Cost Threshold for Parallelism (CTfP) is one of my favorite server level settings in SQL Server. I remember the first time I heard this setting mentioned by Grant Fritchey. I quickly hopped on my servers and found them all set at the default (5) and adjusted them to 50 for the non SSRS servers and 30 for the SSRS ones. That was many years ago, but I had kept those numbers in my head because I didn’t know a better way.
Peter Shore gave an awesome presentation on Waits to our user group last week and reminded me of how much this setting can impact tuning. He also pointed us to a fantastic blog by Jonathan Kehayias about how to know the correct setting for your CTfP.
Peter explained that as I ran Jonathan’s awesome query, I would start to see a point in the StatementSubTreeCost column to help me identify the best CTfP for my environment.
My first thought after looking at this query, “I am so glad Jonathan wrote it because with that much XML, I wouldn’t know if it were safe to run without that trust.”
Today, I gave it a go. I kicked off the query and held my breath. Then I started to turn blue and realized this would probably take a minute. It took about 15 minutes and I was happy I didn’t panic at the wrong disco. It runs in a read uncommitted state which prevents blocking (thank you so much!) and I ran sp_whoisactive over and over to be safe.
This is Jonathan’s query, but I recommend you read his article too because there was so much good information.
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED; WITH XMLNAMESPACES (DEFAULT 'http://schemas.microsoft.com/sqlserver/2004/07/showplan') SELECT query_plan AS CompleteQueryPlan, n.value('(@StatementText)', 'VARCHAR(4000)') AS StatementText, n.value('(@StatementOptmLevel)', 'VARCHAR(25)') AS StatementOptimizationLevel, n.value('(@StatementSubTreeCost)', 'VARCHAR(128)') AS StatementSubTreeCost, n.query('.') AS ParallelSubTreeXML, ecp.usecounts, ecp.size_in_bytes FROM sys.dm_exec_cached_plans AS ecp CROSS APPLY sys.dm_exec_query_plan(plan_handle) AS eqp CROSS APPLY query_plan.nodes('/ShowPlanXML/BatchSequence/Batch/Statements/StmtSimple') AS qn(n) WHERE n.query('.').exist('//RelOp[@PhysicalOp="Parallelism"]') = 1
After running it, I got back 43 records. I felt that was low until I remembered that our CTfP is set higher than my brain standard at 150. After looking over the results, I felt that 150 was about right for this environment. I didn’t stop there.
Jonathan had mentioned how he uses this query to identify what needs to be tuned, and since tuning is my favorite, I started to play with the queries to get them running better.
Huge THANK YOU to the awesome SQL Server Community that is always willing to share and teach! I love being able to find what I need from people that I trust to make my job easier and I couldn’t do it without all of you!
Hugs and please stay safe!
The song for this post is Sunshine by OneRepublic.
Have you read this from MIchael Swart https://michaeljswart.com/2022/01/measure-the-effect-of-cost-threshold-for-parallelism/
I haven’t seen this before, this is an awesome article! Thank you!
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