I am testing the performance of the convert utility in ImageMagick 6.6.0-4 on Debian Squeeze on a dual quad-core (8-cores, 16 core threads) Intel 5620 server.
I tested converting 400MB of JPEG images to a lower resolution. On this server, I found the following timings:
MAGICK_THREAD_LIMIT=1 150 sec
normal 55 sec
waitloop 28 sec
waitloop && MAGICK_THREAD_LIMIT=1 13 sec
Waitloop is a bash script I use to force 16 copies of convert to run in the background. I know convert uses multiple threads automatically to convert an image, so the slow timing of the first item (using only one thread) is expected. It is also expected that using my waitloop tool would improve performance because the normal test only has the CPUs at 40%.
What is surprising is that by disabling threading in convert, and forcing 16 convert processes to run simultaneously, I get a 2x speedup over the waitloop case. Perhaps your documentation should be clearer about the benefits of setting
THREAD_LIMIT=1 when you are already running multiple convert processes, i.e. your documentation isn't clear that setting it to "1" might yield improved performance: http://www.imagemagick.org/script/architecture.php
If you want more details or a self-contained test case, please let me know.