[QGIS-Developer] Caching of raster images

Régis Haubourg regis.haubourg at gmail.com
Thu Oct 5 05:08:17 PDT 2017


Hi Patrick ,
 did you consider using GDAL VRT to avoid opening so much files?
In my experience, this works well.
Régis


2017-10-05 13:48 GMT+02:00 Patrick Dunford <blackwhitekez at gmail.com>:

> Some time ago in a discussion of a particular bug a contributor expressed
> concern that the refresh of background rasters (aerial photography) in
> Windows was too slow.
>
> Maybe this is the reason that recent versions of master appear to be
> loading all of the background imagery into memory (I use a master from
> January this year to work around issues with later ones, and that master
> does not have this feature).
>
> Unfortunately if there are a lot of rasters then the memory demand is
> excessive and unsurprisingly slows down the computer negating any purported
> benefit of caching.
>
> As an example a project I am currently working on has about 900 aerial
> photo images (GeoJpeg). When the layer is turned on for display, Qgis
> requires about 46 GB of virtual memory. Since my computer only has 24 GB of
> physical memory, it is required to dip into the swap space considerably.
> Even with 60 GB of swap space on an SSD, the swapping needed to refresh the
> canvas is substantial and dramatically reduces performance resulting in
> substantial delays. Compare with the January master referred to above which
> only requires about 7 GB of virtual memory total with the aerial photo
> layer displayed. The time needed to refresh the canvas is less than 1
> second, most of the time.
>
> I know that the canvas refresh in Windows with aerial photos can be
> substantially slower than in Linux. This does not affect me, because I
> don't use Windows now that I have a stable platform for running an older
> Linux master alongside the most recent one. What I do know is that the
> memory demands are making it difficult to evaluate the recent masters. I
> need some kind of setting to turn this caching off. With the aerial photo
> layer turned off, the memory usage of the current master is about the same
> as the old one, and it's much quicker to update.
>
>
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