Hi Laura,
Laura Toma wrote:
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.
I haven't rebooted my Linux box with less RAM, but I set up a test
region with about 312 million cells (details below), I think we can
agree that this is for current standards a pretty large region, maybe
not in the future. Your argument still holds true that r.cost may have
some advantage because its temp files are much smaller than the temp
files of r.terracost and therefore a larger proportion can be cached
by
the system (beyond the control of the module). I could however see a
lot
of disk IO on both modules.
For 312 million cells, r.cost needed 51 min, r.terracost needed 24 h
22
min, both got 2GB memory.
Now that sounds like really bad news for r.terracost. But this is not
the whole truth. First, I had to tweak r.cost a little bit in order to
be so fast, still have to come up with a solution to do that
tweaking in
the module. Second, r.cost may suffer more from memory reduction, not
physical RAM reduction, than r.terracost. Reducing the percent_memory
option already slows the module down considerably. But that is also
true
for r.terracost, there the bottleneck seems to be INTERTILE DIJKSTRA
which took well over 12 hours with heavy disk IO and full memory
consumption. Third, r.cost performs better with less start points
keeping region settings constant. I'm not sure if this applies as well
to r.terracost.
In summary, I think that on even larger regions, say >1 billion cells,
and many small separate start points (>100 000), r.terracost should
outperform r.cost, but I would not bet on it ;-) For what I guess is
current everyday use (< 100 million cells), r.cost in grass7 might
most
of the time outperform r.terracost with numtiles>1, sometimes
considerably as in my tests. Speed performance of r.cost is variable
and
dependent on the combination of region size, number and distribution
of
start points, and the amount of memory it is allowed to use. There may
still be some scope for improvement in r.cost, I just did a quick job
there, no in-depth code analysis (yet). The extraordinarily large temp
files of r.terracost (total 64GB, largest single file was about
56GB, no
typo) could be a handicap when processing such large regions. Finally,
the results of the tests I did are valid for my test system only, they
will be different on other systems.
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.
Please test r.cost in grass7 yourself, and maybe share your test
commands, then others can run the tests too and compare.
Here is my test region:
The 312 million cells test region was created in the North Carolina
sample dataset with
g.region rast=elev_state_5...@permanent res=40
Then I created a cost layer with
r.mapcalc "cost = 1"
You wanted many start points, so I generated 10000 start points with
v.random output=start_points_10000 n=10000
and converted this vector to raster with
v.to.rast start_points_10000 use=val val=1 out=start_points_10000 --o
The test command for r.cost was
time r.cost input=cost start_rast=start_points_10000
output=dist_random_10000 percent_memory=40 --o
This setting was equivalent to 2 GB of memory.
time:
real 51m18.172s
user 34m4.067s
sys 0m45.100s
For r.terracost, I used as temp dir again a directory on a separate
hard
drive, faster than the one that r.cost used, so let's say
tmpdir="/path/to/some/fast/dir"
and the test command for r.terracost was
time r.terracost in=cost start_rast=start_points_10000
out=dist_random_10000_terracost STEAM_DIR=$tmpdir VTMPDIR=$tmpdir
memory=2000 numtiles=20788 --o
numtiles=20788 I got with r.terracost -i
time:
real 1453m37.022s
user 513m56.549s
sys 43m38.519s
Sorry for that long post!
Markus M
