Code Sprint: Day 3
Yesterday, I got to sit down and do some real performance testing with the MapServer folks. After rebuilding a local copy of the Boston Freemap on my laptop, I was able to share it with Paul, who ran it through Shark to find out where the performance killers are. The one thing we found was that this 5 year old MapServer ticket was negatively affecting performance on maps with many labels: The labelling code in MapServer right now, if you’re using outlines, draws each glyph 9 times in order to get a nice outline color. After determining this, it was determined that we are going to be working with the GD maintainers to add the support described in #1243 to GD to use Freetype’s internal stroking code to get the same behavior. (At the time, in Freetype *2.0.09*, there was a bug in this code; but we’re now on 2.3.8, so that bug has been long fixed. :)) This change will likely give a 20% increase on map drawing with many outlined labels, as can be seen in maps like the Boston Freemap.
After this, we sat down with MrSID and GDAL/MapServer to figure out if there were performance problems there. One thing we found was that the MapServer code drawing one-band-at-a-time means that there is a significant performance hit. In addition, some other performance enhancement techniques are being looked into at the GDAL level by Frank, thanks to the help of LizardTech developers participating in the sprint. He’s currently looking at improving the way that GDAL reads from MrSID, and was already able to achieve a 25% speed increase by simply changing the size of the internal GDAL buffer size for reading from MrSID to GeoTIFF. More documentation and experimentation is still in order, but there are some possible optimizations to investigate there for users of the library.
We then had a great dinner at Jack Astor’s.
Thanks to our sponsors for today: Bart van den Eijnden from OSGIS.nl and Michael Gerlek from LizardTech — performance improvements in MapServer and GDAL access for label drawing and MrSID are potentially big wins for many users of MapServer.