[GRASS-dev] v.what.rast speedup
hamish_nospam at yahoo.com
Sun Oct 21 18:53:00 EDT 2007
> >> >> > since v.what.rast is (for me) extremely slow, I have added
> >> >> > cover map support to r.random.
> >> >> ..
> >> >> > PS: v.what.rast still running on just 300k points while I
> >> >> > implemented above :-) Anyone who could make v.what.rast faster?
> >> >>
> >> >> Maybe it's the qsort(), maybe it's the i,j loop within a loop.
> >> >
> >> > The loop is certainly inefficient. Rather than shifting the entire
> >> > array down every time it finds a duplicate, it should keep separate
> >> > source and destination indices, e.g. (untested):
> >> >
> >> > for (i = j = 0; j < point_cnt; j++)
> > --- vector/v.what.rast/main.c 17 Oct 2007 14:07:23 -0000 1.26
> > +++ vector/v.what.rast/main.c 21 Oct 2007 17:52:42 -0000
> > ... patch
> Thanks (for your patience): I have made a Spearfish test:
> g.region rast=elevation.10m
> r.random elevation.10m vect=elevpnts n=5000 --o
> v.db.addcol elevpnts col="height double precision"
> # old
> time v.what.rast elevpnts rast=elevation.10m col=height
> real 0m25.253s
> user 0m24.756s
> sys 0m0.308s
> # new
> real 0m24.040s
> user 0m23.707s
> sys 0m0.297s
> Interestingly, timing rather identical (also for 1000). Using the DBF
> driver here.
It is as expected, only a small speed up. While the for i,j loop map may have
been inefficient, it still was fast enough to only take a second to get though.
The bulk of the time was and is being taken up by running db_execute_imediate()
for each point later on in the script. (then db_commit_transaction() is run
after the loop)
To solve this in v.in.garmin and v.in.gpsbabel we wrote all db SET .. to ..
statements to a temporary file, then ran db.execute once for the tmp file
instead of earlier running db.execute for every point. It is not a direct
analogy, but it did made a huge difference there.
Or maybe it is the bsearch() for every point in the db_execute_imediate() loop?
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