*Automatic segment space management (ASSM) is a simpler and more efficient way of managing space within a segment. It completely eliminates any need to specify and tune the pctused,freelists, and freelist groups storage parameters for schema objects created in the tablespace. If any of these attributes are specified, they are ignored. *Oracle introduced Automatic Segment Storage Management (ASSM) as a replacement for traditional freelists management which used one-way linked-lists to manage free blocks with tables and indexes.
ASSM is commonly called "bitmap freelists" because that is how Oracle implement the internal data structures for free block management.
Note:
* Buffer busy waits are most commonly associated with segment header contention onside the data buffer pool (db_cache_size, etc.).
*The most common remedies for high buffer busy waits include database writer (DBWR) contention tuning, adding freelists (or ASSM), and adding missing indexes.