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high cpu and parallelism


high cpu and parallelism

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Oracle_91
Oracle_91
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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
GilaMonster
GilaMonster
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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)
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Abu Dina
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Oracle_91
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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?
GilaMonster
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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

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Oracle_91
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Got it. Brilliant stuff.

Thank you very much Gila.
Jeff Moden
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Abu Dina (9/2/2013)
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