[GRASS-dev] Re: grass-dev Digest, Vol 43, Issue 8

[Laura Toma]

Hi Markus,

How much memory was available  on the machine?  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.

However, I am surprised that withnumtiles=1,  it was slower than  
r.cost.   That's something I'd like to  look into.  Would you mind  
sharing the raster with me, and sending me the exact commands that  
you ran?

A grid with 29M points is pretty small, for today's machines.  I  
suggest running on something 10 times larger.  And use a lot of  
sources, that makes the data access  pattern  less local.

-Laura



> ----------------------------------------------------------------------
>
> Message: 1
> Date: Fri, 06 Nov 2009 09:50:02 +0100
> From: Markus Metz <markus.metz.giswork at googlemail.com>
> Subject: [GRASS-dev] comparing r.cost and r.terracost
> To: GRASS developers list <grass-dev at lists.osgeo.org>
> Message-ID: <4AF3E33A.1090208 at googlemail.com>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
> Same test region as before
>
> North Carolina sample dataset
>
> g.region rast=elev_state_500m at PERMANENT res=100
> # gives about 28 million cells
> v.to.rast nc_state at PERMANENT use=val val=500.0 out=cost --o
> v.to.rast urbanarea at PERMANENT use=val val=1 out=urbanarea --o
> r.cost in=cost start_rast=urbanarea out=dist_urban  
> percent_memory=20 --o
>
> grass7 r.cost
> real    0m55.349s
> user    0m53.360s
> sys    0m1.797s
>
> grass65 r.cost
> real    26m35.166s
> user    2m55.612s
> sys    23m37.921s
>
> r.terracost: check optimal tile size for 400MB memory (default  
> setting;
> r.cost in grass7 used 135MB with 20% of maps in memory)
> r.terracost in=cost start_rast=urbanarea out=dist_urban_terracost -i
> [...]
> TILESIZE: nc_spm_08 N=28064550 elements, M=419430400 bytes, optimal
> numtiles=1870
>
> r.terracost numtiles=1870, intermediate data are stored on disk as
> r.cost does
> real    25m13.593s
> user    22m46.978s
> sys    1m8.059s
>
> r.terracost numtiles=1, all in memory (just fits into 400MB)
> real    0m17.969s
> user    0m17.276s
> sys    0m0.500s
>
> According to Laura Toma, when comparing r.cost with r.terracost,
> numtiles must be >1 for r.terracost in order to compare disk I/O
> algorithms [1]
>
> With these test settings that intentionally reduced memory consumption
> in order to test disk I/O performance, r.terracost is not really  
> faster
> than r.cost in grass65 and much slower than r.cost in grass7.
>
> Markus M
>
> [1] http://lists.osgeo.org/pipermail/grass-dev/2009-July/045157.html
>
>
>
>
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> End of grass-dev Digest, Vol 43, Issue 8
> ****************************************