Oracle Database - parallel_execution_message_size parameter
1 - About
The Parallel servers communicate among themselves and with the Query Coordinator by passing messages via memory buffers.
By default the message size is 2K. Ideally you should increase it to 16k (16384).
However, a larger value for parallel_execution_message_size will increase the memory requirement for the shared pool so if you increase it from 2K to 16K your parallel server memory requirement will be 8 X more.