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[mkgmap-dev] New splitter version, big memory savings

From Lambertus osm at na1400.info on Thu Sep 3 13:38:01 BST 2009

Oh, this is so awesome Chris! Just awesome!!

Chris Miller wrote:
> I've just checked in a new version of the splitter (r84) that requires far 
> less memory and also performs slightly better during the first stage of the 
> split. As an example, it used to take about a 5GB heap to generate areas.list 
> for the whole planet, but now only takes around 300MB(!). An additional advantage 
> is that as the planet grows in size and complexity going forwards, the memory 
> required during the first stage will not increase.
> This change should finally mean that anyone is able to split the planet even 
> on a relatively low end machine (though be prepared for a long wait!). If 
> you try but run out of memory during the second stage of the split (ie after 
> areas.list has been generated), reduce the value of --max-areas. This will 
> reduce the memory required during that second stage, at the cost of additional 
> passes over the data (if you do require multiple passes then I highly recommend 
> you also use the --cache option, it can make a huge difference to performance).
> One possible downside to this new version is the algorithm that decides how 
> to split up the map has been changed somewhat. This results in slightly different 
> tile layouts compared to the old algorithm. This shouldn't be too noticable 
> unless perhaps it puts a tile boundary right through somewhere you care about 
> when the old algorithm didn't. Of course, it may also work in your favour 
> for the same reason! My experiments have shown the number of tiles generated 
> increases by about 2% with the new approach which I think is a small price 
> to pay for the huge memory saving.
> I've put some example kml files online here that show the before and after 
> effects of the change:
> UK  --max-nodes=1600000
> old splitter: http://maps.google.co.uk/maps?q=http:%2F%2Fredyeti.net%2Fosm%2Fuk-original.kml&z=3
> new splitter: http://maps.google.co.uk/maps?q=http:%2F%2Fredyeti.net%2Fosm%2Fuk-density.kml&z=3
> Europe  --max-nodes=1600000
> old: http://maps.google.co.uk/maps?q=http:%2F%2Fredyeti.net%2Fosm%2Feurope-original.kml&z=3
> new: http://maps.google.co.uk/maps?q=http:%2F%2Fredyeti.net%2Fosm%2Feurope-density.kml&z=3
> Europe  --max-nodes=300000
> old: http://maps.google.co.uk/maps?q=http:%2F%2Fredyeti.net%2Fosm%2Feurope-300k-original.kml&z=3
> new: http://maps.google.co.uk/maps?q=http:%2F%2Fredyeti.net%2Fosm%2Feurope-300k-density.kml&z=3
> Planet  --max-nodes=1600000
> old: http://maps.google.co.uk/maps?q=http:%2F%2Fredyeti.net%2Fosm%2Fplanet-original.kml&z=3
> new: http://maps.google.co.uk/maps?q=http:%2F%2Fredyeti.net%2Fosm%2Fplanet-density.kml&z=3
> If you aren't happy with the new tiles, or simply want to compare the new 
> with the old, there's a parameter --legacy-mode=true that will generate areas.list 
> using the old approach. Assuming there aren't any serious problems that come 
> to light I'll be removing that parameter in a future build.
> Where to from here? As some of you may have guessed, this new splitter is 
> based on a 'density map' as discussed in earlier mails. Currently it maps 
> the density of nodes only, and the generated map is only held in memory long 
> enough to calculate the tile boundaries. I indend to write this map out to 
> disk so it can be used by external tools, or reused on successive runs of 
> the splitter to allow extremely quick generation of areas.list with different 
> --max-nodes settings. After that I hope to tackle the quite difficult problem 
> (in terms of performance and memory overhead) of generating density maps 
> for ways and relations too. Once we have density maps for all three element 
> types it will hopefully be possible to generate tiles that are as big as 
> possible but still avoid giving 'Map to big' messages. Another related area 
> I'm starting to look at is new/improved algorithms for arranging the tiles 
> so they eg avoid putting boundaries through the middle of cities, or reduce 
> the overall number of tiles. Any ideas here would be appreciated.
> Enjoy!
> Chris
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