Performance Tuning with SQL Server Dynamic Management Views

WHERE scheduler_id < 255
AND runnable_tasks_count > 0
-- AND pending_disk_io_count > 0
Listing 7.17: Investigating potential disk I/O or CPU pressure.
Insufficient threads
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Chapter 7: OS and Hardware Interaction
If the system isn't experiencing CPU pressure, is not I/O bound, and there aren't
too many other types of waits on the system, then you may need to investigate
the possibility that there are simply too few threads available to the server. The
query in Listing 7.18 comes courtesy of Slava Oks (http://blogs.msdn.com/slavao/
archive/2006/09/28/776437.aspx) and investigates the possible need to adjust the server
threads system configuration, by checking the average value of work_queue_count.
In the absence of CPU pressure, values of more than one for the average of the work_
queue_count column can mean that you need to increase the number of threads
allowable to the server.
SELECT AVG(work_queue_count)
FROM sys.dm_os_schedulers
WHERE status = 'VISIBLE ONLINE'
Listing 7.18: Are there sufficient worker threads for the workload?
Context switching
Context switching occurs when a process fails to complete in a reasonable time, and
the process has to give up control of the scheduler (again, representing a CPU) to let
another process complete. SQL Server generally operates in a non-preemptive mode,
where threads voluntarily give up access to the CPU at the appropriate moment. Some
operations, however, such as calling an extended stored procedure, distributed query,
and so on, cause the scheduler to preempt a context switch. A high number of context