On Thu, Jan 22, 2015 at 2:30 PM, Moritz Lennert
<mlennert@club.worldonline.be> wrote:
I am trying to understand the 'percent_memory' option in r.cost. In the man
page it says: "The amount of map to hold in memory at one time can be
controlled with the percent_memory option. For large maps this value will
have to be set to a lower value.".
The man page is wrong, it is not the amount of the map, but the amount
of the current region.
IIUC, this would mean that if I have a region of X DCELL cells, I would
need: X*8 bytes of memory, or ?
No, it would need X * 3 * 8 bytes (if direction output is requested,
another X * 4 bytes). Considering that you are a power user and
developer, and that you do not understand the 'percent_memory' option,
it is high time to change that option to the standard memory option
where the user specifies the amount of memory in MB that should be
used.
So if memory available is > X*8 I should be able to use percent_memory=100
(or at least 75). Is that correct ?
No, see also above, the input cost surface, the output accumulated
cost surface, and the cell value of the nearest starting point are
held in memory. If a direction output map is requested, this one is
also held in memory.
Is there a special reason why r.cost doesn't use a "memory" parameter
instead of percent_memory ? IMHO, the former is a bit easier to handle for
the average user.
Historical reasons. I wanted to change the 'percent_memory' option to
a 'memory' option, but did not find the time yet. Same for r.walk.
Markus M
Moritz
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