PX Deq wait events
                                                 Last update (2013-10-13 17:38:17)
                                                                                                                   Date added (2010-08-05 13:08:09)

The main wait events related to parallel processing are:
PX Deq: Table Q Normal
PX Deq: Execute Reply
PX Deq Credit: send blkd

PX Deq: Table Q Normal
Indicates that the slave wait for data to arrive on its input table queue. In a parallel execution environment we have a producer-consumer model. One slave set works on the data ( e.g. read data from disk , do a join ) called the producer slave set and the other slave set waits to get the data so can start the work. The slaves in this slave set are called consumer. The wait event "PX Deq: Table Q Normal" means that the slaves in the consumer slave have to wait for rows (data) from the other slave set so they can start their work.

PX Deq: Execute Reply
The QC is expecting a response (acknowledgment) to a control message from the slaves or is expecting to dequeue data from the producer slave set. This means he waits that the slaves finished to execute the SQL statement and that they send the result of the query back to the QC.

PX Deq Credit: send blkd
The wait events "PX Deq Credit: need buffer" and "PX Deq Credit: send blkd" are occur when data or messages are exchanged between process that are part of a px query.

Other links
Monitor parallel sessions
Restrict RAC parallel operations to a limited number of instance
How to setup parallel in an SMP Server