Hi,
If you do a snapshot for dbm (or on Unix, snapshot for db), this is what you get:
Memory Pool Type = Lock Manager Heap
Current size (bytes) = 278528
High water mark (bytes) = 278528
Configured size (bytes) = 278528
The lock manager heap is fully allocated on database activation, so technically that memory is always being used even if there are no locks being held.
Hope that helps!