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Don Burleson Blog 







Oracle CPU Constrained Database

Oracle Tips by Burleson Consulting

In Oracle, the majority of wait time is spent waiting in I/O or performing computations.  As we have stressed, CPU enqueues   can be observed when the CPU run queue exceeds the number of CPUs on the database server, and this can be seen by looking at the “r” column in the vmstat UNIX/Linux utility or within the Windows performance manager. 

If the system is already optimized, having CPU time as a top wait event is a positive because the addition of faster CPUs or more CPUs will relieve the bottleneck. 

However, high CPU usage is also indicative of excessive logical I/O   (consistent gets ) against the data buffers which might indicate the need for SQL tuning or shared pool and library cache tuning.  High CPU usage will be reported as a top 5 timed even in any STATSPACK or AWR report as shown below:


Top 5 Timed Events
~~~~~~~~~~~~~~~~~~                                                     % Total
Event                                               Waits    Time (s) Ela Time
-------------------------------------------- ------------ ----------- --------
db file sequential read                           568,948       4,375    66.15
CPU time                                                        1,983    29.99
db file scattered read                            106,287          65      .99
log file sync                                       7,053          50      .75
log buffer space                                    1,717          47      .71
You can see “real” enqueues on CPU resources when the runqueue (r)column in vmstat exceeds the cpu_count parameter value, and you can also detect an overloaded CPU when you see the “resmgr:cpu quantum” event in a top-5 timed event on a AWR or STATSPACK report.


This is an excerpt from my latest book "Oracle Tuning: The Definitive Reference". 

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts:




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