[GRASS-dev] Re: [GRASS-user] Benchmarking Grass 4.3, 5.4, 6.0, 6.2 raster commands

Glynn Clements glynn at gclements.plus.com
Tue May 1 01:10:29 EDT 2007

Helena Mitasova wrote:

> does the null implementation affect also the runs with rasters that  
> have no nulls and there is no MASK present?

It affects any raster which has a null bitmap, even if no cells are
actually null.

> 10x faster is a huge difference - it may be worthwhile to find out
> whether it is true for integer maps without nulls and whether it
> is really nulls slowing it down so badly.

It's relatively easy to test the effect of the null bitmap:
delete/rename the cell_misc/<name>/null file. There will still be some
residual overhead due to the conversion of zeroes to nulls, but this
will determine the cost of the filesystem calls.

> There were many discussions about the null implementation and as  
> Glynn correctly
> points out the main driver for the current design was to sacrifice  
> the performance
> to preserve the backwards compatibility. Wishes of old users (many of  
> whom
> contributed funds to GRASS development) were given very high priority.

It's possible to embed nulls while retaining compatibility, but the
result is that most CELL maps will end up using 4 bytes per cell
(prior to RLE or zlib compression).

Glynn Clements <glynn at gclements.plus.com>

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