high cpu and parallelism

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high cpu and parallelism

Oracle_91

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September 2, 2013 at 8:58 am

#298372

Hi Everyone,
I was just doing a read-up upon high-cpu and CXPACKET wait/parallelism.
One thing, which is over my reach is below statement.
"worker time is greater than its elapsed time is a great indication of parallelism"
Can anyone please explain an example how could it be possible cpu> total elapsed time. How this is possible? Again i am just learner and I wanted to know who this can happen in real time?
Looking for brief explanation on this.
Thanks in Advance

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Gail Shaw SSC Guru Points: 1004504 More actions · Answer 1

Query parallels, spends 10 seconds executing on 4 CPUs. Elapsed time, 10 seconds. CPU time used 10 seconds * 4 CPUs = 40 seconds.

Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability

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Abu Dina SSChampion Points: 14155 More actions · Answer 2

You no longer MCM?!

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Oracle_91 SSChampion Points: 14548 More actions · Answer 3

Thanks Gila for quick response.

one more clarification, in that case, SQL doesn't SUM up all the execution times for all the CPU's as it is one query which is splitted into parallel threads and will be the Total elapsed time of the actual query?

Gail Shaw SSC Guru Points: 1004504 More actions · Answer 4

Elapsed time is literally that. It's the actual, real time that elapsed while the query was running. Measured by taking the actual real time that the query started, the actual, real time that the query finished and taking the difference.

If a query started executing at 10:00:02 and completed at 10:00:05, then it took 3 seconds to run, the elapsed time is 3 seconds. If that query paralleled 4 ways (ran on 4 CPUs) for all 3 of those seconds, then the total CPU time used by that query was 12 seconds

Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability

We walk in the dark places no others will enter
We stand on the bridge and no one may pass

Oracle_91 SSChampion Points: 14548 More actions · Answer 5

Oracle_91

SSChampion

Points: 14548

September 2, 2013 at 9:34 am

#1646775

Got it. Brilliant stuff.

Thank you very much Gila.

Jeff Moden SSC Guru Points: 1004356 More actions · Answer 6

Abu Dina (9/2/2013)
You no longer MCM?!

They're cancelling the MCM cert.

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RBAR is pronounced "ree-bar" and is a "Modenism" for Row-By-Agonizing-Row.
First step towards the paradigm shift of writing Set Based code:
________Stop thinking about what you want to do to a ROW... think, instead, of what you want to do to a COLUMN.

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