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This is a feature request to reduce memory usage on page load for large taxonomy. It would improve scalability.
To reproduce this issue use a 20MB taxonomy and monitor memory usage. Taxonomy is 3 levels deep.
On page load Hierarchical Select 6.x-3.x-dev (2009-Jul-29) uses 287MB of memory (RAM). And this is for only one visitor.
Most share hosting providers have 32MB limit. Most populare VPS hosting plans are between 360MB & 720MB limit.
To get above memory usage result I type in the following command in Ubuntu terminal. This will output the memory usage every 1 second for 50 iterations (50 secondes). Display in megabyte (M).
vmstat -S M 1 50
Another option to monitor module memory usage is the Devel module. Under 'Performance Logging' maximum memory usage. http://drupal.org/project/devel
To quickly and easily create a 20MB taxonomy I use the Devel module. And its 'Generate content' feature. Hope this help.