[GRASS-dev] grass 7 and pixel random access

[Hamish]

> Yann Chemin wrote:
> >  I would like to know the planned changes for the raster library,
> >  especially the random access of pixels in the raster.
Markus:
> Not sure if all of those are actually planned, but here is a list:
> http://grass.osgeo.org/wiki/GRASS_7_ideas_collection#Raster
> 
> >  I wanted to work on it some months back, but my daily job got more
> >  intense.
> >  In the coming future, we will need to access easily any row for
> >  parallel processing.


One thing I wonder about for parallel processing of (serial) raster
modules- do we really need random read access to send each individual row
into a separate thread? The overhead with that seems counter-productive.
Couldn't we read some GRASS_NPROC envrio variable and then split the
overall number of rows by that number and create a small number of
threads, ie matching the system.


like: ceil(rows/nproc)
nproc=4
rows=1035
chunk size=259


the last thread reads/processes/writes fewer rows than the others as it
finds the EOF.



another thing I still wonder about (see thread from a month or so back)
is  where to start? Modify the libs to support the concept, then tackle
each module on their own? ie concentrate on the non-I/O limited and
can't-do- much-about-it but throw more processor at the problem modules,
and leave non-number crunching modules alone? -- concentrate on areas
where we'll get the most bang for the buck / pick off low hanging fruit /
etc?


Hamish
(showing off his multi-proc naivet'e)



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