[QGIS-Developer] Caching of raster images

[Patrick Dunford]

I don't know what this is.


On 06/10/17 01:08, Régis Haubourg wrote:
> 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 
> <mailto: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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