通过案例学调优之--Oracle Latch基础知识

通过案例学调优之--Oracle Latch基础知识

一、什么是 Latch
1.1、 Latch 是一种低级排队(串行)机制,用于保护 SGA 中共享内存结构。

1.2、 Latch 是一种快速的被获取和释放的内存锁,用于防止共享内存结构被多个用户同时 访问。

1.3、 Latch 请求的类型:  willing-to-wait 和 immediate。

1.3.1 对于 willing-to-wait 类型的 latch:

如果一个进程在第一次尝试中没有获得 latch,那么它会等待并且再尝试一次,如果经过_spin_count 次争夺不能获得 latch, 然后该进程转入睡眠状态,睡眠醒来后,按顺序重复以 前的步骤。睡眠的时间会越来越长。

1.3.2 对于immediate类型的latch:

如果该闩不能立即得到的话,那么该进程就不会为获得该 latch 而等待。它将继续执行 另一个操作。

1.3.3 可以通过 v$latch 视图查看 latch 的具体信息,在 v$latch 中:
(以下字段反映了 Willing-to-wait 类型请求)
GETS:成功地以 Willing-to-wait 请求类型请求一个 latch 的次数。
MISSES:初始以 Willing-to-wait 请求类型请求一个 latch 不成功的次数。

SLEEPS:初始以 Willing-to-wait 请求类型请求一个 latch 不成功后,进程等待获取 latch的次数。

(以下字段反映了 Immediate 类型请求)
IMMEDIATE_GETS:以 Immediate 请求类型成功地获得一个 latch 的次数。

IMMEDIATE_MISSES:以 Immediate 请求类型请求一个 latch 不成功的次数。

12:27:09 SYS@ prod >desc v$latch
 Name                                                              Null?    Type
 ----------------------------------------------------------------- -------- --------------------------------------------
 ADDR                                                                       RAW(4)
 LATCH#                                                                     NUMBER
 LEVEL#                                                                     NUMBER
 NAME                                                                       VARCHAR2(64)
 HASH                                                                       NUMBER
 GETS                                                                       NUMBER
 MISSES                                                                     NUMBER
 SLEEPS                                                                     NUMBER
 IMMEDIATE_GETS                                                             NUMBER
 IMMEDIATE_MISSES                                                           NUMBER
 WAITERS_WOKEN                                                              NUMBER
 WAITS_HOLDING_LATCH                                                        NUMBER
 SPIN_GETS                                                                  NUMBER
 SLEEP1                                                                     NUMBER
 SLEEP2                                                                     NUMBER
 SLEEP3                                                                     NUMBER
 SLEEP4                                                                     NUMBER
 SLEEP5                                                                     NUMBER
 SLEEP6                                                                     NUMBER
 SLEEP7                                                                     NUMBER
 SLEEP8                                                                     NUMBER
 SLEEP9                                                                     NUMBER
 SLEEP10                                                                    NUMBER
 SLEEP11                                                                    NUMBER
 WAIT_TIME                                                                  NUMBER

1.4、 与队列锁(enqueue)不同的是:

     对于 Latch 的请求,不用排队等待资源,由于 Latch的分配相当随机,所以当 Latch 释放后,不管哪个会话请求都可能得到它,当不能获得 Latch的会话,就会在后续不断的重试(spin->sleep->spin...->get latch)。
1.4.1 为什么自旋(spin)?

在一台多 CPU的机器上,如果 Latch 不能立即获得,那么会话就会自旋,即在循环中反复地 尝试来或得 Latch。如果不这样的话,那么就会导致上下文切换(context switching),这样对 资源开销更大。

1.4.2 得到Latch的伪代码如下:

If Latch gotten
Then
Return SUCCESS
Else
Misses on that Latch = Misses + 1
Loop
For I in 1 .. 2000(_spin_count)
Loop
Attempt to get Latch
If Latch gottenThen
Return SUCCESS
End if
End loopGo to sleep for short period
Sleep on Latch = Sleeps + 1
End loop
End if

图示如下:

所以,可以知道,当尝试获得 Latch 的时候,可能会消耗大量的 CPU 时间。系统看上去很忙, 但并没有做多少实际工作。

1.5、相关视图

1.5.1 v$latch :
V$LATCH shows aggregate latch statistics for both parent and child latches, grouped by latch name.
13:20:55 SYS@ prod >desc v$latch
 Name                                                              Null?    Type
 ----------------------------------------------------------------- -------- --------------------------------------------
 ADDR                                                                       RAW(4)
 LATCH#                                                                     NUMBER
 LEVEL#                                                                     NUMBER
 NAME                                                                       VARCHAR2(64)
 HASH                                                                       NUMBER
 GETS                                                                       NUMBER
 MISSES                                                                     NUMBER
 SLEEPS                                                                     NUMBER
 IMMEDIATE_GETS                                                             NUMBER
 IMMEDIATE_MISSES                                                           NUMBER
 WAITERS_WOKEN                                                              NUMBER
 WAITS_HOLDING_LATCH                                                        NUMBER
 SPIN_GETS                                                                  NUMBER
 SLEEP1                                                                     NUMBER
 SLEEP2                                                                     NUMBER
 SLEEP3                                                                     NUMBER
 SLEEP4                                                                     NUMBER
 SLEEP5                                                                     NUMBER
 SLEEP6                                                                     NUMBER
 SLEEP7                                                                     NUMBER
 SLEEP8                                                                     NUMBER
 SLEEP9                                                                     NUMBER
 SLEEP10                                                                    NUMBER
 SLEEP11                                                                    NUMBER
 WAIT_TIME

1.5.2 v$latch_children:V$LATCH_CHILDREN contains statistics about child latches.
13:20:15 SYS@ prod >desc v$latch_children
 Name                                                              Null?    Type
 ----------------------------------------------------------------- -------- --------------------------------------------
 ADDR                                                                       RAW(4)
 LATCH#                                                                     NUMBER
 CHILD#                                                                     NUMBER
 LEVEL#                                                                     NUMBER
 NAME                                                                       VARCHAR2(64)
 HASH                                                                       NUMBER
 GETS                                                                       NUMBER
 MISSES                                                                     NUMBER
 SLEEPS                                                                     NUMBER
 IMMEDIATE_GETS                                                             NUMBER
 IMMEDIATE_MISSES                                                           NUMBER
 WAITERS_WOKEN                                                              NUMBER
 WAITS_HOLDING_LATCH                                                        NUMBER
 SPIN_GETS                                                                  NUMBER
 SLEEP1                                                                     NUMBER
 SLEEP2                                                                     NUMBER
 SLEEP3                                                                     NUMBER
 SLEEP4                                                                     NUMBER
 SLEEP5                                                                     NUMBER
 SLEEP6                                                                     NUMBER
 SLEEP7                                                                     NUMBER
 SLEEP8                                                                     NUMBER
 SLEEP9                                                                     NUMBER
 SLEEP10                                                                    NUMBER
 SLEEP11                                                                    NUMBER
 WAIT_TIME                                                                  NUMBER
 
1.5.3 v$latch_misses :This view contains statistics about missed attempts to acquire a latch.
13:20:10 SYS@ prod >desc v$latch_misses
 Name                                                              Null?    Type
 ----------------------------------------------------------------- -------- --------------------------------------------
 PARENT_NAME                                                                VARCHAR2(64)
 WHERE                                                                      VARCHAR2(80)
 NWFAIL_COUNT                                                               NUMBER
 SLEEP_COUNT                                                                NUMBER
 WTR_SLP_COUNT                                                              NUMBER
 LONGHOLD_COUNT                                                             NUMBER
 LOCATION                                                                   VARCHAR2(80)
 
1.5.4 v$latchname :This view contains information about decoded latch names for the latches shown in
13:19:11 SYS@ prod >desc v$latchname
 Name                                                              Null?    Type
 ----------------------------------------------------------------- -------- --------------------------------------------
 LATCH#                                                                     NUMBER
 NAME                                                                       VARCHAR2(64)
 HASH                                                                       NUMBER

---原作者：[email protected]（感谢作者）