通过案例学调优之--Oracle Latch基础知识

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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 gottenThenReturn SUCCESSElseMisses on that Latch = Misses + 1LoopFor I in 1 .. 2000(_spin_count)LoopAttempt to get LatchIf Latch gottenThenReturn SUCCESSEnd ifEnd loopGo to sleep for short periodSleep on Latch = Sleeps + 1End loopEnd 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_TIME1.5.2 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$latchname :This view contains information about decoded latch names for the latches shown in13:19:11 SYS@ prod >desc v$latchname Name                                                              Null?    Type ----------------------------------------------------------------- -------- -------------------------------------------- LATCH#                                                                     NUMBER NAME                                                                       VARCHAR2(64) HASH                                                                       NUMBER

---原作者：zhuxu@taobao.com（感谢作者）

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