【凤凰社】SQL Server CPU 利用率毛刺的分析定位与解决

一.背景

1.1 问题描述

近期发现一台SQL Server的CPU利用率很不稳定,发现不定时的飙升到100%,更可怕的是在业务繁忙时,影响了业务调用,失败率明显增加,So,减低CPU的利用率,是迫切需要解决的问题。 

CPU升高的原因直观上来说,就是CPU(中央处理器)的负载过高, 中央处理器忙不过来。进一步分析的话,可以从两个角度优化,1.减少单个CPU 的处理时间;2.减少单个任务占有的CPU核数,即一个任务不要分配太多的CPU核数。

1.2 优化的方法

1.表结构的优化,例如索引是否合理、关联表字段的定义是否一致等;

2.SQL 语句的优化;

3.表数据量是否归档、缩减;

4.将数据缓存到缓存层(如,Reids),减少对DB的访问;

5.DB实例配置是否需要优化;

6.升级硬件。 

二. 问题处理过程

 2.1 优化前

从这个监控图可以看出,CPU最大值为100%,平均值为19%,毛刺比较明显。

2.2 定位SQL语句

通过常用的SP,很快定位到了SQL语句,是关于一张表的查询,语句简单,But表的数据量比较大(7600W),查询出的数据有(4000W)。这张表每天的写入量<100W。

而且和研发确认,此SQL的调用也是周期性的,比如5分钟查询一次,基本符合Zabbix周期性CPU毛刺突起。

2.3 处理步骤

Step 1 .考虑到,CPU突然飙升,毛刺陡然加剧,冲刺到100%,而且问题SQL 不是最近新上线,So,我们的第一反映是 索引走偏,统计信息失效了。But 刷新了 表统计信息 ,情况没有好转。

Step 2. 考虑到是不是索引失效了,我们坚持到业务低峰期,重建了表的索引,情况 依然没有好转。

Step 3.考虑是不是表的数据量到了一定规模,才导致的此问题,和业务研发确认后,将历史数据归档,归档了2800W,数据量由7600W减少到4800W。数据量减少后,情况有所好转,SQL事务的排队和阻塞 明显减少。But毛刺突起依然明显,CPU 100% 依然存在。

..........

无语

.........

Step 4 这时想到,最大并行度 。当SQL Server 发现一条指令比较复杂(不仅仅是SQL语句复杂,查询的数据量比较大也是复杂),会决定用多少个线程并行Run,从而提高整体相应时间。如果指令复杂,甚至需要所有CPU来运行这些线程,别的用户发过来的指令会受到影响,甚至可可以拿不到CPURun。即需要调整max degree of parallelism的值

查看问题实例 发现没有设置,即可以使用所有的CPU。修改参数,将最大并行度将至4.Run以下命令:

exec sp_configure 'max degree of parallelism',4
go
RECONFIGURE
GO

此时 毛刺消失了,问题解决了。

2.4 优化后

 优化后,从监控图中可以看出,CPU的最大值降至了25%,平局值为7%。

 三.定位问题SQL常用的SP

当我们遇到DB性可以问题或DBServer监控指标异常时,以下四个SP,可以帮忙我们快速定位SQL语句。

 3.1 查看当前阻塞排队的情况

/*  
---------------------------------------------------------------------------------  
uspm_Block  
功可以:查看阻塞和锁,阻塞源头 
参数:无 
---------------------------------------------------------------------------------  
*/  
create PROCEDURE  [dbo].[uspm_Block]  
as  

--查找有关被阻塞的请求的信息(含用户)
SELECT  s.loginame
       ,[Individual Query] = SUBSTRING (qr.text,qs.statement_start_offset/2, 
         (CASE WHEN qs.statement_end_offset = -1 THEN LEN(CONVERT(NVARCHAR(MAX), qr.text)) * 2 
               ELSE qs.statement_end_offset 
          END - qs.statement_start_offset)/2) 
       ,qs.session_id ,s.counts AS [进程个数],qs.status ,qs.blocking_session_id
       ,qs.wait_type ,qs.wait_time ,qs.wait_resource 
       ,qs.transaction_id 
FROM SYS.DM_EXEC_REQUESTS qs (nolock)
  LEFT JOIN (
             SELECT spid,MAX(loginame)AS loginame,COUNT(0)AS counts FROM SYS.SYSPROCESSES (nolock) GROUP BY spid
            ) s ON qs.session_id=s.spid
  OUTER APPLY SYS.DM_EXEC_SQL_TEXT(qs.sql_handle) AS qr
WHERE qs.status = N'suspended'
--and s.loginame<>''
ORDER BY qs.wait_time DESC

--查找阻塞源头v3.0
SELECT SP.spid
      ,CASE WHEN ST1.text IS NULL THEN ST2.text
            ELSE SUBSTRING (ST1.text,SR.statement_start_offset/2, 
                 (
                  CASE WHEN SR.statement_end_offset = -1 THEN LEN(CONVERT(NVARCHAR(MAX), ST1.text)) * 2 
                       ELSE SR.statement_end_offset 
                  END - SR.statement_start_offset)/2
                  )
        END AS [T-sql]
      ,SP.loginame
      ,DB_NAME(SP.dbid) AS [db_name]
      ,SP.open_tran,SP.hostname,SP.program_name,SP.waitresource,SP.*
FROM SYS.SYSPROCESSES SP (nolock) 
  LEFT JOIN SYS.DM_EXEC_REQUESTS SR (nolock) ON SP.spid=SR.session_id
  LEFT JOIN SYS.DM_EXEC_CONNECTIONS SC (nolock) ON SP.spid=SC.session_id
  OUTER APPLY SYS.DM_EXEC_SQL_TEXT(SC.most_recent_sql_handle) AS ST2
  OUTER APPLY SYS.DM_EXEC_SQL_TEXT(SR.sql_handle) AS ST1
WHERE SP.spid IN
      (
         SELECT BLOCKED FROM SYS.SYSPROCESSES (nolock) WHERE BLOCKED<>0
      )
  AND SP.BLOCKED=0
GO

 3.2 查看 当前最消耗CPU的SQL

/*
---------------------------------------------------------------------------------
uspm_perf_topcpu
功可以:取当前N个最耗CPU的SQL
参数:@topCount   --N的具体数量,默认取前20条
示例:
     1.取前10条
      exec uspm_perf_topcpu 

     2.取前20条最耗CPU的SQL
      exec uspm_perf_topcpu @topCount=20 
---------------------------------------------------------------------------------
*/
CREATE PROCEDURE  [dbo].[uspm_perf_topcpu]
(@topCount int=10)
as 
set nocount on
declare @cmd varchar(1000)
select @cmd='
        SELECT TOP '+ CAST(@topCount AS VARCHAR)+'  SUBSTRING(text, (statement_start_offset/2) + 1,
            ((CASE statement_end_offset
                                WHEN -1 THEN DATALENGTH(text)
                                ELSE statement_end_offset
                                END - statement_start_offset)/2) + 1
                            ) AS query_text
                            ,b.hostname
                            ,b.loginame
                            ,a.*
                            ,qr.text
                            ,qt.query_plan
            FROM sys.dm_exec_requests a (nolock)
               INNER JOIN sys.sysprocesses b (nolock) on a.session_id=b.spid
               CROSS APPLY sys.dm_exec_sql_text(a.sql_handle) as qr
               CROSS APPLY sys.dm_exec_query_plan(a.plan_handle)as qt
            ORDER BY a.total_elapsed_time DESC'
        
exec(@cmd)

GO

 3.3 查看Run时间最长的SQL

/*
---------------------------------------------------------------------------------
uspm_perf_topduration 
功可以:取N个Run时间最长的SQL
参数:@topCount   --N的具体数量,默认取前50条
示例:
      --1.取前50条
      exec uspm_perf_topduration 

      --2.取前10条Run时间最长的SQL
      exec uspm_perf_topduration @topCount=10
---------------------------------------------------------------------------------
*/
CREATE PROCEDURE  [dbo].[uspm_perf_topduration] 
(@topCount int=50)
as 
set nocount on
declare @cmd varchar(600)
select @cmd='
            select  
                highest_cpu_queries.plan_handle,  
                highest_cpu_queries.total_worker_time, 
                q.dbid, 
                q.objectid, 
                q.number, 
                q.encrypted, 
                q.[text] 
            from  
                (select top '+ cast(@topCount as varchar)+'  
                    qs.plan_handle,  
                    qs.total_worker_time 
                from  
                    sys.dm_exec_query_stats qs  (nolock)
                order by qs.total_worker_time desc) as highest_cpu_queries 
                cross apply sys.dm_exec_sql_text(plan_handle) as q 
            order by highest_cpu_queries.total_worker_time desc'
exec(@cmd)
GO

 3.4 当前SQLRun概览

/*  
---------------------------------------------------------------------------------  
uspm_perf_cpudetail  
功可以:查看CPU的任务数量,使用率,CPU瓶颈  
参数:无参  
---------------------------------------------------------------------------------  
*/  
CREATE PROCEDURE  [dbo].[uspm_perf_cpudetail]  
as   
set nocount on  
--1.Cpu_Task  
SELECT '查看cpu任务'  
SELECT scheduler_id, current_tasks_count, runnable_tasks_count  
FROM sys.dm_os_schedulers (nolock)  
WHERE scheduler_id < 255  
  
---2.CUP_USING  
SELECT '查看cpu使用情况'  
declare @ts_now bigint  
 --select @ts_now = cpu_ticks / convert(float, cpu_ticks_in_ms) from sys.dm_os_sys_info (nolock)  
 select @ts_now = cpu_ticks/(cpu_ticks/ms_ticks) from sys.dm_os_sys_info (nolock)  
 
 select top 50 record_id,  
  dateadd(ms, -1 * (@ts_now - [timestamp]), GetDate()) as EventTime,   
  SQLProcessUtilization as [CPU使用率,不可以始终处于高位],  
  SystemIdle,  
  100 - SystemIdle - SQLProcessUtilization as OtherProcessUtilization  
 from (  
  select   
   record.value('(./Record/@id)[1]', 'int') as record_id,  
   record.value('(./Record/SchedulerMonitorEvent/SystemHealth/SystemIdle)[1]', 'int') as SystemIdle,  
   record.value('(./Record/SchedulerMonitorEvent/SystemHealth/ProcessUtilization)[1]', 'int') as SQLProcessUtilization,  
   timestamp  
  from (  
   select timestamp, convert(xml, record) as record   
   from sys.dm_os_ring_buffers  (nolock)  
   where ring_buffer_type = N'RING_BUFFER_SCHEDULER_MONITOR'  
   and record like '%<SystemHealth>%') as x  
  ) as y   
 order by record_id desc  
  
  
--3.CPU_NECK  
SELECT '查看CPU瓶颈'  
select cast([signal_wait_time_ms] as decimal(30,2))/[wait_time_ms] as [百分比] ,*   
from sys.dm_os_wait_stats (nolock)   
where [wait_time_ms]<>0    
and cast([signal_wait_time_ms] as decimal(30,2))>([wait_time_ms]*0.25 )  
order by 1 desc  
  
SELECT '查看百分比是否>10%,如果大于10%,考虑降低并行度'  
select cast([signal_wait_time_ms] as decimal(30,2))/[wait_time_ms] as [百分比],*   
from sys.dm_os_wait_stats (nolock)  
where [wait_time_ms]<>0 AND  wait_type='CXPACKET'
GO

 


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