生产上数据库大量的latch free 导致的CPU资源耗尽的问题的解决

Posted 2020-09-19 mfmdaoyou

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中午的时候，我们生产上的某个数据库，cpu一直居高不下

技术分享

通过例如以下的sql语句，我们查看当时数据库的等待，争用的情况：

select s.SID,
       s.SERIAL#,
       'kill -9 ' || p.SPID,
       s.MACHINE,
       s.OSUSER,
       s.PROGRAM,
       s.USERNAME,
       s.last_call_et,
       a.SQL_ID,
       s.LOGON_TIME,
       a.SQL_TEXT,
       a.SQL_FULLTEXT,
       w.EVENT,
       a.DISK_READS,
       a.BUFFER_GETS
  from v$process p, v$session s, v$sqlarea a, v$session_wait w
 where p.ADDR = s.PADDR
   and s.SQL_ID = a.sql_id
   and s.sid = w.SID
   and s.STATUS = 'ACTIVE'
 order by s.last_call_et desc;

从event能够看到，是latch 的争用导致的原因

技术分享

通过假设的sql，查看是什么样的latch

select * from v$session_wait 
where event  like 'latch free';

技术分享

P2就是这个latch的name。通过v$latchname这个视图就能够知道哪个详细的latch

1:45:55 PM SQL> select * from v$latchname where latch#=164;
 
    LATCH# NAME                                                                   HASH
---------- ---------------------------------------------------------------- ----------
       164 simulator hash latch                                             2233208730

查看latch的历史情况

2:11:59 PM SQL> select name,gets,misses,sleeps from v$latch where sleeps >0 order by sleeps desc;
 
NAME                                                                   GETS     MISSES     SLEEPS
---------------------------------------------------------------- ---------- ---------- ----------
simulator hash latch                                             4827860212  135426899   10890947
cache buffers chains                                             1619822817 2850976006    4747728
gc element                                                       4660052091   25748270     175073
resmgr:schema config                                               91872524     153968      95708
ges resource hash list                                            174151449    1070556      55459
Real-time plan statistics latch                                    40953155     651496      44527
call allocation                                                     3301878     265908      43501
row cache objects                                                 336300485    4970324      19366

这个simulator hash latch已经是显著的latch部分

eagle在他的站点上有篇文章讲到了关于simulator这个

http://www.eygle.com/archives/2011/11/simulator_lru_latch.html

simulator意为模拟。也就是说当Oracle在内存中进行数据块处理时。实际上还会在预先分配的Buffer中进行相关信息记录，如DBA信息，当数据块被老化之后，下次读取时。假设请求的数据在Simulator内存中存在，则觉得继续缓存该数据块是有意义的，通过监控并模拟统计这些操作，并对计算结果加权运算。就能够实现对于内存的调整建议。

在模拟过程中。也是通过Latch来实现的，相关的Latch就有 simulator lru latch 、 simulator hash latch等.

就Buffer Cache而言。假设系统中该类争用严重，则能够考虑关闭db_cache_advice。消除这部分内部操作对于性能的影响。
下面是一个相关BUG。在该Bug中，因为DB_CACHE_ADVICE的开启导致了严重的simulator lru latch的竞争：

Bug 5918642 Heavy latch contention with DB_CACHE_ADVICE on
This note gives a brief overview of bug 5918642.
The content was last updated on: 01-APR-2008
Click html?
cs=UTF-8" style="text-decoration:none; color:rgb(0,130,255); outline-width:medium; padding-bottom:0px; padding-top:0px; outline-style:none; padding-left:0px; outline-color:invert; padding-right:0px">here for details of each of the sections below.

Affects:

Product (Component) Oracle Server (Rdbms)

Range of versions believed to be affected Versions < 11.2

Versions confirmed as being affected

10.2.0.3

Platforms affected Generic (all / most platforms affected)

Fixed:

This issue is fixed in

11.2 (Future Release)
10.2.0.4 (Server Patch Set)
11.1.0.7 (Server Patch Set)

Symptoms:

Related To:

Latch Contention
Waits for "latch free"

Performance Monitoring
DB_CACHE_ADVICE

Description
High simulator lru latch contention can occur when db_cache_advice is
set to ON if there is a large buffer cache.


Workaround:
  Set db_cache_advice to OFF

当然，这个仅仅是治标不治本的做法，这个是显现的表象的问题。根源的问题还是这个sql语句有问题

当一个数据块读入到sga中时，该块的块头(buffer header)会放置在一个hash bucket的链表(hash chain)中。该内存结构由一系列cache buffers chains子latch保护（又名hash latch或者cbc latch）。对Buffer cache中的块，要select或者update、insert,delete等。都得先获得cache buffers chains子latch，以保证对chain的排他訪问。

若在过程中发生争用，就会等待latch:cache buffers chains事件。

产生原因： 1. 低效率的SQL语句（主要体如今逻辑读过高）在某些环境中，应用程序打开运行同样的低效率SQL语句的多个并发会话。这些SQL语句都设法得到同样的数据集，每次运行都带有高 BUFFER_GETS(逻辑读取)的SQL语句是基本的原因。

相反，较小的逻辑读意味着较少的latch get操作，从而降低锁存器争用并改善性能。注意v$sql中BUFFER_GETS/EXECUTIONS大的语句。 2.Hot block 当多个会话反复訪问一个或多个由同一个子cache buffers chains锁存器保护的块时。热块就会产生。

当多个会话争用cache buffers chains子锁存器时，就会出现这个等待事件。有时就算调优了SQL，但多个会话同一时候运行此SQL，那怕仅仅是扫描特定少数块，也是也会出现HOT BLOCK的。

SELECT P935.SEQUENCEID,
       null FA_SEQUENCEID,
       P935.ORDERID,
       P935.ORGORDERID,
       P935.PRODUCTNAME,
       P935.PRODUCTNUM,
       P935.ORDERTIME,
       P935.LASTUPDATETIME,
       P935.ORDERSTATUS,
       P935.MEMO,
       935 orderCode,
       P935.PAYERACCTCODE,
       P935.PAYERACCTTYPE,
       P935.PAYEEACCTCODE PLATACCTCODE,
       P935.PAYEEACCTTYPE PLATACCTTYPE,
       P936.PAYEEACCTCODE,
       P936.PAYEEACCTTYPE,
       EXT935.PAYER_DISPLAYNAME,
       EXT935.PAYER_NAME,
       EXT935.PAYER_IDC,
       EXT935.PAYER_MEMBERTYPE,
       EXT936.PAYER_DISPLAYNAME PLAT_DISPLAYNAME,
       EXT936.SUBMITNAME PLAT_NAME,
       EXT936.PAYER_IDC PLAT_IDC,
       EXT936.PAYER_MEMBERTYPE PLAT_MEMBERTYPE,
       EXT936.PAYEE_DISPLAYNAME,
       EXT936.PAYEE_NAME,
       EXT936.PAYEE_IDC,
       EXT936.PAYEE_MEMBERTYPE,
       P935.PAYEEDISPLAYNAME WEBSITENAME,
       CASE
         WHEN (SELECT count(*)
                 FROM PAYMENTORDER P936
                WHERE P936.Ordercode = 936
                  and P936.Orderstatus = 0
                  AND <span style="color:#ff0000;">P936.Relatedsequenceid = P935.SEQUENCEID</span>) > 0 THEN
          0
         ELSE
          1
       END AS SHARINGRESULT,
       CASE D935.Dealcode
         WHEN 210 then
          14
         else
          D935.DEALTYPE
       end PAYMETHOD,
       D935.DEALAMOUNT,
       G935.EXT1,
       G935.Ext2,
       G935.PAYERCONTACTTYPE,
       G935.PAYERCONTACT,
       NVL(D935.PAYEEFEE, 0) PAYEEFEE,
       NVL(D935.PAYERFEE, 0) PAYERFEE,
       nvl(MS936.PAYEEFEE, 0) PLATFORMFEE,
       P935.VERSION
  FROM PAYMENTORDER          P935,
       PAYMENTORDER          P936,
       DEAL                  D935,
       GATEWAYORDER          G935,
       MSGATEWAYSHARINGORDER MS936,
       PAYMENTORDEREXT       EXT935,
       PAYMENTORDEREXT       EXT936
 WHERE P936.ORDERCODE = 936
   AND P935.ORDERCODE = 935
   AND P936.RELATEDSEQUENCEID = to_char(P935.SEQUENCEID)
   AND P935.SEQUENCEID = G935.SEQUENCEID(+)
   AND P935.SEQUENCEID = D935.ORDERSEQID(+)
   AND P935.SEQUENCEID = EXT935.ORDERSEQID(+)
   AND P936.SEQUENCEID = EXT936.ORDERSEQID(+)
   AND P936.SEQUENCEID = MS936.SEQUENCEID(+)
   AND MS936.SHARINGTYPE = 1
   AND P935.SEQUENCEID = :1
UNION
SELECT P938.SEQUENCEID,
       P935.SEQUENCEID FA_SEQUENCEID,
       P938.ORDERID,
       P938.ORGORDERID,
       P935.PRODUCTNAME,
       P935.PRODUCTNUM,
       P938.ORDERTIME,
       P938.LASTUPDATETIME,
       P938.ORDERSTATUS,
       P938.MEMO,
       938 orderCode,
       P938.PAYERACCTCODE,
       P938.PAYERACCTTYPE,
       P938.PAYEEACCTCODE PLATACCTCODE,
       P938.PAYEEACCTTYPE PLATACCTTYPE,
       P938.PAYEEACCTCODE,
       P938.PAYEEACCTTYPE,
       EXT938.PAYER_DISPLAYNAME,
       EXT938.PAYER_NAME,
       EXT938.PAYER_IDC,
       EXT938.PAYER_MEMBERTYPE,
       EXT938.PAYEE_DISPLAYNAME PLAT_DISPLAYNAME,
       EXT938.SUBMITNAME PLAT_NAME,
       EXT938.PAYEE_IDC PLAT_IDC,
       EXT938.PAYEE_MEMBERTYPE PLAT_MEMBERTYPE,
       EXT938.PAYEE_DISPLAYNAME,
       EXT938.PAYEE_NAME,
       EXT938.PAYEE_IDC,
       EXT938.PAYEE_MEMBERTYPE,
       P935.PAYEEDISPLAYNAME WEBSITENAME,
       null SHARINGRESULT,
       D938.DEALTYPE PAYMETHOD,
       D938.DEALAMOUNT,
       G935.EXT1,
       G935.Ext2,
       G935.PAYERCONTACTTYPE,
       G935.PAYERCONTACT,
       NVL(D938.PAYEEFEE, 0) PAYEEFEE,
       NVL(D938.PAYERFEE, 0) PAYERFEE,
       0 PLATFORMFEE,
       P935.VERSION
  FROM PAYMENTORDER    P935,
       PAYMENTORDER    P938,
       DEAL            D938,
       GATEWAYORDER    G935,
       PAYMENTORDEREXT EXT938
 WHERE P935.ORDERCODE = 935
   AND P938.ORDERCODE = 938
   AND P938.RELATEDSEQUENCEID = to_char(P935.SEQUENCEID)
   AND P935.SEQUENCEID = G935.SEQUENCEID(+)
   AND P938.SEQUENCEID = D938.ORDERSEQID(+)
   AND P938.SEQUENCEID = EXT938.ORDERSEQID(+)
   AND P935.SEQUENCEID = :2

分析上面的sql，上面标红的地方。等号左边是varchar2的数据类型，括号右边是number的数据类型。会导致数据类型的隐式转换，造成极大的性能影响

联系研发。改动了sql语句，问题解决

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*Product (Component)*	Oracle Server (Rdbms)
*Range of versions believed* to be affected**	Versions < 11.2
*Versions confirmed* as being affected**	10.2.0.3
Platforms affected	Generic (all / most platforms affected)

Symptoms:	Related To:
Latch Contention Waits for "latch free"	Performance Monitoring DB_CACHE_ADVICE