Checking for CPU pressure via sys.dm_os_schedulers

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Do you suspect you are encountering CPU pressure?  Some of the more common signs of CPU pressure include:

  • Sustained "% Processor Time" values (>75% for extended periods)
  • High signal wait time percentage from sys.dm_os_wait_stats
  • SOS_SCHEDULER_YIELD waits

One way to measure if you have several tasks waiting for CPU time is to query the sys.dm_os_schedulers DMV.  This DMV has several interesting columns - however one in particular is useful for measuring queued tasks - the "runnable_tasks_count" column.  The "runnable_tasks_count" value indicates the count of workers that have tasks assigned to them that are just waiting for their share of time on the scheduler (logical CPU).  I checked various busy OLTP hosting SQL Server instances, and even though these systems are quite active, the value for runnable_tasks_count was very low (almost always showing zero values). 

Yesterday I did encounter a SQL Server instance where the value of runnable_tasks_count averaged about 13 across all schedulers with a status of "VISIBLE ONLINE".  The value of runnable_tasks_count should be as low as possible - and if it shows ongoing non-zero values, you have further evidence of CPU pressure.

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