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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