Buffer cache学习（四）--实验Cache Buffers Chains Latch

测试：模拟Cache Buffers Chains Latch竞用 
1：创建表test，总共一条记录，共1个Block。
SCOTT@ prod>select * from test;
        ID
----------
         1


SCOTT@ prod>select dbms_rowid.rowid_relative_fno(rowid) file#,
  2  dbms_rowid.rowid_block_number(rowid) block# from test;
     FILE#     BLOCK#
---------- ----------
         4        523
2：创建存储过程，模拟对块不断查询
create or replace procedure latch is
 i number;
begin 
 loop
   select id into i from test;
 end loop;
end;
/
3:session 47上执行
SCOTT@ prod>select userenv('sid') from dual;
USERENV('SID')
--------------
            47
查看会话event
SYS@ prod>select sid,event,p1,p1raw,p1text,p2,p2text from v$session where sid in (47,33);
       SID EVENT             P1 P1RAW            P1TEXT             P2 P2TEXT
---------- ---------------------------------------- ---------- ---------------- ---------- ---------- ----------
33 SQL*Net message from client  1650815232 0000000062657100 driver id        1 #bytes
47 SQL*Net message from client  1650815232 0000000062657100 driver id        1 #bytes
在执行期间可见没有miss
SYS@ prod>select gets,misses,sleeps,spin_gets,wait_time from v$latch where name ='cache buffers chains';


      GETS     MISSES     SLEEPS  SPIN_GETS  WAIT_TIME
---------- ---------- ---------- ---------- ----------
 156829087          0          0          0          0
4：在session 33上在执行存储过程latch
再次查看
SYS@ prod>select sid,event,p1,p1raw,p1text,p2,p2text from v$session where sid in (47,33);
       SID EVENT             P1 P1RAW            P1TEXT             P2 P2TEXT
---------- ---------------------------------------- ---------- ---------------- ---------- ---------- ----------
33 latch: cache buffers chains    1042883468 000000003E29238C address       150 number
47 latch: cache buffers chains   1043032304 000000003E2B68F0 address        150 number
SYS@ prod>/


       SID EVENT              P1 P1RAW            P1TEXT             P2 P2TEXT
---------- ---------------------------------------- ---------- ---------------- ---------- ---------- ----------
33 cursor: pin S                3777061920 00000000E1216420 idn        3080192 value
47 latch: cache buffers chains   1042883468 000000003E29238C address     150 number
查看
SYS@ prod>select gets,misses,sleeps,spin_gets,wait_time from v$latch where name ='cache buffers chains';
GETS     MISSES     SLEEPS  SPIN_GETS  WAIT_TIME
---------- ---------- ---------- ---------- ----------
 241954233        157         70         89    7021403


可以看到出现latch: cache buffers chains等待事件。这就是2个session要同时访问同一个Block，这个时候在一个会话持有Latch的时候，另一个会话必须Spin等待获得Latch。
也会看到latch: library cache等待事件。这是由于在共享池进行软解析的时候需要获得library cache latch来扫描library cache中相应Bucket的Chain来获得执行计划来执行SQL。因为并发性高，导致library cache latch的争用。