[ORACLE]Cache Buffers LRU Chain 闩锁竞争与解决 latch: cache buffers lru chain

　　Oracle 的 Buffer Cache 是共享内存，可以为众多并发进程并发访问，所以在搜索的过程中必须获取 Latch（Latch是 Oracle 的一种串行锁机制，用于保护共享内存结构），锁定内存结构，防止并发访问损坏内存中的数据（我们必须认识到对于数据的访问、Buffer 的存取就意味着多次的 Latch 访问，而过于严重的 Latch 竞争常常是系统的瓶颈所在

latch: cache buffers lru chain事件

SQL> select * from v$event_name where NAME='latch: cache buffers lru chain';

EVENT# EVENT_ID   NAME                            PARAMETER1  PARAMETER2 PARAMETER3 WAIT_CLASS_ID WAIT_CLASS# WAIT_CLASS DISPLAY_NAME                   CON_ID
------ ---------- ------------------------------- ----------  --------- ----------- ------------- ----------- ---------- ----------------------------   ----
781   3401628503  latch: cache buffers lru chain  address     number     why        1893977003     0          Other      latch: cache buffers lru chain  0 

SQL>  col name for a25
SQL> select addr,latch#,name,gets,misses,immediate_gets,immediate_misses from v$latch where name = 'cache buffers lru chain';

ADDR             LATCH# NAME                GETS     MISSES IMMEDIATE_GETS IMMEDIATE_MISSES
---------------- ---------- ------------------------- ---------- ---------- -------------- ----------------
0000000060167918    322 cache buffers lru chain       29612     12         80511        153

Cache Buffers Lru Chain Latch 存在多个子 Latch，其数量受隐含参数_db_block_lru_latches控制

SQL> SELECT x.ksppinm NAME, y.ksppstvl VALUE, x.ksppdesc describ FROM x$ksppi x,x$ksppcv y  WHERE x.inst_id =USERENV ('Instance')  AND y.inst_id =USERENV ('Instance')  AND x.indx = y.indx  AND x.ksppinm LIKE '%lru_latches%';

NAME                         VALUE                DESCRIB
------------------------    ------------------    ------------------------------
_db_block_lru_latches        36                   number of lru latches

从 v$latch_children 视图察看当前各子 Latch 使用情况：

SQL> select addr,child#,name,gets,misses,immediate_gets igets,immediate_misses imisses from v$latch_children where name = 'cache buffers lru chain';

ADDR             CHILD# NAME                GETS     MISSES     IGETS      IMISSES
---------------- ---------- ------------------------- ---------- ---------- ---------- ----------
00000000A870A718     72 cache buffers lru chain           0      0         0        0
00000000A870A640     71 cache buffers lru chain          18      0         1        0
00000000A8709E60     70 cache buffers lru chain           0      0         0        0
00000000A8709D88     69 cache buffers lru chain          18      0         1        0
00000000A87095A8     68 cache buffers lru chain           0      0         0        0
00000000A87094D0     67 cache buffers lru chain          18      0         1        0
00000000A8708CF0     66 cache buffers lru chain           0      0         0        0
00000000A8708C18     65 cache buffers lru chain          18      0         1        0
00000000A8708208     64 cache buffers lru chain           0      0         0        0
00000000A8708130     63 cache buffers lru chain          18      0         1        0
00000000A8707950     62 cache buffers lru chain           0      0         0        0
00000000A8707878     61 cache buffers lru chain          18      0         1        0
00000000A8707098     60 cache buffers lru chain           0      0         0        0
00000000A8706FC0     59 cache buffers lru chain          18      0         1        0
00000000A87067E0     58 cache buffers lru chain           0      0         0        0
00000000A8706708     57 cache buffers lru chain          18      0         1        0
00000000A8705CF8     56 cache buffers lru chain           0      0         0        0
00000000A8705C20     55 cache buffers lru chain          18      0         1        0
00000000A8705440     54 cache buffers lru chain           0      0         0        0
00000000A8705368     53 cache buffers lru chain          18      0         1        0
00000000A8704B88     52 cache buffers lru chain           0      0         0        0
00000000A8704AB0     51 cache buffers lru chain          18      0         1        0
00000000A87042D0     50 cache buffers lru chain           0      0         0        0
00000000A87041F8     49 cache buffers lru chain          18      0         1        0
00000000A87037E8     48 cache buffers lru chain           0      0         0        0
00000000A8703710     47 cache buffers lru chain          18      0         1        0
00000000A8702F30     46 cache buffers lru chain           0      0         0        0
00000000A8702E58     45 cache buffers lru chain          18      0         1        0
00000000A8702678     44 cache buffers lru chain           0      0         0        0
00000000A87025A0     43 cache buffers lru chain          18      0         1        0
00000000A8701DC0     42 cache buffers lru chain           0      0         0        0
00000000A8701CE8     41 cache buffers lru chain          18      0         1        0
00000000A87012D8     40 cache buffers lru chain           0      0         0        0
00000000A8701200     39 cache buffers lru chain          18      0         1        0
00000000A8700A20     38 cache buffers lru chain           0      0         0        0
00000000A8700948     37 cache buffers lru chain          18      0         1        0
00000000A8700168     36 cache buffers lru chain           0      0         0        0
00000000A8700090     35 cache buffers lru chain          18      0         1        0
00000000A86FF8B0     34 cache buffers lru chain           0      0         0        0
00000000A86FF7D8     33 cache buffers lru chain          18      0         1        0
00000000A86FEDC8     32 cache buffers lru chain           0      0         0        0
00000000A86FECF0     31 cache buffers lru chain          18      0         1        0
00000000A86FE510     30 cache buffers lru chain           0      0         0        0
00000000A86FE438     29 cache buffers lru chain          18      0         1        0
00000000A86FDC58     28 cache buffers lru chain           0      0         0        0
00000000A86FDB80     27 cache buffers lru chain          18      0         1        0
00000000A86FD3A0     26 cache buffers lru chain           0      0         0        0
00000000A86FD2C8     25 cache buffers lru chain          18      0         1        0
00000000A85C8A98     24 cache buffers lru chain           0      0         0        0
00000000A85C89C0     23 cache buffers lru chain        7410      5     20378           36
00000000A85C81E0     22 cache buffers lru chain           0      0         0        0
00000000A85C8108     21 cache buffers lru chain        7263      2     20272           39
00000000A85C7928     20 cache buffers lru chain           0      0         0        0
00000000A85C7850     19 cache buffers lru chain        7239      2     20229           36
00000000A85C7070     18 cache buffers lru chain           0      0         0        0
00000000A85C6F98     17 cache buffers lru chain        7097      3     20069           42
00000000A85C6588     16 cache buffers lru chain           0      0         0        0
00000000A85C64B0     15 cache buffers lru chain          18      0         1        0
00000000A85C5CD0     14 cache buffers lru chain           0      0         0        0
00000000A85C5BF8     13 cache buffers lru chain          18      0         1        0
00000000A85C5418     12 cache buffers lru chain           0      0         0        0
00000000A85C5340     11 cache buffers lru chain          18      0         1        0
00000000A85C4B60     10 cache buffers lru chain           0      0         0        0
00000000A85C4A88      9 cache buffers lru chain          18      0         1        0
00000000A85C4078      8 cache buffers lru chain           0      0         0        0
00000000A85C3FA0      7 cache buffers lru chain          18      0         1        0
00000000A85C37C0      6 cache buffers lru chain           0      0         0        0
00000000A85C36E8      5 cache buffers lru chain          18      0         1        0
00000000A85C2F08      4 cache buffers lru chain           0      0         0        0
00000000A85C2E30      3 cache buffers lru chain          18      0         1        0
00000000A85C2650      2 cache buffers lru chain           0      0         0        0
00000000A85C2578      1 cache buffers lru chain          18      0         1        0

72 rows selected.

如果该 Latch 竞争激烈，通常有如下方法可以采用：

（1） 适当增大 Buffer Cache，这样可以减少读数据到 Buffer Cache 的机会，减少扫Lru List 的竞争。

（2） 可以适当增加 LRU Latch 的数量，修改_db_block_lru_latches 参数可以实现，但是该参数通常来说是足够的，除非在 Oracle Support 的建议下或确知该参数将带来的影响，否则不推荐修改。

（3） 通过多缓冲池技术，可以减少不希望的数据老化和全表扫等操作对于 Default池的冲击，从而可以减少竞争。

（4） 优化 SQL，减少数据读取，从而减少对于 LRU List 的扫᧿。