Things SQL Server vNext Should Address: Add Cost Threshold For Parallelism To Setup Options

Cracky


Unless you’re running a data warehouse, I can’t think of a good reason to leave this at the default (5) for most any workload.

Look at any SQL Server setup checklist not written by SharePoint admins, and you’ll see people tell you to change this setting to something higher than 5.

What you change it to is not what I’m here to talk about. I’m Team Start With 50, but you can be whatever team you want and we can still be friends.

I mean, unless you’re going to tell me I should look at the plan cache to figure that out, then we are mortal enemies because you are full of shabby ideas. The plan cache is full of lies and bad estimates, and totally unreliable in the long term.

You could probably make better guesses based on Query Store, but Cost Threshold For Parallelism is, unfortunately, not a database-level setting, and they’d still just be guesses. About estimates. So, you know… Cool your jets, hotshot.

But since MAXDOP is not only available, but also offers guidance for a correct setting to the installer, why not this one? It is, after all, an important counterpart.

If anything, it’d be nice to give folks who care about automation one less post-setup step to handle. To me, that’s a glaring omission.

Costy Things


Of course, maybe it’s time to get Cost Threshold For Parallelism some help. Cost is, after all, just a reference metric.

It can be good, it can be bad. High cost plans can run fast, low cost plans can run slow.

With all the intelligent stuff being built into the product, perhaps it’s time for it to consider things in addition to plan cost for whether a query is eligible for parallelism or not.

Imagine this scenario: You set Cost Threshold For Parallelism to 50, and a really important query that costs 20 query bucks comes along and runs slowly and single threaded every single time it executes. It never stands a chance at going parallel, unless you drop Cost Threshold For Parallelism way low for the whole server.

Your only option other than lowering Cost Threshold For Parallelism is using an unsupported trace flag (8649), or an unsupported USE hint (ENABLE_PARALLEL_PLAN_PREFERENCE).

It sure would be nice if there were a supported override that you could set, say a maximum CPU threshold for a serial plan. I don’t think you could change this in flight, but you could potentially have it act like memory grant feedback, and adjust between executions.

Thanks for reading!

Going Further


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