知识大全 oracle性能调整—诊断latch竞争

Posted 2022-07-16 竞争

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　　概念 Latch是简单的低层次的序列化技术用以保护SGA中的共享数据结构比如并发用户列表和buffer cache里的blocks信息一个服务器进程或后台进程在开始操作或寻找一个共享数据结构之前必须获得对应的latch 在完成以后释放latch 不必对latch本身进行优化如果latch存在竞争表明SGA的一部分正在经历不正常的资源使用

　　）Latch的作用 A 序列化访问保护SGA中的共享数据结构保护共享内存的分配 B 序列化执行避免同时执行某些关键代码避免互相干扰

　　）Latch请求的两种类型 A willing to wait 请求的进程经过短时间的等待后再次发出请求直到获得latch B immediate 如果没有获得latch 请求的进程不等待而是继续处理其他指令检查Latch竞争检查latch free是不是主要的wait event Select * from v$system_event order by time_waited;

　　检查latch的使用情况 Select * from v$latch: 与willing to wait请求有关的列 gets misses sleeps wait_time cwait_time spin_gets 与immediate请求有关的列 immediate_gets immediate_misses

　　Gets: number of successful willing to wait requests for a latch; Misses: number of times an initial wiling to wait request was unsuccessful; Sleeps: number of times a process waited after an initial willing to wait request; Wait_time: number of milliseconds waited after willing to wait request; Cwait_time: a measure of the cumulative wait time including the time spent spinning and sleeping the overhead of context switches due to OS time slicing and page faults and interrupts; Spin_gets: gets that misses first try but succeed after spinning

　　Immediate_gets: number of successful immediate requests for each latch; Immediate_misss: number of unsuccessful immediate requests for each latch;

　　一般无需调整latch 但是下列的措施是有用的 A 对处于竞争中的latch做进一步的调查 B 如果竞争主要存在于shared pool和library cache中可以考虑调整应用 C 如果进一步的调查显示需要调整shared pool和buffer cache 就进行调整

　　Select * from v$latch where name like %shared pool% or name like %library cache%

　　如果竞争是在shared pool或library cache上表示下列集中情况

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