Why does SQL Server switch parallell processing into single threaded execution

  • Hello!

    This is impossible to answer for our specific problem, I know. But does anyone know what I should look for on our server to determine the cause on our server?

    Case description:

    A web service has more than 10000 id's to run through a stored procedure on the SQL Server. It divides this into 10 buckets and send 10 requests to the server with 1000 id's each. The processing starts OK with 3 threads finishing after a reasonable time. Then 6 threads stop processing and the 7th run to completion. Then the next thread picks up again and runs to completion, and so on.

    I would like to see why SQL Server determines it is best to suddenly suspend 7 threads. My current idea is to query for the execution plan of each batch and look for changes and look at some performance counters at the same time. Hopefully I see something changes at the time the parallell execution stops and enter single threaded execution.

    Does anyone have a better idea of what to look for?

    Best regards,

    Vemund Haga

  • Can you please post the actual execution plan?

    If you want to capture all the details including the performance counters then use SQLDiag with perfstats script. A little bit of googling will give you all the details.

  • where do you download sqldiag and perfstats from?

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