Hi Markus,
How much memory was available on the machine?
8 GB
If the machine had more than 512MB RAM, it is not fair to run
terracost running with mem=400MB, and compare it with an algorithm
that can use more memory.
I don't understand, both modules can use more than 400MB of memory. I
set r.terracost to use 400MB max and r.cost to use 135MB max. If
anything, this is not fair for r.cost and gives an advantage to
r.terracost. I did not mention that I gave r.terracost another
advantage
by assigning the temporary directories to a folder on a separate, very
fast hard drive that had nothing else to do but manage the temp
files of
r.terracost. The temp files of r.cost are in the standard grass
.tmp/$HOST directory, in my case that (slower) hard drive also had
other
things to do than just manage r.cost's temp files. I really tried to
give r.terracost a head start ;-)
my experience is that , if you want to see how an application would
behave with 500 MB of RAM, you have to physically reboot the machine
with 500 MB of RAM (it's very easy to do this on a Mac, and
relatively easy on Linux. on windows, i don't know).
if the machine has more than 500MB RAM, even if you restrict the
application to use less, the system gives it all it can. in your
setup, it is almost as if r.cost would run fully in memory, because
even it it places the segments on disk, the system file cache fits all
segments in memory. the same is true for terracost, its streams fit
in memory. but using tiles has a big CPU overhead, which is why it
is slower.
when i did some preliminary testing, i rebooted the machine with 512MB
RAM, and ran r.cost on grids of 50M-100M cells. it was slow,
completely IO bound, and took several hours or more. or if you use
1GB of RAM, you may need to go to larger grids.
However, I am surprised that withnumtiles=1, it was slower than
r.cost.
it looks like i misread your numbers.
BTW, I took the liberty to fix r.terracost, it works now with
numtiles>1. See changelog for r39684
https://trac.osgeo.org/grass/changeset/39684
great, thanks! let's see if terracost is worthwhile :)
-Laura
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