[QGIS-Developer] Preview job and slow datasources
Matthias Kuhn
matthias at opengis.ch
Thu Nov 16 08:51:00 PST 2017
Hi
they are triggered by QgsMapCanvas (in the gui library), so I would be
very surprised if they were triggered on the server.
Matthias
On 11/16/2017 02:29 PM, kimaidou wrote:
> Hi,
>
> I was wondering if these preview jobs are enabled in QGIS Server
> context, when requesting a GetMap from any client. I am pretty sure
> the context of the preview jobs execution is related to Desktop, but I
> would like to be sure.
> Any hint on this ?
>
> Regards,
> Michaël
>
> 2017-11-16 11:15 GMT+01:00 Matthias Kuhn <matthias at opengis.ch
> <mailto:matthias at opengis.ch>>:
>
> 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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>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
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>>>>>
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