什么是buffer busy wait?
A session that reads or modifies a buffer in the SGA must first acquire the cache buffers chains latch and traverse the buffer chain until it finds the necessary buffer header. Then it must acquire a buffer lock or a pin on the buffer header in shared or exclusive mode, depending on the operation it intends to perform. Once the buffer header is pinned, the session releases the cache buffers chains latch and performs the intended operation on the buffer itself. If a pin cannot be obtained, the session waits on the buffer busy wait event. This wait event does not apply to read or write operations that are performed in sessions’ private PGAs.
一个会话读或者修改一个SGA中的buffer，首先必须获得cache buffers chains latch和遍历缓冲区链表，直到找到所需的缓冲区头。
会话必须获得一个buffer lock或者pin在 shared or exclusive 模式下,这取决于它打算执行的操作。一旦缓冲区头被pin住，会话释放的超高速缓冲存储器锁存器链，并且执行在缓冲本身预定的操作。如果不能获得pin后，会话的缓冲区忙等待事件等待。这个等待事件不适用于读取或写入都在会话的私有PGA的执行的操作。
读写时兼容的，写写是不兼容的:
此实验是不断的去更新同一个块中的不同记录,通过这种方式可以造成buffer busy wait等待.
--创建测试表
create table test_db
(
ID NUMBER,
NICK VARCHAR2(30)
);
SQL> insert into test_db values(1,'zhaolin');
1 row created.
SQL> commit;
Commit complete.
SQL> insert into test_db values(2,'nature');
1 row created.
SQL> commit;
Commit complete.
SQL> select rowid,t.* from test_db t;
ROWID ID NICK
------------------ ---------- ------------------------------
AAATKfAAEAAAtZHAAA 1 zhaolin
AAATKfAAEAAAtZHAAB 2 nature
--检查这两条记录是否在同一个块上
SQL> select dbms_rowid.ROWID_RELATIVE_FNO(rowid) file#,dbms_rowid.ROWID_BLOCK_NUMBER(rowid) block# from test_db;
FILE# BLOCK#
---------- ----------
4 185927
4 185927
--session 1
SQL> select sid from v$mystat where rownum<2;
SID
----------
33
declare
i number:=0;
begin
loop
update test_db set nick='session1' where id=1;
i:=i+1;
if mod(i,100)=0 then
commit;
end if;
end loop;
end;
--session 2
SQL> select sid from v$mystat where rownum<2;
SID
----------
1
declare
i number:=0;
begin
loop
update test_db set nick='session2' where id=2;
i:=i+1;
if mod(i,100)=0 then
commit;
end if;
end loop;
end;
--我们再启动会话session3
SQL> col name format a30
SQL> select SESSION_ID,NAME,P1,P2,P3,WAIT_TIME,CURRENT_OBJ#,CURRENT_FILE#,CURRENT_BLOCK#
from v$active_session_history ash, v$event_name enm
where ash.event#=enm.event#
and ash.session_id in (1,33) 2 3 4
5 ;
SESSION_ID NAME P1 P2 P3 WAIT_TIME CURRENT_OBJ# CURRENT_FILE# CURRENT_BLOCK#
---------- ------------------------------ ---------- ---------- ---------- ---------- ------------ ------------- --------------
33 latch: In memory undo latch 836651180 243 0 0 78495 4 185927
33 buffer busy waits 4 185927 1 0 78495 4 185927
1 latch: cache buffers chains 884779988 150 0 0 -1 0 0
1 buffer busy waits 4 185927 1 0 78495 4 185927
33 latch: cache buffers chains 884779988 150 0 0 -1 0 0
1 buffer busy waits 4 185927 1 0 78495 4 185927
1 latch: cache buffers chains 884150708 150 0 0 -1 0 0
1 latch: cache buffers chains 884779988 150 0 0 -1 0 0
1 latch: cache buffers chains 884649516 150 0 0 -1 0 0
1 buffer busy waits 4 185927 1 0 78495 4 185927
1 buffer busy waits 4 185927 1 0 78495 4 185927
SESSION_ID NAME P1 P2 P3 WAIT_TIME CURRENT_OBJ# CURRENT_FILE# CURRENT_BLOCK#
---------- ------------------------------ ---------- ---------- ---------- ---------- ------------ ------------- --------------
33 latch: cache buffers chains 884779988 150 0 0 -1 0 0
1 latch: cache buffers chains 884779988 150 0 0 -1 0 0
33 latch: In memory undo latch 836651180 243 0 0 78495 4 185927
1 latch: cache buffers chains 884779988 150 0 0 -1 0 0
1 buffer busy waits 4 185927 1 0 78495 4 185927
33 buffer busy waits 4 185927 1 0 78495 4 185927
33 buffer busy waits 4 185927 1 0 78495 4 185927
1 latch: In memory undo latch 836651280 243 0 0 78495 4 185927
1 latch: cache buffers chains 884779988 150 0 0 -1 0 0
1 buffer busy waits 4 185927 1 0 78495 4 185927
33 latch: cache buffers chains 884709944 150 0 0 -1 0 0
SESSION_ID NAME P1 P2 P3 WAIT_TIME CURRENT_OBJ# CURRENT_FILE# CURRENT_BLOCK#
---------- ------------------------------ ---------- ---------- ---------- ---------- ------------ ------------- --------------
1 latch: cache buffers chains 884779988 150 0 0 -1 0 0
33 latch: In memory undo latch 836651180 243 0 0 78495 4 185927
33 latch: cache buffers chains 884862440 150 0 0 -1 0 0
1 latch: cache buffers chains 884779988 150 0 0 -1 0 0
1 latch: cache buffers chains 884779988 150 0 0 -1 0 0
1 buffer busy waits 4 185927 1 0 78495 4 185927
1 buffer busy waits 4 185927 1 0 78495 4 185927
33 latch: cache buffers chains 884779988 150 0 0 -1 0 0
33 db file sequential read 1 7802 1 0 40 1 7802
33 db file sequential read 1 57936 1 0 62 1 57936
1 db file sequential read 2 5549 1 0 -1 0 0
查看p1 p2参数
select owner,segment_name,segment_type from dba_extents
where file_id = 4 and 185927between block_id and block_id+blocks-1; SQL> SQL> 2
OWNER SEGMENT_NAME SEGMENT_TYPE
------------------------------ --------------------------------------------------------------------------------- ------------------
SCOTT TEST_DB TABLE