[GRASS-dev] r.external performance

[Alessandro Frigeri]

Hi,

I've set up a small experiment to verify how r.external registered
maps perform over native grass raster maps.  I've prepared a shell
script that performs r.slope.aspect one time on the imported dem map
(through r.in.gdal) and one time on the same map, but registered with
r.external.  The execution time is registered via gnu/time utility and
differences are calculated. I'm working with geotiff maps (gtopo30),
GRASS 6.4.0+42329, GDAL 1.7.2. .

Today I made the first experiments and in general it seems that for
small maps (~1200x1500) we have a better performance with GRASS native
maps (about 10%), while for bigger maps (18000x43000) I have found a
better performance on external registered maps.  Markus suggested me
that the speedup of the process for big maps may be explained by GDAL
caching mechanism.  These first numbers support the idea that we have
a very good flexibility in GRASS, so we can choose from time to time
the solution that best fits our need (to me 10% is not critical).  But
I'm sure there are issues I did not take into account, as for example
working with compressed data, or other you might highlight.

I'd like to share the script and have some hints from you, and ideas
to improve it, in order to have more meaningful results.  For example,
the gnu/time utility offers several 'times', and I've chosen 'real
time', but I could be wrong.  Moreover, I noted that results within
single runs differ a lot, but the mean value of several n-runs output
coherent values.  Why that?

Find the script attached, it requires to input the grass native map as
first argument, the external registered map as second argument and the
number of runs that will be used to calculate a mean of the results.

Regards,

Alessandro Frigeri

-- 
Alessandro Frigeri, PhD
-------------- next part --------------
A non-text attachment was scrubbed...
Name: rexternal.perf.slope.sh
Type: application/x-sh
Size: 1636 bytes
Desc: not available
Url : http://lists.osgeo.org/pipermail/grass-dev/attachments/20100921/52313a8c/rexternal.perf.slope.sh