Finding CPU Pressure using wait_stats DMV

CPU Pressure forms vital part in performance monitoring and sys.dm_os_wait_stats is a good place to check. Some background info before we look at the DMV.

Waiting time for a SQL Process is divided into two. They are Resource waits and signal waits( CPU waits ). When a process/query is submitted to SQL Server, SQL Server checks whether all resources required for the process are available.Resources would include things like IO, Network, Memory, Locks, Latch waits etc. For Ex: If a process is waiting for another process to release its lock, then it becomes a resource wait because of locks. If a process wants to write something to a disk but its has to wait for some other IO activity to complete then its a resource wait because of IO. So all these waiting time gets categorized as resource waits..

After a process grabs all the resources it requires, the process gets scheduled to run/executed by the Processor/ CPU. So, it enters the CPU queue. If there are already many processes executing, then it has to wait to get the CPU. The time spent waiting for the CPU is termed as signal waits.

sys.dm_os_wait_stats DMV provides the cumulative sum of the waiting time experienced by all processes at each wait type. dm_os_wait_stats DMV is a SQL Server 2005/2008 equivalent of DBCC SQLPERF(waitstats). A lot more accurate explanation about the DMV is provided here . dm_os_wait_stats maintains these stats since last restart of the server or since the last execution DBCC SQLPERF ('sys.dm_os_wait_stats', CLEAR);

sys.dm_os_wait_stats provides about 485 wait types/wait events and waiting time spent at each wait type. Two interesting columns of this DMV are

* wait_time_ms
* signal_wait_time_ms

Wait_time_ms gives the total time spent at each wait type ( Resource + cpu wait ). signal_wait_time_ms specifies the time spent on waiting for CPU. Resource waittime can be found by wait_time_ms - signal_wait_time_ms.


SELECT SUM(signal_wait_time_ms) * 100 / SUM(wait_time_ms) AS
       cpu_pressure_percentage,
       SUM(signal_wait_time_ms)                           AS cpu_wait,
       SUM(wait_time_ms - signal_wait_time_ms)            AS resource_wait,
       SUM(wait_time_ms)                                  AS total_wait_time
FROM   sys.dm_os_wait_stats 




Consider the query above. cpu_pressure_percentage provides percentage of time spent waiting for CPU, by finding the ratio of signal_time and total wait time. Microsoft recommends this value to be less than 25% for a normal system. Value any higher would indicate a system suffering likely from a CPU stress.
Most common causes for CPU stress/high signal waits can be

* Poor and ineffective query plans / missing indexes / table scans
* Too many Plan Recompliations
* Slow CPU and requires a upgrade

As a production DBA, its a good idea to collect the resultset dm_os_wait_stats DMV once few hours or twice or thrice a day and perhaps analyze the results to notice any surge in wait time. This can be one proactive approach to detect potential major problems.