[gdal-dev] Weird gdal2tiles performance inconsistency

[mladen-g at distributel.net]

Hi all,

I'm seeing some very strange performance from gdal2tiles on a  
particular grid, so I'm wondering if someone has an insight into why  
this is happening.

The problem is that when I tile the grid at zoom level 7, I get much  
better performance than with other zoomlevels, including lower ones (!).

The grid is this (zipped PNG, ~2.7Mb):
http://dl.dropbox.com/u/53500018/ontarioOrigSizeTrimReprojected.zip

So, let's say I run this command:
gdal2tiles.bat -w google -s EPSG:4326 -z 1-7 -v  
C:\temp\ontarioOrigSizeTrimReprojected.png
C:\temp\outputTiles

This takes about 16 seconds on my machine.

But if I replace the last line to be zoom level 1:
gdal2tiles.bat -w google -s EPSG:4326 -z 1-1 -v  
C:\temp\ontarioOrigSizeTrimReprojected.png
C:\temp\outputTiles

This takes 32 seconds, or about double the time of tiling zoomlevels  
1-7.  Note also that I've experimented with other maximum zoomlevels  
(5,6,8,9,10,11) and they all run slower than 7.

So, does anyone know Why does this strange behaviour happen on zoom  
level 7?  And is there any way to optimize the input to gdal2tiles for  
better performance?

Regards,
Mladen

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