In short, high THREADPOOL waits can happen when SQL Server doesn’t have enough “worker threads” to handle new tasks, which could cause SQL Server to hang and refuse connections. When a task is waiting for a worker thread to become available, that wait type is called THREADPOOL wait.
A background process, called “Scheduler Monitor“, will identify when the same worker threads are “stuck” in the same state for 60 seconds or more. In which case it will resolve the issue as a Deadlocked Scheduler, and that’ll cause dropped connections, rollbacks, and even fail-overs.
Read on to understand what causes this as well as why we always fumble our keys under the car as the scary monster approaches.