[GRASS-dev] Re: big region r.watershed

[Hamish]

Markus Metz wrote:
> The original version uses very little memory, so assuming that GRASS
> runs today on systems where at least 500MB RAM are available I changed
> the parameters for the seg mode, more data are kept in memory, speeding
> up the seg mode. Looking at other modules using the segment library
> (e.g. v.surf.contour, r.cost), it seems that there is not one universally
> used setting, instead the segment parameters are tuned to each module.
> The new settings work for me, but not necessarily for others, and maybe
> using 500MB is a bit much.

fwiw r.terraflow has a memory= option, the default is 300mb.
AFAIU, the bigger you make that, the smaller the on-disk temp files need
to be (ie work-around to keep tmp files <2gb for 32bit filesystems). 

a number of modules like r.in.poly have a rows= option, which I didn't
really understand until I got into the code. (hold at most that many
region rows (all columns) in memory at once). Interestingly the default
value has scaled quite well over the years.

and other modules like r.in.xyz have percent= (0-100) for how much of the
map to keep in memory at once.

For GRASS 7, a consistent approach (user option) among modules would be
nice to ease the learning curves.


Hamish