The adaptive multiuser algorithm, which is enabled by default, reduces the degree of parallelism as the load on the system increases. When using Oracle Database adaptive parallelism capabilities, the database uses an algorithm at SQL execution time to determine whether a parallel operation should receive the requested DOP or have its DOP lower to ensure the system is not overloaded.
In a system that makes aggressive use of parallel execution by using a high DOP, the adaptive algorithm adjusts the DOP down when only a few operations are running in parallel. While the algorithm still ensures optimal resource utilization, users may experience inconsistent response times. Using solely the adaptive parallelism capabilities in an environment that requires deterministic response times is not advised. Adaptive parallelism is controlled through the database initialization parameter PARALLEL_ADAPTIVE_MULTI_USER.