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Diagnosing Stalled Dispatchers

Sean Gallardy has another memory dump to investigate:

If you’ve been following with the series of demystifying dumps then you’re probably thinking or have thought something along the lines of, “All of these issues seem to be around stalled or non-yielding things…” and you’d be correct. If you revisit the non-yielding scheduler dump post, I explain a little about cooperative scheduling and how things need to place nice, if one piece isn’t playing nice by doing its part in a timely manner or not giving up time to others, that’s going to be a very large problem for the system in general. The stalled dispatcher falls into the same set of issues.

There are various dispatcher pools in SQL Server, though the most widely known one is for extended events. Dispatcher pools are items that hold pieces of work to be completed, like a dumping ground for “someone pick this up and do this” type of tasks. If it helps, you can think of them in a similar way as connection pooling, where there are background threads which will do something (like making or updating connections) when needed. It can respond to pressure by making new threads if needed or trim threads when the work slows down. In SQL Server dispatchers set their own attributes including when an item is considered stuck, the number of threads to use, etc., which means some may never be considered stalled as they are able to disable their own timeouts.

Read on to see what might cause a stalled dispatcher and what a memory dump for one can tell us.