Oracle等待事件详细分析

SweetHarbour 2012-09-16

从今天起,木木同学要认真整理一下Oracle中常见的等待事件,通过这部分的学习,希望自己能对oracle内部的结构能有一个更清晰的认识,有兴趣的童鞋一起来哇。

1、latch:cache buffers chains从oracle 9i开始,以只读为目的的查询chains时,可以将cache buffers chains锁存器以shared模式共享,因此有助于减少争用。(我们需要注意,若能共享cache buffer chains 锁存器,理论上理论上不应该发生同时执行select 操作引起cbc锁存器的争用,但实际的测试结果表明,同时执行select依然会发生cbc锁存器争用,其理由是与buffer lock相关:为了读取工作,以shared模式已经获得锁存器,但是读取实际缓冲区过程中,还要以shared 模式获取buffer lock,在此过程真呢过需要部分修改缓冲区头信息。因此在获取buffer lock过程中,需要将cbc锁存器修改为exclusive 模式,在释放buffer lock期间也需要exclusive模式获取cbc锁存器,在此过程中会发生争用。)

发生cache buffers chains 锁存器争用代表性的情况如下:低效的SQL    和     hot block(热块)
低效SQL引起的cbc争用先介绍视图:v$latch_children 数据库中有些类别的latches拥有多个。v$latch中提供了每个类别的总计信息。如果想查看单个latch,可以通过查询本视图:查询数据库中所有latch的名字和个数:SQL> select name,count(*) ct from v$latch_children group by name order by ct desc;

NAME                                                                             CT
----------------------------------------                               ----------
cache buffers chains                                               1024
SQL memory manager workarea list latch           67
channel operations parent latch                            65
global tx hash mapping                                           47
message pool operations parent latch               34
name-service namespace bucket                         32
simulator hash latch                                             32
row cache objects                                                  29
redo allocation                                                        20
In memory undo latch                                           18
checkpoint queue latch                                         16


NAME                                                              CT
----------------------------------------               ----------
msg queue                                                  15
JS queue access latch                             13
commit callback allocation                       11
transaction allocation                              11
buffer pool                                                 8
cursor bind value capture                         8
simulator lru latch                                      8
object queue header operation                8
object queue header heap                        8
cache buffers lru chain                               8
business card                                             8


NAME                                                      CT
----------------------------------------        ----------
shared pool                                             7
flashback copy                                       6
virtual circuit queues                            6
post/wait queue                                    5
slave class                                             5
JS slv state obj latch                            4
redo copy                                                4
session switching                                 4
parallel query alloc buffer                       4
job workq parent latch                            3
undo global data                                  3

    
NAME                                                     CT
---------------------------------------       -----------
library cache pin allocation                      3
library cache pin                                      3
library cache hash chains                   3
peplm                                                     3
library cache lock                                 3
library cache lock allocation                 3
library cache                                           3
Shared B-Tree                                     2
session idle bit                                     2
parallel query stats                              2
longop free list parent                           2


NAME                                                   CT
----------------------------------------       ----------
latch wait list                                        2
ksfv messages                                     2
enqueue hash chains                          2client/application info                           2
channel handle pool latch                    1
granule operation                                 1
logminer context allocation                   1
session queue latch                           1
sim partition latch                                 1
msg queue latch                                   1
done queue latch                                  1


已选择55行。
如此说来,oracle10g中有55个有名字的latch,拥有量最大的就是我们的cbc latch,正好1024个。
我通过构建测试环境,创建了表,加上索引。并且创建一个全表扫描的过程:SQL> create table  cbc_test(id number, name char(100));SQL> insert into cbc_test(id,name) select object_id, object_name from dba_objects;SQL> cretate index cbc_test_idx on cbc_test(id);
好了,下面进行不必要的广泛扫描索引:创建一个过程:create or replace procedure cbc_do_select is
  begin
    for x in(select /*+index(cbc_test cbc_test_idx)*/ *
    from cbc_test where id>=0) loop
     null;
    end loop;
  end;

反复执行此过程2000次:var job_no number;
  begin
   for idx in 1..2000 loop
     dbms_job.submit(:job_no,'cbc_do_select;');
     commit;
   end loop;
 end;

查看一下cbc 锁存器对应的CHILD#,GETS, SLEEPS判断子锁存器上使用的次数和争用是否集中:
select * from
  2   (select child#,name,gets,sleeps from v$latch_children
  3     where name='cache buffers chains'
  4     order by sleeps desc
  5    )where rownum<=20;


    CHILD# NAME                                 GETS     SLEEPS
---------- ------------------------------ ---------- ----------
       837 cache buffers chains                34466         28
        67 cache buffers chains                23994         15
       684 cache buffers chains                 6288         14
       238 cache buffers chains                 3823         12
       898 cache buffers chains                 4868         12
       908 cache buffers chains                32807         10
       288 cache buffers chains                 3956          8
       737 cache buffers chains                 3865          8
       412 cache buffers chains                 1671          8
       968 cache buffers chains                 2706          7
       420 cache buffers chains                 2998          6


    CHILD# NAME                                 GETS     SLEEPS
---------- ------------------------------ ---------- ----------
       460 cache buffers chains                 3912          6
         1 cache buffers chains                 2564          4
       839 cache buffers chains                 1119          4
       951 cache buffers chains                21741          4
        33 cache buffers chains                 3786          3
       251 cache buffers chains                 1997          2
       578 cache buffers chains                 3857          2
       733 cache buffers chains                 3554          2
       280 cache buffers chains                 4549          2

已选择20行。

看来837号cbc 子锁存器争用比较多,可以判定是比较热的块。
我们也可以通过v$latch_children视图,确定热块的cbc 锁存器的地址:SQL> select * from
  2   (select latch#,child#,addr,gets,sleeps from v$latch_children
  3     where name='cache buffers chains'
  4     order by sleeps desc
  5    )where rownum<=20;


    LATCH#     CHILD# ADDR           GETS     SLEEPS
---------- ---------- -------- ---------- ----------
       116        894 6CB7F7F0     554344         90
       116        837 6CFFF70C     727379         80
       116        951 6CB839D8     471117         74
       116         56 6C783224     107910         60
       116        341 6C797BAC     144056         47
       116        458 6C7A02F4      37434         38
       116        240 6C7906E4      99251         37
       116        297 6C7948CC      98903         36
       116        566 6C7A7FD4      94932         36
       116        790 6CFFC0B4     107997         36
       116        280 6C793524      87455         34


    LATCH#     CHILD# ADDR           GETS     SLEEPS
---------- ---------- -------- ---------- ----------
       116        919 6CB814D8      49667         31
       116        583 6C7A937C      85883         29
       116        113 6C78740C     120322         28
       116        627 6C7AC65C      91840         28
       116        534 6C7A5AD4     112523         27
       116        270 6C792994      50302         26
       116        418 6C79D4B4      37697         26
       116        680 6C7B03A4     104550         26
       116        676 6C7AFF04     103297         26


已选择20行。
通过上面红色的锁存器地址,结合x$bh 视图,查看tch的次数,确定热快:
SQL> select hladdr,obj,(select object_name from dba_objects
  2            where (data_object_id is null and object_id=x.obj)
  3            or data_object_id=x.obj
  4            and rownum=1 )as object_name,dbarfil,dbablk,tch
  5            from x$bh x
  6          where hladdr in('6CB7F7F0','6CFFF70C')
  7      order by hladdr,obj;


HLADDR          OBJ             OBJECT_NAME      DBARFIL     DBABLK        TCH
--------                ----------          -------------              ----------       ----------          ----------
6CB7F7F0         18                 OBJ$                          1             47810          0
6CB7F7F0        109              I_SYSAUTH1             1               827            15
6CB7F7F0      57855           CBC_TEST                1             58171           0
6CB7F7F0      57855           CBC_TEST               1              58655           0
6CFFF70C          2                ICOL$                         1             42272          17
6CFFF70C         18              OBJ$                           1              47811           0
6CFFF70C        109             I_SYSAUTH1             1             828               15
6CFFF70C      57855           CBC_TEST               1             58172           0
6CFFF70C      57855           CBC_TEST               1             58656           0

但是很不幸,我这里也没有出现热块的迹象,因为tch竟然都是0.我也不知咋回事。。

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