Parallelizing calls to msDrawLayer()

[Ed McNierney]

Dave -

"If the files are located on separate disks, of course, it's game over
in favor of the threaded model."

Unfortunately, the world isn't always that simple.  I make some pretty
large images (up to a few hundred megabytes), and if I have to create a
raster buffer per layer for each request, it's possible I'm going to
start swapping (I'm serving many simultaneous requests) when I'm not
swapping now.  That's a different kind of "game over".  Yes, if my
parallelism allows my process to finish faster then I'll see a
counteracting reduction in the number of simultaneous requests I need to
handle.  It's hard to predict which will win in a general sense.

Our map-serving systems are complex and not at all homogeneous;
MapServer is used in a very wide variety of environments.  I have found,
from this list, that a surprising number of users are on "low end"
hardware (e.g. data on one local IDE disk).

I've been trying to pay attention to software optimization for a very
long time, across a variety of applications.  Some aspects are easy,
others are not.  The most dangerous thing, I have found, is to think one
knows the answer.  When that happens, one tends to stop paying attention
and looking hard for even better answers.  If you're interested in
working on parallel layer rendering, that's great and there are
certainly scenarios that would benefit from it.  Your original post
asked if any major drawbacks were evident, and those of us who've been
using and optimizing MapServer for a long time have tried to offer our
input and experience.

	- Ed