A working set refers to the data actually pulled into memory so the server can respond to client applications. The data set here is then the entries in the directory that you see are in fact getting used due to client traffic. The data set may include every entry in the directory, or may be composed most of the time of the some smaller number of entries, such as entries corresponding to people in a time zone where users are active.
We define three data set sizes, based on how much of the directory data set fits into available physical memory:
The data set fits entirely into physical memory with fully-loaded database and entry caches.
The data set fits in physical memory, and extra physical memory can be dedicated to entry cache.
The data set is too small to fit completely in available physical memory.
The ideal case is of course the small data set. If your data set is small, set database cache size and entry cache size such that all entries fit in both the database cache and the entry cache.
The following sections provide recommendations for medium and large data sets where the server performs either all searches or all modify operations.