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Don Burleson Blog 







  MMON suspending action tips

Oracle Database Tips by Donald BurlesonMarch 29, 2015

Question:  I am getting "MMON suspending action" messages in my alert log and my instance is not allowing new SQL*Plus sessions.  A trace files also shows ORA-12751 errors. "DDE rules only execution for: ORA 12751 ".  So, how so I address this MMON suspending action errors and connect to my instance?

Answer:  You can connect to your instance using the -prelim option or by killing your oracle instance and re-starting the instance.

The MMON background process is responsible for the collection of statistics for the Automatic Workload Repository (AWR).  As Oracle becomes over-stressed, he shuts-down non-critical background processes such as MMON.

Error occurred while spawning process m000; error = 12751

Suspending MMON action 'Block Cleanout Optim, Undo Segment Scan' for 82800 seconds
Suspending MMON action 'tablespace alert monitor' for 82800 seconds

Suspending MMON action 'recovery area alert action' for 82800 seconds
Suspending MMON action 'AWR Auto Flush Task' for 82800 seconds
Suspending MMON action 'AWR Auto CPU USAGE Task' for 82800 seconds
Suspending MMON slave action kewfmadsa_ for 82800 seconds
Suspending MMON action 'undo usage' for 82800 seconds

Suspending MMON action 'undo usage' for 82800 seconds
Suspending MMON action 'metrics monitoring' for 82800 seconds

As we see, MMON will suspend actions where there is a large enqueue of background tasks waiting for service, or any case of server resource exhaustion.  MMON is not critical to Oracle operations, hence it is suspended as a instance survival mechanism.

The solution is tuning SQL to use less computing resources or adding either RAM or CPU to the server . . . 

In this case the root cause appears to be CPU exhaustion, a case of CPU exhaustion where the runqueue exceeds the number of CPU's on the server.  See my notes on high CPU usage. The MMON process can also be suspended for a shortage of RAM resources (see swap RAM and total RAM usage) or a shortage of other server resources.  See my book Oracle Tuning: The Definitive Reference 3rd edition" for isolating the exact server resource problem

For more details see these MOSC notes:

  •  MOSC note 1478691.1 titled "Minact-scn: Useg Scan Erroring Out With Error E:12751".
  •  MOSC Note 761298.1 titled "MMON Suspended Due to ORA-12751 "cpu time or run time policy violation".
  •  MOSC Note 1291296.1 titled "ORA-12751 "cpu time or run time policy violation" in the MMON Slave Process File during Feature Usage Statistics [FUS]".

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