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AQ Server EMON Processes Get Error ORA-04021 (Doc ID 1921934.1)

Last updated on JULY 23, 2021

Applies to:

Oracle Database Cloud Schema Service - Version N/A and later
Gen 1 Exadata Cloud at Customer (Oracle Exadata Database Cloud Machine) - Version N/A and later
Oracle Cloud Infrastructure - Database Service - Version N/A and later
Oracle Database Exadata Express Cloud Service - Version N/A and later
Oracle Database Backup Service - Version N/A and later
Information in this document applies to any platform.


See errors in database alert log on qxxx processes:

ORA-04021: timeout occurred while waiting to lock object
ORA-00447: fatal error in background process
Emon timeout encountered error 4021

AQ messages may stop being processed.

You have confirmed streams pool size>0.


A systemstate dump may show an EMON worker (Exxx) blocking other sessions waiting for Library Cache lock and not processing.

Example from systemstate:

   OSD pid info: Unix process pid: 79628, image: oracle@<NODE NAME> (E003) <<================EMON worker
   Short stack dump:

Current Wait Stack:

     Not in wait; last wait ended 65 min 48 sec ago     <<=====================================NOT IN WAIT
   There are 64 sessions blocked by this session.   <<================================BLOCKING OTHER SESSIONS


NOTE that the emon stack contains kponexpnfy which calls kkcnsli.


A sql trace of the emon worker may also show repeated looping of statement:

select location_name from loc$ where emon# = :1 and connection# = :2








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