Oracle数据库经常会遇到CPU利用率很高的情况，这种时候大都是数据库中存在着严重性能低下的SQL语句，这种SQL语句大大的消耗了CPU资源，导致整个系统性能低下。当然，引起严重性能低下的SQL语句的原因是多方面的，具体的原因要具体的来分析，下面通过一个实际的案例来说明如何来诊断和解决CPU利用率高的这类问题。
操作系统：Linux7.0
数据库：Oracle11.2.0.4
问题描述：现场工程师汇报数据库非常慢，几乎所有应用操作均无法正常进行。不久后，系统断开连接，宕机。
首先重启系统后，启动数据库。执行top发现CPU资源几乎消耗殆尽，存在很多占用CPU很高的进程，而内存和I/O都不高，具体如下：
last pid: 26136; load averages: 8.89, 8.91, 8.12
216 processes: 204 sleeping, 8 running, 4 on cpu
CPU states: 0.6% idle, 97.3% user, 1.8% kernel, 0.2% iowait, 0.0% swap
Memory: 8192M real, 1166M free, 14M swap in use, 8179M swap free
PID USERNAME THR PRI NICE SIZE RES STATE TIME CPU COMMAND
25725 oracle 1 50 0 4550M 4508M cpu2 12:23 11.23% oracle
25774 oracle 1 41 0 4550M 4508M run 14:25 10.66% oracle
26016 oracle 1 31 0 4550M 4508M run 5:41 10.37% oracle
26010 oracle 1 41 0 4550M 4508M run 4:40 9.81% oracle
26014 oracle 1 51 0 4550M 4506M cpu6 4:19 9.76% oracle
25873 oracle 1 41 0 4550M 4508M run 12:10 9.45% oracle
25723 oracle 1 50 0 4550M 4508M run 15:09 9.40% oracle
26121 oracle 1 41 0 4550M 4506M cpu0 1:13 9.28% oracle
25745 oracle 1 41 0 4551M 4512M run 9:33 9.28% oracle
26136 oracle 1 41 0 4550M 4506M run 0:06 5.61% oracle
409 root 15 59 0 7168K 7008K sleep 173.1H 0.52% picld
25653 oracle 1 59 0 4550M 4508M sleep 1:01 0.46% oracle
25565 oracle 1 59 0 4550M 4508M sleep 0:07 0.24% oracle
25703 oracle 1 59 0 4550M 4506M sleep 0:08 0.13% oracle
25701 oracle 1 59 0 4550M 4509M sleep 0:23 0.10% oracle
于是先查看数据库的告警日志ALERT文件，并没有发现有什么错误存在，日志显示数据库运行正常，排除数据库本身存在问题。
然后查看这些占用CPU资源很高的Oracle进程究竟是在做什么操作，使用如下SQL语句：
select sql_text,spid,v$session.program,process from
v$sqlarea,v$session,v$process
where v$sqlarea.address=v$session.sql_address
and v$sqlarea.hash_value=v$session.sql_hash_value
and v$session.paddr=v$process.addr
and v$process.spid in (PID);
用top中占用CPU很高的进程的PID替换脚本中的PID,得到相应的Oracle进程所执行的SQL语句，发现占用CPU资源很高的进程都是执行同一个SQL语句：
select username "username", to_char(timestamp,'DD-MON-YYYY HH24:MI:SS') "time_stamp", action_name "statement", os_username "os_username", userhost "userhost", returncode||decode(returncode,'1004','-Wrong Connection','1005','-NULL Password','1017','-Wrong Password','1045','-Insufficient Priviledge','0','-Login Accepted','--') "returncode" from sys.dba_audit_session where (sysdate - timestamp)*24 < 1 and returncode <> 0 order by timestamp;
基本上可以肯定是这个SQL引起了系统CPU资源大量被占用，那究竟是什么原因造成这个SQL这么大量占用CPU资源呢，从上面的SQL语句中我们可以看到sys.dba_audit_session这张表，由此可以确定是由于审计的原因导致数据库占用大量CPU。
查看数据库审计信息：
SQL> show parameter audit
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
audit_file_dest string /u01/app/oracle/admin/orcl/adump
audit_sys_operations boolean FALSE
audit_syslog_level string
audit_trail string DB
可以看到数据库审计为开启状态，并且将audited record的存放在数据库里（sys.aud$）中。
问题处理方法：
1.如果审计不是必须的，可以关掉审计功能；
SQL> alter system set audit_trail=none scope=spfile;
SQL>showdown immediate;
SQL>startup
2.删除已有的审计信息
可以直接truncate表aud$,
或者采取dbms_audit_mgmt来清除。
3.或者将aud$表移到另外一个表空间下，以减少system表空间的压力和被撑爆的风险。
附：11g中有关audit_trail参数的设置说明：
AUDIT_TRAIL
| Property | Description |
|---|---|
| Parameter type | String |
| Syntax | AUDIT_TRAIL = { none | os | db [, extended] | xml [, extended] } |
| Default value | none |
| Modifiable | No |
| Basic | No |
AUDIT_TRAIL enables or disables database auditing.
Values:
-
none
Disables standard auditing. This value is the default if the AUDIT_TRAIL parameter was not set in the initialization parameter file or if you created the database using a method other than Database Configuration Assistant. If you created the database using Database Configuration Assistant, then the default is db.
-
os
Directs all audit records to an operating system file. Oracle recommends that you use the os setting, particularly if you are using an ultra-secure database configuration.
-
db
Directs audit records to the database audit trail (the SYS.AUD$ table), except for records that are always written to the operating system audit trail. Use this setting for a general database for manageability.
If the database was started in read-only mode with AUDIT_TRAIL set to db, then Oracle Database internally sets AUDIT_TRAIL to os. Check the alert log for details.
-
db, extended
Performs all actions of AUDIT_TRAIL=db, and also populates the SQL bind and SQL text CLOB-type columns of the SYS.AUD$ table, when available. These two columns are populated only when this parameter is specified.
If the database was started in read-only mode with AUDIT_TRAIL set to db, extended, then Oracle Database internally sets AUDIT_TRAIL to os. Check the alert log for details.
-
xml
Writes to the operating system audit record file in XML format. Records all elements of the AuditRecord node except Sql_Text and Sql_Bind to the operating system XML audit file.
-
xml, extended
Performs all actions of AUDIT_TRAIL=xml, and populates the SQL bind and SQL text CLOB-type columns of the SYS.AUD$table, wherever possible. These columns are populated only when this parameter is specified.
You can use the SQL AUDIT statement to set auditing options regardless of the setting of this parameter.