Waits: External But Not Preemptive

Ewald Cress has some thoughts on external wait types:

Previously I dug into preemptive waits in SQLOS, and to be honest, I equated “preemptive” with “external”. For the most part the two go hand in hand after all.

To recap, a preemptive wait isn’t necessarily a wait at all. What happens is that a worker needs to run some code that can’t be trusted to play by cooperative scheduling rules. And rather than put the SQLOS scheduler (and all its sibling workers) at the mercy of that code, the worker detaches itself from the scheduler and cedes control to a sibling runnable worker.

Read the whole thing.

Related Posts

How LSNs Get Generated

Stuart Moore looks at how SQL Server builds log sequence numbers: If you’ve ever dug down in the SQL Server transaction logs or had to build up restore chains, then you’ll have come across Log Sequence Numbers (LSNs). Ever wondered why they’re so large, why they all look suspiciously the same, why don’t they start […]

Read More

Row Goals On Anti-Joins

Paul White continues his row goals series: The optimizer assumes that people write a semi join (indirectly e.g. using EXISTS) with the expectation that the row being searched for will be found. An apply semi join row goal is set by the optimizer to help find that expected matching row quickly. For anti join (expressed e.g. using NOT EXISTS) the optimizer’s assumption is that […]

Read More