[mkgmap-dev] Commit: r1566: Drop all tags from the osm file thatare not used

Mon Feb 8 13:30:13 GMT 2010

What might be nice is if this stripping can be done before (or during) the 
split. That way the splitter would have to do less work, and possibly would 
also be able to perform a better split. The downside would be that any style 
changes might necessitate another splitter run. Would it be difficult to 
refactor out the filtering logic from mkgmap so splitter could use it? Thoughts?

Chris

sc> Version 1566 was commited by steve on 2010-02-08 12:33:09 +0000
sc> (Mon, 08 Feb 2010) BRANCH: style-speed
sc> 
sc> Drop all tags from the osm file that are not used
sc> in the applied style.
sc> Whether this makes a big difference or not depends on the country.
sc> In the UK which is mostly manually taged it doesn't make a great
sc> deal
sc> of difference.  There are only an average of 2 tags per way and
sc> about zero per
sc> node.
sc> In countries with data imports, there are typically many tags per
sc> way (and even nodes)
sc> that refer back to the original data.  Having tags on nodes is
sc> particularly bad for memory
sc> consumption since there are more nodes than ways and most are not
sc> POIs.  We are forced
sc> to allocate a Tags object even though there is likely to be nothing
sc> to do as none of the tags
sc> are used.
sc> Denmark is the worst in this respect and so will show the best
sc> improvement with this patch. See also:
sc> http://www.mkgmap.org.uk/pipermail/mkgmap-dev/2009q3/003597.html
sc> 
sc> For this to be successful you need an acurate list of all the tags
sc> that could be used.  Tags are
sc> 
sc> examined not just in the style itself, but there are hardwired tag
sc> lookups in the style system itself
sc> 
sc> and there are non-style related usages as well.  For the moment all
sc> the built in usages of tags
sc> 
sc> are held in the builtin-tag-list file.  I think I have got them all
sc> for normal use, but there may be
sc> 
sc> options that require ones I've missed.
sc> 