You may see in the error log stating that "search is not indexed". The Directory Server referential integrity plug-in is automatically enabled by the Access Manager. But no indexes exist for Access Manager's attributes such as iplanet-Access Manager-static-group-dn and iplanet-Access Manager-modifiable-by. Access Manager does not configure the arguments of the plug-in, but uses the default arguments (update interval=0). Every deletion causes an immediate integrity check, which consumes a lot of system resources when the search is not indexed.
Solution: Be sure the referential integrity plug-in is enabled and configured, and that the attributes to be maintained are indexed. Be sure the referential integrity is configured to be executed synchronously or with a delay. A delay will remove the thread shortage per application.
If you observe that one or more of the deleted entries are repeatedly added or deleted, over time these entries may trigger non-indexed searches to the database. This issue is addressed in later versions of Directory Server. Upgrade to Directory Server 5.2 Patch 5 or Directory Server 6.0.