Oracle数据库经常CPU利用率很高的问题分析

发布时间:2020-04-21编辑:脚本学堂
本文详细介绍下,在oracle数据库中经常遇到CPU利用率很高的问题的解决方法,这里做了详细的分析与探讨,有兴趣的朋友可以参考研究下。

本节主要内容:
Oracle数据库 CPU利用率太高 问题分析与解决方法

问题描述:
oracle数据库经常会遇到CPU利用率很高的情况,大多是数据库中存在着严重性能低下的sql语句,大大的消耗了CPU资源,导致整个系统性能低下。

以下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:solairs8
数据库:Oracle9.2.0.4

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。
首先,登陆主机,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

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$sqlarea.hashvalue=v$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 d.domainname,d.mswitchdomainid, a.SERVICEID,a.SERVICECODE,a.USERTYPE,a.STATUS,a.NOTIFYSTATUS,to_char(a.DATECREATED,'yyyy-mm-dd hh24:mi:ss') DATECREATED,VIPFLAG,STATUS2,CUSTOMERTYPE,CUSTOMERID FROM service a, gatewayloc b, subbureaunumber c, mswitchdomain d WHERE b.mswitchdomainid = d.mswitchdomainid and b.gatewaysn = c.gatewaysn AND a.ServiceCode like c.code||'%' and a.serviceSpecID=1 and a.status!='4' and a.status!='10' and a.servicecode like '010987654321%' and SubsidiaryID=999999999

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?
先来看看数据库的进程等待事件都有些什么:
 

 SQL> select sid,event,p1,p1text from v$session_wait;
   SID EVENT P1 P1TEXT
  ---------- ----------------------------------------------------------------
   12 latch free 4.3982E+12 address
   36 latch free 4.3982E+12 address
   37 latch free 4.3982E+12 address
   84 latch free 4.3982E+12 address
   102 latch free 4.3982E+12 address
   101 latch free 4.3982E+12 address
   85 latch free 4.3982E+12 address
   106 latch free 4.3982E+12 address
   155 latch free 4.3982E+12 address
   151 latch free 4.3982E+12 address
   149 latch free 4.3982E+12 address
   147 latch free 4.3982E+12 address
   1 pmon timer 300 duration

从上面的查询,可以看出,大都是latch free的等待事件,然后接着查一下这些latch的等待都是什么进程产生的:
 

  SQL> select spid from v$process where addr in
  (select paddr from v$session where sid in(84,102,101,106,155,151));
  SPID
  ------------
  25774
  26010
  25873
  25725

由此看出latch free这个等待事件导致了上面的那个SQL语句都在等待,占用了大量的CPU资源。
究竟主要是那种类型的latch的等待,根据下面的SQL语句:
 

 SQL> SELECT latch#, name, gets, misses, sleeps
   FROM v$latch
   WHERE sleeps>0
   ORDER BY sleeps;
  LATCH# NAME GETS MISSES SLEEPS
  ---------- ----------------------------------------------------------------
   15 messages 96876 20 1
   159 library cache pin allocation 407322 43 1
   132 dml lock allocation 194533 213 2
   4 session allocation 304897 48 3
   115 redo allocation 238031 286 4
   17 enqueue hash chains 277510 85 5
   7 session idle bit 2727264 314 16
   158 library cache pin 3881788 5586 58
   156 shared pool 2771629 6184 662
   157 library cache 5637573 25246 801
   98 cache buffers chains 1722750424 758400 109837

由上面的查询可以看出最主要的latch等待是cache buffers chains,这个latch的等待表明数据库存在单独的BLOCK的竞争这些latch,
来看下这个latch存在的子latch及其对应的类型:
  

SQL> SELECT addr, latch#, gets, misses, sleeps
   FROM v$latch_children
   WHERE sleeps>0
   and latch# = 98
   ORDER BY sleeps desc;
  ADDR LATCH# GETS MISSES SLEEPS
  ---------------- ---------- ---------- ---------- ----------
  000004000A3DFD10 98 10840661 82891 389
  000004000A698C70 98 159510 2 244
  0000040009B21738 98 104269771 34926 209
  0000040009B227A8 98 107604659 35697 185
  000004000A3E0D70 98 5447601 18922 156
  000004000A6C2BD0 98 853375 7 134
  0000040009B24888 98 85538409 25752 106
  ……………

接着,来查看sleep较多的子latch对应都有哪些对象:
 

 SQL> select distinct a.owner,a.segment_name,a.segment_type from
   dba_extents a,
  (select dbarfil,dbablk
  from x$bh
  where hladdr in
   (select addr
   from (select addr
   from v$latch_children
   order by sleeps desc)
   where rownum < 5)) b
  where a.RELATIVE_FNO = b.dbarfil
  and a.BLOCK_ID <= b.dbablk and a.block_id + a.blocks > b.dbablk;
  OWNER SEGMENT_NAME SEGMENT_TYPE
  ---------------------------------------------------------------------------
  TEST I_SERVICE_SERVICESPECID INDEX
  TEST I_SERVICE_SUBSIDIARYID INDEX
  TEST SERVICE TABLE
  TEST MSWITCHDOMAIN TABLE
  TEST I_SERVICE_SC_S INDEX
  …………………

看到在开始的那个SQL语句中的几个对象都有包括在内,于是来看看开始的那个SQL的执行计划:
 

 SQL> set autotrace trace explain
  SQL>SELECT d.domainname,d.mswitchdomainid, a.SERVICEID,a.SERVICECODE,a.USERTYPE,a.STATUS,a.NOTIFYSTATUS,to_char(a.DATECREATED,'yyyy-mm-dd hh24:mi:ss') DATECREATED,VIPFLAG,STATUS2,CUSTOMERTYPE,CUSTOMERID FROM service a, gatewayloc b, subbureaunumber c, mswitchdomain d WHERE b.mswitchdomainid = d.mswitchdomainid and b.gatewaysn = c.gatewaysn AND a.ServiceCode like c.code||'%' and a.serviceSpecID=1 and a.status!='4' and a.status!='10' and a.servicecode like '010987654321%' and SubsidiaryID=999999999;
  Execution Plan
  ----------------------------------------------------------
   0 SELECT STATEMENT Optimizer=CHOOSE
   1 0 NESTED LOOPS
   2 1 NESTED LOOPS
   3 2 NESTED LOOPS
   4 3 TABLE ACCESS (FULL) OF 'SUBBUREAUNUMBER'
   5 3 TABLE ACCESS (BY INDEX ROWID) OF 'GATEWAYLOC'
   6 5 INDEX (UNIQUE SCAN) OF 'PK_GATEWAYLOC' (UNIQUE)
   7 2 TABLE ACCESS (BY INDEX ROWID) OF 'MSWITCHDOMAIN'
   8 7 INDEX (UNIQUE SCAN) OF 'PK_MSWITCHDOMAIN' (UNIQUE)
   9 1 TABLE ACCESS (BY INDEX ROWID) OF 'SERVICE'
   10 9 AND-EQUAL
   11 10 INDEX (RANGE SCAN) OF 'I_SERVICE_SERVICESPECID' (NON
   -UNIQUE)
   12 10 INDEX (RANGE SCAN) OF 'I_SERVICE_SUBSIDIARYID' (NON-
   UNIQUE)

根据开始查到的引起latch free等待中的对象和SQL语句的执行计划,觉得SERVICE表上的索引有问题,似乎存在了过多的扫描,于是将同样的SQL语句在别的地市的同样的数据库上执行一下,查看相应的执行计划:
 

 SQL> set autotrace trace explain
  SQL>SELECT d.domainname,d.mswitchdomainid, a.SERVICEID,a.SERVICECODE,a.USERTYPE,a.STATUS,a.NOTIFYSTATUS,to_char(a.DATECREATED,'yyyy-mm-dd hh24:mi:ss') DATECREATED,VIPFLAG,STATUS2,CUSTOMERTYPE,CUSTOMERID FROM service a, gatewayloc b, subbureaunumber c, mswitchdomain d WHERE b.mswitchdomainid = d.mswitchdomainid and b.gatewaysn = c.gatewaysn AND a.ServiceCode like c.code||'%' and a.serviceSpecID=1 and a.status!='4' and a.status!='10' and a.servicecode like '010987654321%' and SubsidiaryID=999999999;
  Execution Plan
  ----------------------------------------------------------
   0 SELECT STATEMENT Optimizer=CHOOSE
   1 0 TABLE ACCESS (BY INDEX ROWID) OF 'SERVICE'
   2 1 NESTED LOOPS
   3 2 NESTED LOOPS
   4 3 NESTED LOOPS
   5 4 TABLE ACCESS (FULL) OF 'SUBBUREAUNUMBER'
   6 4 TABLE ACCESS (BY INDEX ROWID) OF 'GATEWAYLOC'
   7 6 INDEX (UNIQUE SCAN) OF 'PK_GATEWAYLOC' (UNIQUE)
   8 3 TABLE ACCESS (BY INDEX ROWID) OF 'MSWITCHDOMAIN'
   9 8 INDEX (UNIQUE SCAN) OF 'PK_MSWITCHDOMAIN' (UNIQUE)
   10 2 INDEX (RANGE SCAN) OF 'I_SERVICE_SC_S' (NON-UNIQUE)

对比两个执行计划,发现索引I_SERVICE_SERVICESPECID和I_SERVICE_SUBSIDIARYID是不应该走的,于是又对比了两个地方SERVICE表上的索引个数:
 

  SQL> select index_name from user_indexes where table_name='SERVICE';
  INDEX_NAME
  ------------------------------
  I_SERVICE_ACCOUNTNUM
  I_SERVICE_CID
  I_SERVICE_DATEACTIVATED
  I_SERVICE_PRICEPLANID
  I_SERVICE_SC_S
  I_SERVICE_SERVICECODE
  I_SERVICE_SERVICESPECID
  I_SERVICE_SUBSIDIARYID
  PK_SERVICE_SID
  SQL> select index_name from user_indexes where table_name='SERVICE';
  INDEX_NAME
  ------------------------------
  I_SERVICE_ACCOUNTNUM
  I_SERVICE_CID
  I_SERVICE_DATEACTIVATED
  I_SERVICE_SC_S
  I_SERVICE_SERVICECODE
  PK_SERVICE_SID

发现存在问题的数据库中的SERVICE表上不知道怎么多出了I_SERVICE_PRICEPLANID、I_SERVICE_SERVICESPECID 、I_SERVICE_SUBSIDIARYID三个索引,而这些索引就是导致了开始那个SQL语句用了不该用的索引,引起latch free等待和CPU占用很高的罪魁祸首,于是删除了那三个索引,重新执行相应的SQL语句,很快就得出了结果,CPU的利用率也马上下降为正常了,观察结果如下:
 

 last pid: 26387; load averages: 1.61, 1.38, 1.21
  195 processes: 194 sleeping, 1 on cpu
  CPU states: 96.2% idle, 1.6% user, 1.7% kernel, 0.5% iowait, 0.0% swap
  Memory: 8192M real, 1183M free, 14M swap in use, 8179M swap free
  PID USERNAME THR PRI NICE SIZE RES STATE TIME CPU COMMAND
  26383 oracle 1 59 0 4550M 4506M sleep 0:12 4.52% oracle
   409 root 15 59 0 7168K 7008K sleep 173.1H 0.53% picld
  25653 oracle 1 59 0 4550M 4508M sleep 2:12 0.48% oracle
  26384 root 1 59 0 2800K 1912K cpu2 0:00 0.21% top-3.5b8-sun4u
  25569 oracle 1 59 0 4550M 4508M sleep 0:12 0.09% oracle
  25717 oracle 1 59 0 4550M 4507M sleep 0:07 0.05% oracle
  25571 oracle 1 59 0 4550M 4507M sleep 0:10 0.04% oracle
  25681 oracle 1 59 0 4550M 4508M sleep 0:10 0.04% oracle
  25544 oracle 1 58 0 4554M 4501M sleep 0:14 0.03% oracle
  25703 oracle 1 59 0 4550M 4506M sleep 0:23 0.03% oracle
  ………………
 

对于CPU利用率过高的情况,如果是SQL语句性能比较低下引起的基本上都可以按照这个思路来诊断和解决问题。