SQL Server threads architecture


SQL Server retrieves threads from Windows. The SQL Server configuration setting max worker threads (set at instance level) determines how many threads will be retrieved. SQL Server has its own internal scheduling system, independent of the scheduling performed by the operating system.

Instead of using Windows threads directly, SQL Server creates a pool of worker threads that are mapped to Windows threads whenever work needs to be performed.
When a SQL Server component needs to execute code, the component creates a task that represents the unit of work to be done.

For example, if you send a batch of Transact-SQL commands to the server, it’s
likely that the batch will be executed within a task.
When a SQL Server component creates a task, it is assigned the next available worker thread that is not in use. If no worker threads are available, SQL Server will try to retrieve another Windows thread, up to the point that the max worker threads configuration limit is reached.

At that point, the new task would need to wait to get a worker thread. THREADPOOL wait type is when there are no available threads in the server’s thread pool, during parallelism multiple worker threads are required. So, this wait type ir related with CXPACKET.

I suggest reducing parallelism not setting MAXDOP 1. Try to increase Cost threshold for parallelism if the setting is too low (default is 5) and analyze the queries with parallelism, see the indexes and statistics to understand if the query optimizer is estimating right.


Wait wait wait…

Let’s talk about why we have to wait and how to understand the wait types.

Paul Randal in his post Wait statistics, or please tell me where it hurts said:

A thread is using the CPU (called RUNNING) until it needs to wait for a resource. It then moves to an unordered list of threads that are SUSPENDED. In the meantime, the next thread on the FIFO (first-in-first-out) queue of threads waiting for the CPU (called being RUNNABLE) is given the CPU and becomes RUNNING. If a thread on the SUSPENDED list is notified that it’s resource is available, it becomes RUNNABLE and is put on the bottom of the RUNNABLE queue. Threads continue this clockwise movement from RUNNING to SUSPENDED to RUNNABLE to RUNNING again until the task is completed

That’s explain a lot, because the SQL Server threads doesn’t run all in the same time. A good example is when our query is doing physical reads. The IO subsystem is the slowest part of our resources and probably will take some time if the query is reading gigabytes of data.


After the CPU request the data from the disk, the disk will run for it, but before send the data back. All data need to go to memory first and that may don’t have the necessary space.  The thread is going to wait until some resources been released first. There are many scenarios, for example, how many threads are running this query? How long will take to the application to show that data?

So, every time a thread needs to wait for a resource it will increase a wait time type, such as PAGEIOLATCH_XX , PAGELATCH_XX, ASYNC_NETWORK_IO, CXPACKET, RESOURCE_SEMAPHORE. I will talk more about waits in the next posts.