[QGIS-Developer] Preview job and slow datasources

[Patrick Dunford]

What is this "preview job" function?


On 16/11/17 03:56, Even Rouault wrote:
>
> Hi,
>
> I'm working currently on improving raster performance per
>
> https://github.com/qgis/QGIS/pull/5632 , and while testing with a 
> variety of GDAL datasources, I found that the "preview job" (*) 
> mechanism that is currently enabled on the main canvas (pre-rendering 
> the 8 zones around the canvas extent) can have adverse performance 
> impact with very slow datasources. Particularly those doing network 
> accesses, where the bandwidth consumed by the preview jobs prevents 
> the main rendering job to being served first (due to the fact the the 
> preview jobs of the previous interaction are not immediately cancelled 
> by the next one since, because being busy in libcurl calls)
>
> Wouldn't it make sense to offer an option to disable preview jobs on 
> the main canvas for such use case ?
>
> I guess this could also be useful for complex projects with many 
> layers of "fast datasources", but where the global rendering time is 
> large.
>
> A clever variant would measure the execution time of those preview 
> jobs, and if, for example, it is above the 250 ms delay in which they 
> are fired by QgsMapCanvas::schedulePreviewJob(), it would 
> automatically disable it (potentially, temporarily, until the number 
> of layers changes). In the case I mentionned above, each preview job 
> takes several seconds to complete.
>
> Or a simpler alternative would be to schedule the next preview job 
> after the previous preview job has completed (instead of starting them 
> every 250 ms). This would not be completely ideal with my above 
> mentionned network case, compared to completely disabling preview 
> jobs, but that could be a reasonable compromise, and would limit the 
> CPU load of preview jobs to at most one core (currently if the preview 
> jobs run slowly, they can "eat" up to 8 cores, since they are 8 
> preview jobs)
>

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