On most platforms, the default value is 2148bytes if PARALLEL_AUTOMATIC_TUNING is set to false, and 4096 bytes if PARALLEL_AUTOMATIC_TUNING is set to true. The default value is adequate for most applications. Larger values require a larger shared pool. Larger values result in better performance at the cost of higher memory use. For this reason, replication gets no benefit from increasing the size.
Set PARALLEL_EXECUTION_MESSAGE_SIZE = 4096
When using parallel media recovery or parallel standby recovery, increasing the PARALLEL_EXECUTION_MESSAGE_SIZE database parameter to 4K (4096) can improve parallel recovery by as much as 20 percent. Set this parameter on both the primary and standby databases in preparation for switchover operations. Increasing this parameter requires more memory from the shared pool by each parallel execution slave process.
The PARALLEL_EXECUTION_MESSAGE_SIZE parameter is also used by parallel query operations and should be tested with any parallel query operations to ensure there is sufficient memory on the system. A large number of parallel query slaves on a 32-bit installation may reach memory limits and prohibit increasing the PARALLEL_EXECUTION_MESSAGE_SIZE from the default 2K (2048) to 4K.
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