通过案例学调优之--和SHAREDPOOL相关的主要Latch
通过案例学调优之--和 SHARED POOL 相关的主要 Latch 3.1 、和 SHARED POOL 相关的主要 Latch 有: Latch: shared pool Latch: library cache 我们知道 Oracle 通过 SHARED POOL 来实现 SQL 共享,减少硬解析等。而 SQL 的相关信息,如: SQL 语句文本, SQL 执行
通过案例学调优之--和 SHARED POOL 相关的主要 Latch
3.1、和 SHARED POOL 相关的主要 Latch 有:
Latch: shared pool
Latch: library cache
我们知道 Oracle 通过 SHARED POOL 来实现 SQL 共享,减少硬解析等。而 SQL 的相关信息,
如:SQL 语句文本,SQL 执行计划等都存放在 SHARED POOL 的 Library Cache 部分。
3.2、其中 Library Cache 的结构如下图:
可以看到其结构和 BUFFER CACHE 类似,为了能够在 Library Cache 中快速的查找到对应的 SQL, 也是通过将不同的 SQL 语句通过 HASH 函数 HASH 后放置到对应 Hash Bucket 来保存的。
下面看看图中***的块(右上角标注着:Object Handle):
1) 这个块也就是所谓的 Library Cache Object Handle,这个 Handle 描述 Library Cache 中对象的一些属性,如名称(Name),所属的命名空间(Namespace)、标记(Flags)、指向对象所处的内存地址的指针(Heap 0)等。对应 SQL 来说,这个可以算是父游标。
2) Heap 0 用来存放与对象有直接关系的一些信息,比如对象类型、对象相关的表、实际的执行计划等。
3) 同一个 Hash Bucket 中的 Object Handle 相互链接形成一条 Chain。
关于 Library Cache 更详细的可以查阅 Julian Dyke 的 Library Cache Internals.ppt。
Eygle 网站上也有一张简洁的图:
3.3下面先看SQL的的整个执行过程来,然后再看看执行过程中是怎么用到SHARED POOL的相 关 Latch。
1) 当客户端执行一条 SQL,这时候 Oracle 首先将 SQL 文本转换成 ASCII 值,然后根据 HASH函数计算该 SQL 对应的 Hash Value。
-
2) 根据得到的 Hash Value 到 Library Cache 中查找对应的 Bucket,然后查找 Bucket 里是否存
在该 SQL?
(Y) 如果存在,则接下来查找对应的子游标,这个时候将一直持有 Library Cache Latch,直到找到对应的执行计划。然后释放 Latch。(软解析)
(N) 如果不存在,就要去 SHARE POOL 里面获得可用空间,来生生成对应的 Library Cache 对象。这个时候就要获得 Shared Pool Latch 在 SHARE POOL 的 Free Lis(SHRAE POOL 通过 Free List 管理 Free Chunk)查找可用的空间,之后释放 Shared Pool Latch。 接下来就开始进行硬解析过程,将执行解析后的执行计划等信息记录到 Library Cache 中,这个整个过程消耗大量的 CPU,同时将一直持有 Library Cache Latch,一 直到硬解析结束。(硬解析) 3) 根据获得的执行计划,开始执行 SQL,如:到 BUFFER CACHE 查询数据等。
3.4 整个逻辑如下如:
3.5 当出现Latch竞争严重的时候:
3.5.1如果同时出现大量的 Share Pool Latch 和 Library Cache Latch 的话,根据上面的逻辑那说明数
据库中存在大量的硬解析,这个时候就要查找那些 SQL 没有绑定变量。
3.5.2如果只是出现大量的 Library Cache Latch 的话,那么可能有两种情况:
1) 当持有 Library Cache Latch 查找 Bucket 对应的 Chain 时候,发现存在高 Version 的 SQL,这个时候就要扫描这些对应的子游标,整个过程将一直持有 Latch,导致其他会话获取不到 Latch 进行操作。
2) 大量的并发请求,而且不能实现 SQL 一次 Parse Call 多次 Execution。
案例分析：
3.6 测试模拟为硬解析和 SQL 的 Version Count 高的情况。
3.6.1Oracle 10g 有方法可以让 SQL 产生很多的子游标,必须具备下面几种的条件:
1)cursor_sharing = similar
2)收集了列上的 histogram
3)SQL 中使用到了此列作为条件,并且条件是“等于”
4)这个 SQL 是没有绑定变量的
这时候,Oracle 会认为每条 SQL 的 literal 变量都是 unsafe 的,因此就不重用以前的 cursor而新产生一个 version,也就会重新硬解析一次。
10:56:01 SCOTT@ prod >show parameter cursor
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
cursor_sharing string similar
cursor_space_for_time boolean FALSE
open_cursors integer 300
session_cached_cursors integer 50
1、session1：以scott建立测试表
11:44:26 SYS@ prod >conn scott/tiger
Connected.
11:01:41 SCOTT@ prod >select * from v$mystat where rownum=1;
SID STATISTIC# VALUE
---------- ---------- ----------
1 0 0
10:56:09 SCOTT@ prod >create table test as select rownum as col1 ,rownum col2 from user_objects
10:58:38 2 ;
Table created.
2、建立测试表直方图
10:58:51 SCOTT@ prod >exec dbms_stats.gather_table_stats(user,'TEST',method_opt=>'for columns col1 size 3');
PL/SQL procedure successfully completed.
10:59:36 SCOTT@ prod >select column_name,num_buckets,histogram from user_tab_col_statistics
11:00:43 2 where table_name='TEST';
COLUMN_NAME NUM_BUCKETS HISTOGRAM
------------------------------ ----------- ---------------
COL1 3 HEIGHT BALANCED
11:01:35 sys@ prod >ALTER SYSTem flush shared_pool;
System altered.
3、session 2:以scott建立另一个会话
11:03:44 SCOTT@ prod >select * from v$mystat where rownum=1;
SID STATISTIC# VALUE
---------- ---------- ----------
44 0 0
11:04:01 SCOTT@ prod >create table test1 as select rownum as col1 ,rownum col2 from user_objects;
Table created.
11:04:36 SCOTT@ prod >exec dbms_stats.gather_table_stats(user,'TEST1',method_opt=>'for columns col1 size 3');
PL/SQL procedure successfully completed.
11:05:04 SCOTT@ prod >select column_name,num_buckets,histogram from user_tab_col_statistics
11:05:19 2 where table_name='TEST1';
COLUMN_NAME NUM_BUCKETS HISTOGRAM
------------------------------ ----------- ---------------
COL1 3 HEIGHT BALANCED
11:05:30 sys@ prod >ALTER SYSTem flush shared_pool;
System altered.
4、在session 1执行以下操作
11:02:42 SCOTT@ prod >begin
11:06:28 2 for i in 1..50000 loop
11:06:40 3 execute immediate 'select * from test where col1='||i;
11:07:08 4 end loop;
11:07:11 5 end;
11:07:13 6 /
在session 2执行同样地操作
11:07:57 SCOTT@ prod >begin
11:08:01 2 for i in 1..50000 loop
11:08:01 3 execute immediate 'select * from test1 where col1='||i;
11:08:01 4 end loop;
11:08:01 5 end;
11:08:02 6 /
5、查看session event
11:11:36 sys@ prod > select sid,event,p1,p1text,p2,p2text from v$session where sid in (1,44)
SID EVENT P1 P1TEXT P2 P2TEXT
---------- ------------------------------ ---------- ------------------------------ ---------- ------------------------------
1 latch: shared pool 537557404 address 293 number
44 latch: shared pool 537557404 address 293 number
Elapsed: 00:00:00.00
11:11:38 sys@ prod >/
SID EVENT P1 P1TEXT P2 P2TEXT
---------- ------------------------------ ---------- ------------------------------ ---------- ------------------------------
1 latch: shared pool 537557404 address 293 number
44 latch: row cache objects 828539960 address 270 number
Elapsed: 00:00:00.00
11:11:39 sys@ prod >/
SID EVENT P1 P1TEXT P2 P2TEXT
---------- ------------------------------ ---------- ------------------------------ ---------- ------------------------------
1 latch: shared pool 537557404 address 293 number
44 latch: shared pool 537557404 address 293 number
Elapsed: 00:00:00.00
11:11:41 sys@ prod >/
SID EVENT P1 P1TEXT P2 P2TEXT
---------- ------------------------------ ---------- ------------------------------ ---------- ------------------------------
1 latch: shared pool 537557404 address 293 number
44 latch: row cache objects 828007508 address 270 number
Elapsed: 00:00:00.00
11:11:42 sys@ prod >/
SID EVENT P1 P1TEXT P2 P2TEXT
---------- ------------------------------ ---------- ------------------------------ ---------- ------------------------------
1 latch: shared pool 537557404 address 293 number
44 latch: shared pool 537557404 address 293 number
11:12:32 sys@ prod >/
SID EVENT P1 P1TEXT P2 P2TEXT
---------- ------------------------------ ---------- ------------------------------ ---------- ------------------------------
1 latch free 821793596 address 274 number
44 latch: shared pool 537557404 address 293 number
sys@ prod >select sid,event,p1,p1text,p2,p2text from v$session where sid in (1,44)
SID EVENT P1 P1TEXT P2 P2TEXT
---------- ------------------------------ ---------- ------------------------------ ---------- ------------------------------
1 latch: shared pool 537557404 address 293 number
44 library cache: mutex X 1307903034 idn 65536 value
11:14:58 sys@ prod >select sid,event,p1,p1text,p2,p2text from v$session where sid in (1,44)
SID EVENT P1 P1TEXT P2 P2TEXT
---------- ------------------------------ ---------- ------------------------------ ---------- ------------------------------
1 library cache: mutex X 3413592168 idn 2883584 value
44 latch: row cache objects 828539960 address 270 number
11:15:18 sys@ prod >select sid,event,p1,p1text,p2,p2text from v$session where sid in (1,44)
SID EVENT P1 P1TEXT P2 P2TEXT
---------- ------------------------------ ---------- ------------------------------ ---------- ------------------------------
1 SQL*Net message from client 1650815232 driver id 1 #bytes
44 SQL*Net message from client 1650815232 driver id 1 #bytes
从上面的过程可以看到,大量的硬解析将导致严重的 library cache latch（mutex） 和 shared pool latch竞争。
6、查看Library cache中sql情况
sys@ prod >select *
2 from (select sql_id,child_number,child_latch,executions,sql_text
3 from v$sql
4 where sql_text like '%select * from test1 where col1%'
5 and sql_text not like '%v$sql%'
6 and sql_text not like '%begin%'
7 order by child_number desc)
8* where rownum <10
SQL_ID CHILD_NUMBER CHILD_LATCH EXECUTIONS SQL_TEXT
------------- ------------ ----------- ---------- --------------------------------------------------
6tsrjxza4gvur 1987 0 1 select * from test1 where col1=:"SYS_B_0"
6tsrjxza4gvur 1988 0 1 select * from test1 where col1=:"SYS_B_0"
6tsrjxza4gvur 1989 0 1 select * from test1 where col1=:"SYS_B_0"
6tsrjxza4gvur 1990 0 1 select * from test1 where col1=:"SYS_B_0"
6tsrjxza4gvur 1991 0 1 select * from test1 where col1=:"SYS_B_0"
6tsrjxza4gvur 1992 0 1 select * from test1 where col1=:"SYS_B_0"
6tsrjxza4gvur 1993 0 1 select * from test1 where col1=:"SYS_B_0"
6tsrjxza4gvur 1994 0 1 select * from test1 where col1=:"SYS_B_0"
6tsrjxza4gvur 1995 0 1 select * from test1 where col1=:"SYS_B_0"
6tsrjxza4gvur 1996 0 1 select * from test1 where col1=:"SYS_B_0"
6tsrjxza4gvur 1997 0 1 select * from test1 where col1=:"SYS_B_0"
6tsrjxza4gvur 1998 0 1 select * from test1 where col1=:"SYS_B_0"
6tsrjxza4gvur 1999 0 1 select * from test1 where col1=:"SYS_B_0"
6tsrjxza4gvur 2000 0 1 select * from test1 where col1=:"SYS_B_0"
6tsrjxza4gvur 2001 0 1 select * from test1 where col1=:"SYS_B_0"
6tsrjxza4gvur 2002 0 1 select * from test1 where col1=:"SYS_B_0"
6tsrjxza4gvur 2003 0 1 select * from test1 where col1=:"SYS_B_0"
11:46:50 SYS@ prod >select sql_id,hash_value,address,version_count from v$sqlarea where sql_id='6tsrjxza4gvur';
SQL_ID HASH_VALUE ADDRESS VERSION_COUNT
------------- ---------- -------- -------------
6tsrjxza4gvur 3561484119 2E8CF368 3885 可以看到 SQL 的 Version_Count 很高,而且 V$SQL 视图里面也能查到对应的子游标。
案例分析：
模拟高并发下,对 Version Count 高 SQL 查询:
session 1：
11:42:41 SYS@ prod >conn scott/tiger Connected. 12:05:10 SCOTT@ prod >select * from v$mystat where rownum=1; SID STATISTIC# VALUE ---------- ---------- ---------- 38 0 0 12:06:03 SCOTT@ prod >begin 12:06:23 2 for i in 1..500000 loop 12:06:23 3 execute immediate 'select * from test1 where col1=1'; 12:06:23 4 end loop; 12:06:23 5 end; 12:06:23 6 / PL/SQL procedure successfully completed.Copy after loginsession 2： 11:30:32 SYS@ prod >conn scott/tiger Connected. 12:05:15 SCOTT@ prod >select * from v$mystat where rownum=1; SID STATISTIC# VALUE ---------- ---------- ---------- 39 0 0 12:06:11 SCOTT@ prod >begin 12:06:29 2 for i in 1..500000 loop 12:06:29 3 execute immediate 'select * from test1 where col1=1'; 12:06:29 4 end loop; 12:06:29 5 end; 12:06:29 6 / PL/SQL procedure successfully completed.Copy after login12:07:17 SYS@ prod >col event for a30 12:07:27 SYS@ prod >col p1text for a20 12:07:33 SYS@ prod >col p2text for a20 12:07:38 SYS@ prod >select sid,event,p1,p1text,p2,p2text from v$session where sid in (38,39) SID EVENT P1 P1TEXT P2 P2TEXT ---------- ------------------------------ ---------- -------------------- ---------- -------------------- 38 library cache: mutex X 3561484119 idn 2621440 value 39 cursor: mutex S 3561484119 idn 2490369 value Elapsed: 00:00:00.00 12:07:38 SYS@ prod >/ SID EVENT P1 P1TEXT P2 P2TEXT ---------- ------------------------------ ---------- -------------------- ---------- -------------------- 38 library cache: mutex X 3561484119 idn 2621440 value 39 library cache: mutex X 3561484119 idn 2490368 value Elapsed: 00:00:00.00 12:07:43 SYS@ prod >/ SID EVENT P1 P1TEXT P2 P2TEXT ---------- ------------------------------ ---------- -------------------- ---------- -------------------- 38 library cache: mutex X 3561484119 idn 2555904 value 39 cursor: mutex S 3561484119 idn 2490368 value Elapsed: 00:00:00.00 12:07:45 SYS@ prod >/ SID EVENT P1 P1TEXT P2 P2TEXT ---------- ------------------------------ ---------- -------------------- ---------- -------------------- 38 latch: shared pool 537557404 address 293 number 39 library cache: mutex X 3561484119 idn 2490368 value Elapsed: 00:00:00.00 12:07:46 SYS@ prod >/ SID EVENT P1 P1TEXT P2 P2TEXT ---------- ------------------------------ ---------- -------------------- ---------- -------------------- 38 library cache: mutex X 3561484119 idn 2621440 value 39 library cache: mutex X 3561484119 idn 2621440 value Elapsed: 00:00:00.00 12:07:47 SYS@ prod >/ SID EVENT P1 P1TEXT P2 P2TEXT ---------- ------------------------------ ---------- -------------------- ---------- -------------------- 38 library cache: mutex X 3561484119 idn 2621440 value 39 cursor: mutex S 3561484119 idn 2490368 value Elapsed: 00:00:00.00 12:07:49 SYS@ prod >/ SID EVENT P1 P1TEXT P2 P2TEXT ---------- ------------------------------ ---------- -------------------- ---------- -------------------- 38 library cache: mutex X 3561484119 idn 2621440 value 39 library cache: mutex X 3561484119 idn 2621440 value Elapsed: 00:00:00.00 12:07:50 SYS@ prod >/ SID EVENT P1 P1TEXT P2 P2TEXT ---------- ------------------------------ ---------- -------------------- ---------- -------------------- 38 library cache: mutex X 3561484119 idn 2621440 value 39 library cache: mutex X 3561484119 idn 2490368 value Elapsed: 00:00:00.00 12:07:51 SYS@ prod >/ SID EVENT P1 P1TEXT P2 P2TEXT ---------- ------------------------------ ---------- -------------------- ---------- -------------------- 38 library cache: mutex X 3561484119 idn 2555904 value 39 cursor: mutex S 3561484119 idn 2490368 value Elapsed: 00:00:00.01 12:08:11 SYS@ prod >/ SID EVENT P1 P1TEXT P2 P2TEXT ---------- ------------------------------ ---------- -------------------- ---------- -------------------- 38 cursor: pin S 3561484119 idn 2555905 value 39 library cache: mutex X 3561484119 idn 2490368 value Elapsed: 00:00:00.00 12:08:15 SYS@ prod >/ SID EVENT P1 P1TEXT P2 P2TEXT ---------- ------------------------------ ---------- -------------------- ---------- -------------------- 38 library cache: mutex X 3561484119 idn 2621440 value 39 library cache: mutex X 64028 idn 2490368 value 12:09:04 SYS@ prod >/ SID EVENT P1 P1TEXT P2 P2TEXT ---------- ------------------------------ ---------- -------------------- ---------- -------------------- 38 cursor: mutex S 3561484119 idn 2555904 value 39 cursor: pin S &n
Copy after login
Hot AI Tools
Undresser.AI Undress
AI-powered app for creating realistic nude photos
AI Clothes Remover
Online AI tool for removing clothes from photos.
Undress AI Tool
Undress images for free
Clothoff.io
AI clothes remover
AI Hentai Generator
Generate AI Hentai for free.
Hot Article
Hot Tools
Notepad++7.3.1
Easy-to-use and free code editor
SublimeText3 Chinese version
Chinese version, very easy to use
Zend Studio 13.0.1
Powerful PHP integrated development environment
Dreamweaver CS6
Visual web development tools
SublimeText3 Mac version
God-level code editing software (SublimeText3)
Hot Topics
Li Auto's L series welcomes new member: Li Auto L6 positioned as a medium-sized five-seat SUV
Jul 02, 2023 pm 11:57 PM
According to news on June 30, Li Auto’s L series models, including L7, L8 and L9, have achieved considerable sales results in their respective price ranges. However, according to the editor’s understanding, Li Auto hopes to further increase sales, and its focus is on the performance of another new car, the Li Li L6. Recently, a blogger allegedly photographed a camouflaged Ideal L6 car in a highway service area. According to the photos taken by the blogger, the camouflaged car suspected to be the Lili L6 is not driving normally, but is parked on a trailer. Compared with the white Weilai SUV next to it, even as the lowest-positioned model in the L series, the Lideal L6 appears to be quite large. It is reported that the Lili L6 is positioned as a medium-sized five-seat SUV. Although these photos don't provide much information about the appearance details
Analysis of common web standards and their actual cases
Jan 13, 2024 pm 03:50 PM
Understand common Web standards and their practical application cases In today's digital era, the World Wide Web has become an important platform for people to obtain information, communicate, and conduct business activities. Web standards are the basis for ensuring that web pages display normally and operate stably on different browsers. This article will introduce some common web standards and illustrate their importance through practical application cases. First of all, HTML (Hypertext Markup Language) is the most basic part of the Web standards and is used to describe the structure and content of web pages. HTML uses tags to define whether
Solve the problem of environment detection when reinstalling the system
Jan 08, 2024 pm 03:33 PM
How to solve the problem that the environment test fails when reinstalling the system and needs to be rewritten. The reason is: the mobile phone is poisoned. You can install anti-virus software such as Mobile Manager for anti-virus. 2. Many junk files are stored inside the mobile phone, causing the running memory of the mobile phone to be occupied. Just clear the phone cache to solve this problem. 3. The phone memory is occupied too much by saved software and files. It is no problem to delete unnecessary files and software frequently. As long as your hardware configuration meets the installation requirements, you can use the new one directly. Reinstall the system from the system disk! You can use a USB flash drive or hard disk to install, which is very fast. But the key is to use a system disk with good compatibility (supports installation in IDE, ACHI, and RAID modes), and it can be automatically and permanently activated, which has been verified. so
Take stock of some practical Linux tips
Mar 12, 2024 pm 01:49 PM
Linux is a powerful operating system with many useful commands and tips to help you use it more efficiently. 1. Check the file check value. During the file copying or transmission process, the file may be damaged or modified. In this case, the check value can be used for verification. Usually, we need to use some interface programs provided by other teams in our work. Whenever the running results of these programs are not as expected, we will compare the md5 check values of both parties to confirm the consistency of the data. There are many ways to generate the check value of a file. Commonly used ones include md5sum check, crc check, sum check, etc. The commands are: md5sumfile_namecksumfile_namesum algorithm parameter file
In C++, translate the following into Chinese: Count the number of numbers between L and R that are relatively prime to P
Aug 26, 2023 pm 09:33 PM
In the world of computer programming, finding the number of numbers in a given range that is coprime to a specific number can be a common task. Relatively prime numbers, also known as relative prime numbers, are numbers that have no common factors other than 1. In this article, we will explore finding the number of numbers that are relatively prime to a specific number P between the given integers L and R by using C++ language. Syntax We will first outline the syntax of the method we will use in the following code examples - intcountCoprimes(intL,intR,intP); Algorithm The algorithm we will use to count the number of coprime numbers is as follows − Initialize the variable count as 0, used to store the count of relatively prime numbers. Iterate over each number num starting from L until R. for each
Released IoT cases for four consecutive years, Qualcomm and ecological partners jointly draw a new blueprint for the era of digital-real integration
Oct 13, 2023 pm 08:25 PM
On busy production lines, robotic arms collaborate efficiently, and operators can easily control the entire process through remote real-time control. Under the mine, technical experts wear AR glasses and use the information displayed in real time in the glasses to quickly solve equipment problems with the technical team thousands of miles away. In the park, unmanned delivery vehicles shuttle freely, allowing users to pick up packages without leaving home. These scenarios are not fictitious, but real cases that appear in the "2023 Qualcomm Empowering Enterprise Digital Transformation Case Collection" recently released by Qualcomm. For four consecutive years, Qualcomm has released the "Internet of Things" focusing on the key development directions and highlight implementation scenarios of the Internet of Things industry. "Application Case Collection" displays the industry's latest technological directions and innovative ecological cooperation models in a multi-dimensional and three-dimensional manner. This year’s case collection focuses on Chinese companies’ use of Qualcomm Technologies
PHP development case: making an online question and answer community
Oct 27, 2023 am 11:25 AM
In today's Internet age, more and more people like to find answers and communicate with questions online. As a result, various online question and answer communities emerged. These communities provide a platform for users to ask questions, answer questions, and exchange experiences and knowledge with each other. Today, we will introduce the production process of an online Q&A community developed based on PHP. First, we need to clarify what functions an online Q&A community needs to have. Generally speaking, it should include the following aspects: user registration, login, questions, replies
Detailed introduction to the integration and use of Nginx server's native modules and third-party plug-ins.
Aug 06, 2023 pm 09:13 PM
Nginx server is a high-performance web server software with very strong scalability and supports the integration of native modules and third-party plug-ins. This article will introduce in detail the integration and use of the Nginx server's native modules and third-party plug-ins, and attach code examples to help readers better understand and apply them. 1. Nginx native module Nginx’s native module refers to the functional modules officially developed and maintained by Nginx, including core modules, HTTP modules, mail modules, etc. Configuration in Nginx
