RE: Predicting high "PX Deq Credit" waits in PQ

  • From: "Christian Antognini" <Christian.Antognini@xxxxxxxxxxxx>
  • To: "Rich Jesse" <rjoralist@xxxxxxxxxxxxxxxxxxxxx>
  • Date: Thu, 31 Jul 2008 08:38:01 +0200

Hi Rich

> After that initial burst however my process that's running
> the query does all the IO while two of the parallel procs
> sit in the "PX Deq Credit: send blkd" idle wait for the
> 15-minute duration of the query.

1) The wait "PX Deq Credit: send blkd" simply means that a producer
waits for a consumer. In other words, it happens when a consumer cannot
process data as fast as a producer can. 

2) When I read "my process that's running the query does all the IO" I
understand that you are referring to the QC. 

Based on these two facts, I can only think that an important part of the
processing is serialized. You should be able to see it in the execution
plan by looking at a P->S communication. In a fully parallelized
execution it should be available at the end (or beginning, depending on
the point of view...) of the execution plan. Probably for the operation
"PX SEND QC" just before the operation "PX COORDINATOR". In your case is
probably done "sooner".

Chris Antognini

Troubleshooting Oracle Performance, Apress 2008

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