>>>
>>>>
>>>>
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