診斷和解決CPU利用率高的問題(zt)

Oracle資料庫經常會遇到CPU利用率很高的情況，這種時候大都是資料庫中存在著嚴重效能低下的SQL語句，這種SQL語句大大的消耗了CPU資源，導致整個系統效能低下。當然，引起嚴重效能低下的SQL語句的原因是多方面的，具體的原因要具體的來分析，下面透過一個實際的案例來說明如何來診斷和解決CPU利用率高的這類問題。

作業系統：solairs8

資料庫：Oracle9.2.0.4

問題描述：現場工程師彙報資料庫非常慢，幾乎所有應用操作均無法正常進行。

首先登陸主機，執行top發現CPU資源幾乎消耗殆盡，存在很多佔用CPU很高的程式，而記憶體和I/O都不高，具體如下：

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

於是先檢視資料庫的告警日誌ALERT檔案，並沒有發現有什麼錯誤存在，日誌顯示資料庫執行正常，排除資料庫本身存在問題。

然後檢視這些佔用CPU資源很高的Oracle程式究竟是在做什麼操作，使用如下SQL語句：

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中佔用CPU很高的程式的PID替換指令碼中的PID,得到相應的Oracle程式所執行的SQL語句，發現佔用CPU資源很高的程式都是執行同一個SQL語句：

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

基本上可以肯定是這個SQL引起了系統CPU資源大量被佔用，那究竟是什麼原因造成這個SQL這麼大量佔用CPU資源呢，我們先來看看資料庫的程式等待事件都有些什麼：

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

從上面的查詢我們可以看出，大都是latch free的等待事件，然後接著查一下這些latch的等待都是什麼程式產生的：

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這個等待事件導致了上面的那個SQL語句都在等待，佔用了大量的CPU資源。我們來看看究竟主要是那種型別的latch的等待，根據下面的SQL語句:

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

由上面的查詢可以看出最主要的latch等待是cache buffers chains，這個latch的等待表明資料庫存在單獨的BLOCK的競爭這些latch，我們來看這個latch存在的子latch及其對應的型別：

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

……………

接著我們來檢視sleep較多的子latch對應都有哪些物件：

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

…………………

我們看到在開始的那個SQL語句中的幾個物件都有包括在內，於是來看看開始的那個SQL的執行計劃：

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)

根據開始查到的引起latch free等待中的物件和SQL語句的執行計劃，覺得SERVICE表上的索引有問題，似乎存在了過多的掃描，於是將同樣的SQL語句在別的地市的同樣的資料庫上執行一下，檢視相應的執行計劃:

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)

對比兩個執行計劃，發現索引I_SERVICE_SERVICESPECID和I_SERVICE_SUBSIDIARYID是不應該走的，於是又對比了兩個地方SERVICE表上的索引個數:

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

發現存在問題的資料庫中的SERVICE表上不知道怎麼多出了I_SERVICE_PRICEPLANID、I_SERVICE_SERVICESPECID 、I_SERVICE_SUBSIDIARYID三個索引，而這些索引就是導致了開始那個SQL語句用了不該用的索引，引起latch free等待和CPU佔用很高的罪魁禍首，於是刪除了那三個索引，重新執行相應的SQL語句，很快就得出了結果，CPU的利用率也馬上下降為正常了，觀察結果如下：

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

………………

對於CPU利用率過高的情況，如果是SQL語句效能比較低下引起的基本上都可以按照這個思路來診斷和解決問題，當然具體問題還得具體分析，解決問題的方法也有很多種，這裡不過是拋磚引玉一下，只要能最終達到我們解決問題的目的就可以了。