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Internals of Oracle Segment_header_control

Reducing Segment Header Contention

Segment header contention in Oracle occurs when multiple sessions compete for the same segment header block, leading to performance bottlenecks and potential scalability issues. It is commonly associated with high-concurrency environments, where multiple transactions access the same object, such as a table or index. The following recommendations will help you reduce segment header contention in Oracle:
  1. Monitor contention: Use Oracle's Automatic Workload Repository (AWR) reports, Active Session History (ASH) data, or dynamic performance views, such as V$WAITSTAT and V$SESSION_WAIT, to identify segment header contention. Keep an eye on wait events like "buffer busy waits" and "enq: TX - allocate ITL entry" to pinpoint problematic objects.
  2. Implement Automatic Segment Space Management (ASSM): ASSM simplifies space management by automatically managing the free and used space within segments. This reduces contention by allowing concurrent transactions to allocate space independently. To enable ASSM, set the TABLESPACE parameter to "AUTO" during tablespace creation.
  3. Increase INITRANS parameter: INITRANS determines the initial number of Interested Transaction List (ITL) entries in a data block. By increasing this value, you provide more ITL slots for concurrent transactions, reducing contention. However, do not set the value too high, as it may waste space in each block.
  4. Use partitioning: Break large tables or indexes into smaller, more manageable partitions to distribute contention across multiple segment headers. This approach improves performance by allowing Oracle to read or write data from different partitions concurrently.
  5. Utilize reverse key indexes: For indexes with monotonically increasing key values, such as sequence-based primary keys, consider using reverse key indexes. This approach redistributes index insertions across all leaf blocks, reducing contention on the right-most index blocks.
  6. Use freelist groups: In environments without ASSM, consider using freelist groups to allocate and manage space in segments. Assign each parallel server process to a separate freelist group to minimize contention.
  7. Optimize application design: Review your application's transaction processing logic to identify potential bottlenecks and reduce the frequency of concurrent DML operations on the same object. Consider using techniques like batch processing, bulk binds, or optimized SQL statements to minimize contention.
  8. Employ appropriate isolation levels: Choose the appropriate transaction isolation level to balance concurrency and consistency requirements. For example, the READ COMMITTED isolation level may help reduce contention by allowing non-blocking reads, while SERIALIZABLE provides a higher level of isolation at the cost of increased contention.

By implementing these strategies, you can effectively reduce segment header contention in Oracle, improving performance, scalability, and overall database efficiency. Regularly monitor your system and adjust these parameters as needed to maintain optimal performance in your specific environment.

One benefit of Automatic Segment Space Management is the number of bitmap freelists that are guaranteed to reduce buffer busy waits.
Prior to Oracle9i, buffer busy waits were a major issue for systems with high concurrent inserts. As a review, a buffer busy wait often occurs when a data block is inside the data buffer cache, but it is unavailable because it is locked by another DML transaction. Without multiple freelists, every Oracle table and index has a single data block at the head of the table to manage the free block for the object. After any SQL insert runs, it has to go to this segment header block and get a free data block on which to place its row.
Oracle's ASSM feature claims to improve the performance of concurrent DML operations significantly since different parts of the bitmap can be used, simultaneously eliminating serialization for free block lookups.