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[mkgmap-dev] splitter - performance and memory

From Chris Miller chris.miller at kbcfp.com on Tue Jul 21 15:43:14 BST 2009

> It is also worth noting something that osmcut (an other map splitter)
> does. It has two modes; one where a map is used for when the input
> file is small and another where a list is used and the node-id is the
> index into the list. The later is used for splitting the whole planet
> file.

Ah I didn't even realise there was another map splitter! Thanks for the pointer 
and info. I've already been considering various ways to trade off memory 
& performance based on the size and content of the dataset (including using 
an indexed array), but comparing my ideas with what osmcut does looks helpful.

> The second method uses less memory where the memory wasted by node-ids
> that are not in use is less than the amount of memory used by storing
> the node-id plus the memory wasted in free space in the hash maps.
> As currently most nodes are used (about 88%) this is a big win
> on the whole planet file, while still giving access to the node ids.

Makes sense. From what I can see osmcut appears to be only subdividing into 
evenly sized tiles though, rather than quadtree partitioning. The advantage 
they have there is they only need to hold 2 bytes/node (as a tile ID) rather 
than 4 bytes/node (lat+lon), and they don't need to perform a secondary pass 
to determine which way belongs in which tile. That's OK though, it's a more 
interesting challenge taking the quadtree approach :)

> I've committed the patch.

Thanks! The next patch I produce will likely include a third party XPP parser 
jar file and associated changes to the IDEA project file. What's the best 
way to deal with this - create a build.xml so non-IDEA users can build it 


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