When Movable Type loads any category information from the database, by default, it pre-loads all of the data related to placement of entries within categories (i.e. placement records) into RAM. This has the effect of making rebuilding operations and some uses of the administrative interface more efficient.

However, when the number of placement records grows large (either by quantity of entries or categories), this can have a negative effect on memory usage and system performance. If you are experiencing high memory usage during rebuilds you can disable this functionality by adding the following to your mt-config.cgi file:

NoPlacementCache 1