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 <blackwhite...@gmail.com
<mailto:blackwhite...@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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