如何使用shared pool latch？

Posted 2021-09-04 diuxie

使用那个shared pool latch.txt

--//我的测试环境很小,仅仅1个shared pool latch.如果存在多个,一条sql语句应该会使用那个shared pool latch.
--//按照前面的学习,猜测应该于hash_value , bucket_size , _kghdsidx_count 相关,测试看看。
--//注：一般缺省bucket_size ＝2^(9+8) =131072

1.环境:
SCOTT@book> @ ver1
PORT_STRING VERSION BANNER

---

x86_64/Linux 2.4.xx 11.2.0.4.0 Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production

SCOTT@book> select * from dept where deptno=20;

DEPTNO DNAME          LOC

---

    20 RESEARCH       DALLAS

SCOTT@book> @ hash
HASH_VALUE SQL_ID CHILD_NUMBER HASH_HEX

---

95129850 80baj2c2ur47u 0 5ab90fa

SYS@book> @ hide _kgl_bucket_count
NAME DESCRIPTION DEFAULT_VALUE SESSION_VALUE SYSTEM_VALUE ISSES ISSYS_MOD

---

_kgl_bucket_count Library cache hash table bucket count (2^_kgl_bucket_count * 256) TRUE 9 9 FALSE FALSE

SYS@book> alter system set "_kghdsidx_count"=7 scope=spfile;
System altered.

--//重启数据库略

SELECT addr

    ,latch#
    ,child#
    ,level#
    ,name
    ,gets
    ,sleeps
    ,immediate_gets
    ,immediate_misses
    ,spin_gets
FROM V$LATCH_CHILDREN

WHERE name LIKE \'shared pool\'
ORDER BY addr;

ADDR LATCH# CHILD# LEVEL# NAME GETS SLEEPS IMMEDIATE_GETS IMMEDIATE_MISSES SPIN_GETS

---

000000006010D9A0 336 1 7 shared pool 10126 0 0 0 29
000000006010DA40 336 2 7 shared pool 6220 0 0 0 5
000000006010DAE0 336 3 7 shared pool 8610 0 0 0 7
000000006010DB80 336 4 7 shared pool 7817 0 0 0 21
000000006010DC20 336 5 7 shared pool 7446 0 0 0 18
000000006010DCC0 336 6 7 shared pool 7302 1 0 0 7
000000006010DD60 336 7 7 shared pool 6347 0 0 0 3
7 rows selected.
--//记下7个addr地址. 000000006010D9A0 000000006010DA40 000000006010DAE0 000000006010DB80 000000006010DC20 000000006010DCC0 000000006010DD60.

2.测试:
--//编辑gdb脚本:
$ cat shared_pool.gdb
set pagination off

break kslgetl if $rdi==0X6010D9A0
commands

silent
printf "child#=1 kslgetl %x, %d, %d, %d\\n", $rdi, $rsi, $rdx, $rcx
c

end

break kslgetl if $rdi==0X6010DA40
commands

silent
printf "child#=2 kslgetl %x, %d, %d, %d\\n", $rdi, $rsi, $rdx, $rcx
c

end

break kslgetl if $rdi==0X6010DAE0
commands

silent
printf "child#=3 kslgetl %x, %d, %d, %d\\n", $rdi, $rsi, $rdx, $rcx
c

end

break kslgetl if $rdi==0X6010DB80
commands

silent
printf "child#=4 kslgetl %x, %d, %d, %d\\n", $rdi, $rsi, $rdx, $rcx
c

end

break kslgetl if $rdi==0X6010DC20
commands

silent
printf "child#=5 kslgetl %x, %d, %d, %d\\n", $rdi, $rsi, $rdx, $rcx
c

end

break kslgetl if $rdi==0X6010DCC0
commands

silent
printf "child#=6 kslgetl %x, %d, %d, %d\\n", $rdi, $rsi, $rdx, $rcx
c

end

break kslgetl if $rdi==0X6010DD60
commands

silent
printf "child#=7 kslgetl %x, %d, %d, %d\\n", $rdi, $rsi, $rdx, $rcx
c

end

--//先执行Select * from dept where deptno=20;,desc dept多次,避免一些递归.
--// hash_value % bucket_size % _kghdsidx_count
--// 95129850 % 7 = 4
--// 95129850 % 131072 % 7 = 2

--//验证看看使用那个shared pool latch.
--//session 1:
SCOTT@book(1,7)> @ spid

   SID    SERIAL# PROCESS                  SERVER    SPID       PID  P_SERIAL# C50

---

     1          7 4691                     DEDICATED 4692        24          4 alter system kill session \'1,7\' immediate;

SCOTT@book> select * from dept where deptno=20;

DEPTNO DNAME          LOC

---

    20 RESEARCH       DALLAS

--//session 2:
$ gdb -p 4692 -x shared_pool.gdb

...
Breakpoint 1 at 0x93f97a8
(gdb) c
...
child#=3 kslgetl 6010dae0, 1, 0, 4039
child#=3 kslgetl 6010dae0, 1, 0, 3980
child#=3 kslgetl 6010dae0, 1, 0, 4039
child#=3 kslgetl 6010dae0, 1, 2132183136, 3991
--//6010dae0 对应的就是CHILD#＝3.
--//估计 hash_value % bucket_size % _kghdsidx_count + 1 . 或者 _kghdsidx_count-hash_value % _kghdsidx_count(不对!!)
--//可以大致推测使用的计算公式是 hash_value % bucket_size % _kghdsidx_count + 1
--//多找几条sql语句验证看看。

3.继续验证看看.
--//注意验证前最好执行select sysdate from dual;多次排除链接
--//因为11g下sqlplus执行后不会马上释放光标，导致看到的是前面的语句，这样可能不对。

--//session 3，主要目的确定hash_value:

SCOTT@book> select * from dept where deptno=10;

DEPTNO DNAME          LOC

---

    10 ACCOUNTING     NEW YORK

SCOTT@book> @ hash
HASH_VALUE SQL_ID CHILD_NUMBER HASH_HEX

---

911274289 4xamnunv51w9j 1 3650f131
--// 911274289 % 131072 % 7 +1 = 6

SCOTT@book> select * from dept where deptno=30;

DEPTNO DNAME          LOC

---

    30 SALES          CHICAGO

SCOTT@book>
HASH_VALUE SQL_ID CHILD_NUMBER HASH_HEX

---

69952862 816w0g822qtay 0 42b655e
--// 69952862 % 131072 % 7+1 = 4

SCOTT@book> select * from dept where deptno=40;

DEPTNO DNAME          LOC

---

    40 OPERATIONS     BOSTON

HASH_VALUE SQL_ID CHILD_NUMBER HASH_HEX

---

3912471479 14ymr4znm74xr 0 e93393b7
--// 3912471479 % 131072 % 7 +1 = 4

SCOTT select * from dept where deptno=50;
no rows selected

HASH_VALUE SQL_ID CHILD_NUMBER HASH_HEX

---

1927948053 bswp9zttfn9sp 0 72ea2715
--//1927948053 % 131072 % 7 +1 = 3

--//session 1:
--//再次提醒执行select sysdate from dual;多次。
select sysdate from dual;
select sysdate from dual;
select sysdate from dual;
select sysdate from dual;
select sysdate from dual;
select * from dept where deptno=10;
select sysdate from dual;
select sysdate from dual;
select * from dept where deptno=30;
select sysdate from dual;
select sysdate from dual;
select * from dept where deptno=40;
select sysdate from dual;
select sysdate from dual;
select * from dept where deptno=50;

--//session 2:
$ gdb -p 4692 -x shared_pool.gdb
...
child#=6 kslgetl 6010dcc0, 1, 0, 3980 => select * from dept where deptno=10; => child#=6
...
child#=4 kslgetl 6010db80, 1, 0, 3980 => select * from dept where deptno=30; => child#=4
...
child#=4 kslgetl 6010db80, 1, 0, 3980 => select * from dept where deptno=40; => child#=4
...
child#=3 kslgetl 6010dae0, 1, 0, 4039 => select * from dept where deptno=50; => child#=3

--//OK.都能对上。

总结:
--//说明一条sql语句会使用 其sql语句的 hash_value % bucket_size % _kghdsidx_count +1 的 shared pool latch.