Oracle的X$表系列介绍之-X$KSLLCLASS

Oracle的X$表系列介绍之-X$KSLLCLASS

作者:eygle

出处:http://blog.eygle.com

日期:January 16, 2005

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X$KSLLCLASS--[K]ernel [S]ervice [L]ock [L]atches [CLASS]

从Oracle9iR2开始，Oracle允许对Latch进行分类，不同的分类可以用于不同的_SPIN_COUNT值。
这就避免了之前版本，一旦修改_SPIN_COUNT参数就会影响所有Latch的问题。从而可以在一定程度上控制该参数对于CPU的过量耗用。

以下是x$ksllclass的查询输出:

SQL> select indx, spin, yield, waittime from   x$ksllclass;
      INDX       SPIN      YIELD   WAITTIME
---------- ---------- ---------- ----------
         0      20000          0          1
         1      20000          0          1
         2      20000          0          1
         3      20000          0          1
         4      20000          0          1
         5      20000          0          1
         6      20000          0          1
         7      20000          0          1
8 rows selected.

比如我们的数据库系统经历较为严重的cache buffers chains竞争，为了降低其SLEEEP次数，我们可以对该Latch进行针对性分类邦定，单独修改其_SPIN_COUNT值。

SQL> select latch#,name from v$latchname where name='cache buffers chains';
    LATCH# NAME
---------- ----------------------------------------------------------------
        98 cache buffers chains

我们可以如下修改初始化参数，而后重新启动数据库:

_latch_class_1 = "10000"
_latch_classes = "98:1"

SQL> select latch#,name from v$latchname where name='cache buffers chains';
    LATCH# NAME
---------- ----------------------------------------------------------------
        98 cache buffers chains
SQL> alter system set "_latch_class_1"=10000 scope=spfile;
System altered.
SQL> alter system set "_latch_classes"="98:1" scope=spfile;
System altered.
SQL> startup force;
ORACLE instance started.
Total System Global Area   80811208 bytes
Fixed Size                   451784 bytes
Variable Size              37748736 bytes
Database Buffers           41943040 bytes
Redo Buffers                 667648 bytes
Database mounted.
Database opened.
SQL> show parameter latch
NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
_latch_class_1                       string      10000
_latch_classes                       string      98:1
SQL> select indx, spin, yield, waittime from   x$ksllclass;
      INDX       SPIN      YIELD   WAITTIME
---------- ---------- ---------- ----------
         0      20000          0          1
         1      10000          0          1
         2      20000          0          1
         3      20000          0          1
         4      20000          0          1
         5      20000          0          1
         6      20000          0          1
         7      20000          0          1
8 rows selected.
SQL> 

由此，单独控制一些重要Latch成为可能。

SQL> select a.kslldnam, b.kslltnum, b.class_ksllt 
  2  from   x$kslld a, x$ksllt b 
  3  where  a.kslldadr = b.addr 
  4  and    b.class_ksllt > 0;
KSLLDNAM                                          KSLLTNUM CLASS_KSLLT
---------------------------------------------- ---------- -----------
process allocation                                     3           2
cache buffers chains                                  98           1

更多请参考:
Richmond Shee, Kirtikumar Deshpande and K Gopalakrishnan的《Oracle Wait Interface》

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