[mapserver-dev] fastpath 4326->3857 reprojections

[thomas bonfort]

Hi all,

During the codesprint I talked to Daniel about a possible speedup
patch we could use to bypass going through proj when performing the
common 4326 to 3857 (google mercator) reprojections. During my trip
back from the codesprint I implemented a quick patch that does this,
and I think that the benefits are sufficiently important under this
(rather common) test case to merit being pushed to trunk.

test-case:
 - using the mapserver-utils osm mapfile, we seed the first 6 zoom
levels of the google-mercator grid. Depending on the zoom level, this
mapfile has two (land and borders) or one (borders) layers that need
to go through reprojection. There are between 1 and 4 other layers
that are already in 3857 and therefore do not need reprojecting.
 - the seeding is done with the mapcache seeding tool, which can be
configured to use multiple threads to do parallel rendering. The
seeding is done by directly using the libmapserver api (i.e. direct
calls to msDrawMap, not going through curl http getmap requests).
 - bug 4041 (http://trac.osgeo.org/mapserver/ticket/4041) contains a
patch to avoid locking inside mapserver around proj calls, for trunk
versions of proj. In the following result spreadsheet "with/without
480" refers to the fact that this patch is applied or not (480 stands
for version 4.8.0 of proj)

without further ado, here's the resulting spreadsheet, which gives the
time in seconds needed to entirely seed the first 6 zoom levels:

https://docs.google.com/spreadsheet/pub?key=0AnCiCdIXpTHidHJ0YzVmSW54WncwZ2lGSS1wQ2xNZHc&output=html

tl;dr

- if we factor out thread locking (which concerns the cgi/fastcgi
performance), the speedup is x 2.5
- in the multithreaded seeding case, the speedup is x 37

--
thomas