[gdal-dev] gdal_merge is very slow

[Leith Bade]

That seemed to do the trick. It finished overnight, a lot faster than
before.

Final file was 83GB. (Only 1/2 the dataset was used)

Is there a reason GDAL_CACHEMAX 2147 is the maximum? Under 64-bit a 32-bit
process has 4GB of virtual memory rather than 2GB under 32-bit (or 3GB with
the /3GB Windows boot switch).

I was successfully able to use kdu_compress on the result as well so now I
only need 4GB to store the file! (Although kdu_compress also has a memory
limit at 2GB, but you can flush the codestream after every tile to fix
that).

Thanks,
Leith Bade
leith at leithalweapon.geek.nz



On 17 July 2010 15:29, Greg Coats <gregcoats at mac.com> wrote:

> Since you have 4 GB RAM, I suggest overriding the 40 MB GDAL_CACHEMAX
> default, and increasing it
> gdal_merge.py --config GDAL_CACHEMAX 1000
> The maximum possible GDAL_CACHEMAX is
> gdal_merge.py --config GDAL_CACHEMAX 2147
> Greg
>
> On Jul 16, 2010, at 9:19 PM, Leith Bade wrote:
>
> Hi,
>
> I am trying to use gdal_merge to mosaic a very large topo GeoTIFF set.
>
> Uncompressed the data set is 60GB, but I keep it stored with DEFLATE
> compression which results in a dataset under 10GB.
>
> Mosaicked the uncompressed file will be 125GB because of the large regions
> of nodata generated. Unfortunately this is too big to store on my HDD so I
> need to apply DEFLATE to it as well.
>
> I am experimenting with only 1/2 the data set at the moment with this
> command:
> gdal_merge -co COMPRESS=DEFLATE -co ZLEVEL=9 -co BIGTIFF=YES -o NI-50.tif
> *-00.tif
>
> On my AMD Phenom II X4 @ 3.2GHz, 64 bit Windows 7, 4GB DDR3 CAS-7 RAM
> (basically a decently specced PC gaming machine)
> it has been running for 16 hours and so far has only made it 60% (124 of
> 204 files)
>
> The other issue is so far the file is 54GB so I will likely run out of disc
> space before it finished. This indicates that no DEFLATE compression is
> happening at all!
>
> It currently maxing only 1 CPU core, so I assume it is trying to run
> DEFLATE then somehow failing to compress at all?
>
> Looking at gdal_merge.py I think the major performance issue is the order
> in which it copies the data. Currently it copies an entire image at a time
> (1 colour channel at a time). Thus DEFLATE will not work due to the rather
> random write pattern to each scanline.
>
> I think a much faster method would be to calculate the destination
> rectangles for each file into some sort of list. Aftter this generate the
> destination file 1 scanline at a time, calculating which source images
> intersect, and working left to right filling with solid color or copying
> scanlines from the source image.
>
> This allows the DEFLATE to work far more effciently as the write pattern is
> horizontally linear.
>
> What are your views/suggestions/etc.?
>
> Thanks,
> Leith Bade
> leith at leithalweapon.geek.nz
>
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