Performance issues; waited too long for a row cache enqueue lock!

Question

hi Experts,
OS: Oracle Solaris on SPARC (64-bit)
DB version:
SQL> select * from V$VERSION;
BANNER
Oracle Database 11g Release 11.2.0.1.0 - 64bit Production
PL/SQL Release 11.2.0.1.0 - Production
CORE    11.2.0.1.0      Production
TNS for Solaris: Version 11.2.0.1.0 - Production
NLSRTL Version 11.2.0.1.0 - Production
SQL>We have seen 100% CPU usage and high database load, so I checked the instance and have seen there were many blocking sessions and more than 71 sessions running the same select ;
elect tablespace_name as tbsname from        (select tablespace_name,sum(bytes)/1024/1024 free_mb,0 total_mb,0 max_mb         from dba_free_space         group by tablespace_name         union         select tablespace_name, 0 current_mb,sum(bytes)/1024/1024 total_mb,                sum(decode(maxbytes, 0, bytes, maxbytes))/1024/1024 max_mb         from dba_data_files         group by tablespace_name) group by tablespace_name having round((sum(total_mb)-sum(free_mb))/sum(max_mb)*100) > 95  Blocking sessions are running queries like this;
SELECT * from MYTABLE WHERE MYCOL=:1 FOR UPDATE;This select queries are coming from a cron job running every 10 minutes to check the tablespaces; so I first killed (kill -9 pid) those select statements so the load and CPU decreased to 13% of CPU usage. Blocking sessions still there and I didn't killed them waiting for app guys confirmation... after few hours and the CPU usage never went down the 13%; I have seen many errors;
WAITED TOO LONG FOR A ROW CACHE ENQUEUE LOCK! pid=...System State dumped to trace file .....trcAfter that , we decided to restart the DB to release the locks!
I would like to understand why during loads we were no able to run those select statements, statspack schedule snapshot reports were not able to finish, also automatic
database statistics... why 5 for update statements locked the whole DB?

Answer 1

user12035575 wrote:
SELECT FOR UPDATE will only lock the table row until the transaction is completed.
"WAITED TOO LONG FOR A ROW CACHE ENQUEUE LOCK" happens when it needs to acquire a lock on data dictionary. Did you check the trace file associated with the statement?The trace file is too long, which information I need to focus more?