[20150123]热链竞争.txt

--最近一段时间都在看vage的,提到CBC latch保护多个链表,而且还要保护BH的读和修改操作.而一个链表中,可能
--会有多个BH,这样算下来,一个CBC latch除了操作多个链表以外,还要保护数目更多的BH.因此,当CBC Latch出现竞争时,可能是如下两种
--情况:
1.多个进程频繁地以不兼容的模式申请获得某一CBC LATCH,访问此CBC latch保护的不同链表和不同BH.
2.多个进程频繁地以不兼容的模式申请获得莫伊CBC LATCH,访问此CBC Latch保护的同一链表的同一BH.

在这两种情况中,第一种情况被称为热链竞争,第二种情况被称为热块竞争. P62.

--自己重复测试,出现热链的情况:

1.建立测试环境:
SCOTT@test> @ver1

PORT_STRING                    VERSION        BANNER
------------------------------ -------------- --------------------------------------------------------------------------------
x86_64/Linux 2.4.xx            11.2.0.3.0     Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit Production

SCOTT@test> create table t as select rownum id,'test' data from dual connect by levelTable created.

SCOTT@test> select rowid,t.* from t where id=600;
ROWID                      ID DATA
------------------ ---------- ----
AABIyeAAEAAAAcMABL        600 test

SCOTT@test> @lookup_rowid AABIyeAAEAAAAcMABL
    OBJECT       FILE      BLOCK        ROW DBA                  TEXT
---------- ---------- ---------- ---------- -------------------- ----------------------------------------
    298142          4       1804         75 4,1804               alter system dump datafile 4 block 1804
                                                                 ;
2.查询对应的cbc latch地址:
SCOTT@test> select LRU_flag,hladdr from x$bh where dbarfil=4 and dbablk=1804 and OBJ=298142;
  LRU_FLAG HLADDR
---------- ----------------
         0 00000000BCAC7C58

SCOTT@test> select file#,dbablk,owner,object_id,data_object_id,object_name from x$bh a,dba_objects b where hladdr=hextoraw('00000000BCAC7C58') and a.obj=b.data_object_id;
     FILE#     DBABLK OWNER   OBJECT_ID DATA_OBJECT_ID OBJECT_NAME
---------- ---------- ------ ---------- -------------- --------------------
         1      70369 SYS            20              2 ICOL$
         1       9171 SYS            20              2 ICOL$
         1      70369 SYS            19              2 IND$
         1       9171 SYS            19              2 IND$
         1      70369 SYS            21              2 COL$
         1       9171 SYS            21              2 COL$
         1      70369 SYS             5              2 CLU$
         1       9171 SYS             5              2 CLU$
         1      48446 SYS            36             36 I_OBJ1
         1      70369 SYS             2              2 C_OBJ#
         1       9171 SYS             2              2 C_OBJ#
         1      70369 SYS             4              2 TAB$
         1       9171 SYS             4              2 TAB$
         1      70369 SYS            80              2 LOB$
         1       9171 SYS            80              2 LOB$
         1      70369 SYS            83              2 COLTYPE$
         1       9171 SYS            83              2 COLTYPE$
         1      70369 SYS            86              2 SUBCOLTYPE$
         1       9171 SYS            86              2 SUBCOLTYPE$
         1      70369 SYS            88              2 NTAB$
         1       9171 SYS            88              2 NTAB$
         1      70369 SYS            92              2 REFCON$
         1       9171 SYS            92              2 REFCON$
         1      70369 SYS            95              2 OPQTYPE$
         1       9171 SYS            95              2 OPQTYPE$
         1      70369 SYS           114              2 ICOLDEP$
         1       9171 SYS           114              2 ICOLDEP$
         1      70369 SYS           173              2 VIEWTRCOL$
         1       9171 SYS           173              2 VIEWTRCOL$
         1      17847 SYS           232            232 I_ARGUMENT1
         1      70369 SYS           251              2 LIBRARY$
         1       9171 SYS           251              2 LIBRARY$
         1      70369 SYS           252              2 ASSEMBLY$
         1       9171 SYS           252              2 ASSEMBLY$
         2     100036 SYS           267            267 SMON_SCN_TO_TIME_AUX
         2      95465 SYS           267            267 SMON_SCN_TO_TIME_AUX
         2     100036 SYS           269            267 SMON_SCN_TIME
         2      95465 SYS           269            267 SMON_SCN_TIME
         1      92292 SYS           420            420 C_OBJ#_INTCOL#
         1      79045 SYS           420            420 C_OBJ#_INTCOL#
         1      87721 SYS           420            420 C_OBJ#_INTCOL#
         1      92292 SYS           422            420 HISTGRM$
         1      79045 SYS           422            420 HISTGRM$
         1      87721 SYS           422            420 HISTGRM$
         1      70369 SYS           497              2 ATTRCOL$
         1       9171 SYS           497              2 ATTRCOL$
         1      70369 SYS           502              2 TYPE_MISC$
         1       9171 SYS           502              2 TYPE_MISC$
         2      91127 SYS          5870           5870 SCHEDULER$_EVENT_LOG
         4       1804 SCOTT      298142         298142 T

50 rows selected.

--可以确定这些对象对应的块受到这个CBC latch的保护.

3.获得SYS.SCHEDULER$_EVENT_LOG第0行rowid.

SCOTT@test> select dbms_rowid.rowid_create(1,5870,2,91127,0) from dual ;
DBMS_ROWID.ROWID_C
------------------
AAABbuAACAAAWP3AAA

SCOTT@test> select LOG_DATE  from sys.SCHEDULER$_EVENT_LOG where rowid='AAABbuAACAAAWP3AAA';
LOG_DATE
---------------------------------------------------------------------------
2012-05-12 22:09:39.833944

$ cat f1.sql
declare
m_id number;
m_data varchar2(200);
begin
        for i in 1 .. 1e9 loop
                select data into m_data from t where rowid='AABIyeAAEAAAAcMABL';
end loop;
end ;
/

$ cat g1.sql
declare
m_id number;
m_data varchar2(200);
begin
        for i in 1 .. 1e9 loop
                select to_char(LOG_DATE,'yyyymmdd') into m_data from sys.SCHEDULER$_EVENT_LOG where rowid='AAABbuAACAAAWP3AAA';
end loop;
end ;
/

00000000BCAC7C58

sqlplus scott/btbtms @f1.sql &
sqlplus scott/btbtms @g1.sql &

--也许我的机器太快了,2个会话根本看不到latch: cache buffers chains.

$ cat f.sh
#! /bin/bash
sqlplus -s scott/btbtms @ $1 &
sqlplus -s scott/btbtms @ $1 &
sqlplus -s scott/btbtms @ $1 &
sqlplus -s scott/btbtms @ $1 &

source f.sh f1.sql
source f.sh g1.sql

--启动4+4个会话,如果出现5个以上latch: cache buffers chains等待事件基本可以确定,P1RAW的信息都是00000000BCAC7C58.

SCOTT@test> select p1raw,p2raw,sid,serial#,seq#,event,state,wait_time_micro,seconds_in_wait from v$session where wait_class'Idle' order by event ;
P1RAW            P2RAW                   SID    SERIAL#       SEQ# EVENT                                    STATE               WAIT_TIME_MICRO SECONDS_IN_WAIT
---------------- ---------------- ---------- ---------- ---------- ---------------------------------------- ------------------- --------------- ---------------
0000000062657100 0000000000000001         18        123       2449 SQL*Net message to client                WAITED SHORT TIME                 3               0
0000000080018467 0000000000000002         17        397         76 cursor: pin S                            WAITED KNOWN TIME            100077               0
00000000C12F4A4F 000000CF00000004         20        195        317 cursor: pin S                            WAITED KNOWN TIME             99767               0
00000000BCAC7C58 000000000000009B        207        407        331 latch: cache buffers chains              WAITED KNOWN TIME             69568               0
00000000BCAC7C58 000000000000009B        400         43        293 latch: cache buffers chains              WAITED SHORT TIME               785               0
00000000BCAC7C58 000000000000009B        204        341         74 latch: cache buffers chains              WAITED KNOWN TIME             69055               0
00000000BCAC7C58 000000000000009B        596         23        344 latch: cache buffers chains              WAITED SHORT TIME              4253               0
00000000BCAC7C58 000000000000009B         13         59         88 latch: cache buffers chains              WAITED SHORT TIME               811               0
00000000BCAC7C58 000000000000009B        202         41         66 latch: cache buffers chains              WAITED KNOWN TIME             69574               0
9 rows selected.