怎么解决ORACLE 12C :'acknowledge over PGA limit' Wait Event问题

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用户反映19c生产环境数据库慢，运行简单语句uat只需要2s,但是prod需要40s,对此对数据库进行检查

首先检查数据库的awr报告，发现异常等待acknowledge over PGA limit占用较大

Top 10 Foreground Events by Total Wait Time

查询mos文档，发现此类问题是由于PGA大小达到了PGA_AGGREGATE_LIMIT的值，防止ORA-4036错误，后面进程分配pga时需要等待其他进程释放pga。

12c: 'acknowledge over PGA limit' Wait Event (Doc ID 2138882.1)

CAUSE

The "acknowlege over PGA limit" is a new wait event that was introduced with PGA_AGGREGATE_LIMIT in 12.1,

and it will force a process that wants more PGA to wait a bit if the instance is getting close to hitting the limit.

The hope is some other process will release memory and avoid the ORA-4036 error.

SOLUTION

1. Set PGA_AGGREGATE_LIMIT=0 (as SYS user) to revert to 11g behavior of PGA memory management using PGA_AGGREGATE_TARGET parameter and the wait event will be alleviated.

ALTER SYSTEM SET PGA_AGGREGATE_LIMIT=0 SID='*' SCOPE=BOTH;

2. Set PGA_AGGREGATE_LIMIT to higher than the default value (as SYS user), which was calculated based on the PGA_AGGREGATE_TARGET, PROCESSES or "_pga_limit_target_perc" setting and that will also help to reduce this wait event.

ALTER SYSTEM SET PGA_AGGREGATE_LIMIT= &new_value SID='*' SCOPE=BOTH;

oracle建议设置PGA_AGGREGATE_LIMIT=0或者增大这个参数的值解决

ALTER SYSTEM SET PGA_AGGREGATE_LIMIT=0 SCOPE=BOTH;

但是，这个方法只是暂时解决，究竟是什么导致的呢，需要继续检查pga使用情况

首先判断该应用pga设置是否合理，即连接数数量和pga大小，可以通过查询process数量和pga使用，如果

明显出现process较小而pga使用很大，则有可能是某些应用进程或者数据库进程异常导致，可以通过下面语句查询出是

哪个进程异常导致

1.列出占用pga最大的进程

select pid,spid,substr(username,1,20) "USER" ,program,PGA_USED_MEM,PGA_ALLOC_MEM,PGA_FREEABLE_MEM,PGA_MAX_MEM

from v$process

where pga_alloc_mem=(select max(pga_alloc_mem) from v$process

where program not like '%LGWR%');

2.查看数据库会话所有sid占用pga的的详细情况

set linesize 120

set pagesize 120

column spid heading 'OSpid' format a8

column pid heading 'Orapid' format 999999

column sid heading 'Sess id' format 99999

column serial# heading 'Serial#' format 999999

column status heading 'Status' format a8

column pga_alloc_mem heading 'PGA alloc' format 99,999,999,999

column pga_used_mem heading 'PGA used' format 99,999,999,999

column username heading 'oracleuser' format a12

column osuser heading 'OS user' format a12

column program heading 'Program' format a20

SELECT

p.spid,

p.pid,

s.sid,

s.serial#,

s.status,

p.pga_alloc_mem,

p.PGA_USED_MEM,

s.username,

s.osuser,

s.program

FROM

v$process p,

v$session s

WHERE s.paddr ( + ) = p.addr

and p.BACKGROUND is null /* Remove if need to monitor background processes */

Order by p.pga_alloc_mem desc;

3.检查pga随着时间的增长

select snap_id,round(value/1024/1024,0) from dba_hist_pgastat where name='total PGA allocated' order by snap_id

4.检查应用进程分配的pga,查询出大于50m的

select a.sid,a.status,a.sql_id,a.last_call_et,b.pga_alloc_mem/1024/1024,a.event,a.state from v$session a,v$process b where a.paddr=b.addr and a.status='INACTIVE' and b.pga_alloc_mem/1024/1024 > 50;

5.查看当前pga使用情况

select * from v$pgastat

通过1可以查询出是否有异常的数据库进程占用较大的pga,2可以查询出是否有异常会话占用较大pga,3可以查询出pga在最近一段时间的增长情况，可以判断出什么时候开始出现异常，4则是对应用进程使用Pga大于50m进行排查

如果是数据库进程异常，则需要检查该系统进程对应的trc文件进行分析。

如果是应用进程异常，则通过该sid和spid去分析,对不释放的进程做heapdump,看看内存中导致存储的是什么内容，然后和应用一起解决

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