在项目运行的过程中,死锁不可能完全避免,但要尽可能减少死锁的出现,
产生死锁的原因主要是:
1,系统资源不足。
2,进程运行推进的顺序不合适。
3,资源分配不当等。
产生死锁的四个必要条件:
- 互斥条件:一个资源每次只能被一个进程使用,即在一段时间内某资源仅为一个进程所占有。此时若有其他进程请求该资源,则请求进程只能等待。
- 请求与保持条件:进程已经保持了至少一个资源,但又提出了新的资源请求时,该资源已被其他进程占有,此时请求进程被阻塞,但对自己已获得的资源保持不放。
- 不可剥夺条件:已经分配的资源不能从相应的进程中被强制地剥夺。
- 循环等待条件: 系统中若干进程组成环路,该环路中每个进程都在等待相邻进程正占用的资源。
这四个条件是死锁的必要条件,只要系统发生死锁,这些条件必然成立,而只要上述条件之一不满足,就不会发生死锁。
排查死锁是有哪个SQL导致的,死锁产生后即消失,很难让用户重现死锁问题,虽然可以从日志中分析死锁,但非常繁琐,可以利用下面的SQL SERVER 扩展事件,查询历史死锁,查询原因:
/**************************************************************** * 检查近期死锁;定位到具体的对象。方便排查问题; ********************************************************************/ DECLARE @SessionName SysName SELECT @SessionName = 'system_health' IF OBJECT_ID('tempdb..#Events') IS NOT NULL BEGIN DROP TABLE #Events END DECLARE @Target_File NVarChar(1000) , @Target_Dir NVarChar(1000) , @Target_File_WildCard NVarChar(1000) SELECT @Target_File = CAST(t.target_data as XML).value('EventFileTarget[1]/File[1]/@name', 'NVARCHAR(256)') FROM sys.dm_xe_session_targets t INNER JOIN sys.dm_xe_sessions s ON s.address = t.event_session_address WHERE s.name = @SessionName AND t.target_name = 'event_file' SELECT @Target_Dir = LEFT(@Target_File, Len(@Target_File) - CHARINDEX('\', REVERSE(@Target_File))) SELECT @Target_File_WildCard = @Target_Dir + '\' + @SessionName + '_*.xel' --Keep this as a separate table because it's called twice in the next query. You don't want this running twice. SELECT DeadlockGraph = CAST(event_data AS XML) , DeadlockID = Row_Number() OVER(ORDER BY file_name, file_offset) INTO #Events FROM sys.fn_xe_file_target_read_file(@Target_File_WildCard, null, null, null) AS F WHERE event_data like '<event name="xml_deadlock_report%' ;WITH Victims AS ( SELECT VictimID = Deadlock.Victims.value('@id', 'varchar(50)') , e.DeadlockID FROM #Events e CROSS APPLY e.DeadlockGraph.nodes('/event/data/value/deadlock/victim-list/victimProcess') as Deadlock(Victims) ) , DeadlockObjects AS ( SELECT DISTINCT e.DeadlockID , ObjectName = Deadlock.Resources.value('@objectname', 'nvarchar(256)') FROM #Events e CROSS APPLY e.DeadlockGraph.nodes('/event/data/value/deadlock/resource-list/*') as Deadlock(Resources) ) SELECT * FROM ( SELECT e.DeadlockID , TransactionTime = Deadlock.Process.value('@lasttranstarted', 'datetime') , DeadlockGraph , DeadlockObjects = substring((SELECT (', ' + o.ObjectName) FROM DeadlockObjects o WHERE o.DeadlockID = e.DeadlockID ORDER BY o.ObjectName FOR XML PATH ('') ), 3, 4000) , Victim = CASE WHEN v.VictimID IS NOT NULL THEN 1 ELSE 0 END , SPID = Deadlock.Process.value('@spid', 'int') , ProcedureName = Deadlock.Process.value('executionStack[1]/frame[1]/@procname[1]', 'varchar(200)') , LockMode = Deadlock.Process.value('@lockMode', 'char(1)') , Code = Deadlock.Process.value('executionStack[1]/frame[1]', 'varchar(1000)') , ClientApp = CASE LEFT(Deadlock.Process.value('@clientapp', 'varchar(100)'), 29) WHEN 'SQLAgent - TSQL JobStep (Job ' THEN 'SQLAgent Job: ' + (SELECT name FROM msdb..sysjobs sj WHERE substring(Deadlock.Process.value('@clientapp', 'varchar(100)'),32,32)=(substring(sys.fn_varbintohexstr(sj.job_id),3,100))) + ' - ' + SUBSTRING(Deadlock.Process.value('@clientapp', 'varchar(100)'), 67, len(Deadlock.Process.value('@clientapp', 'varchar(100)'))-67) ELSE Deadlock.Process.value('@clientapp', 'varchar(100)') END , HostName = Deadlock.Process.value('@hostname', 'varchar(20)') , LoginName = Deadlock.Process.value('@loginname', 'varchar(20)') , InputBuffer = Deadlock.Process.value('inputbuf[1]', 'varchar(1000)') FROM #Events e CROSS APPLY e.DeadlockGraph.nodes('/event/data/value/deadlock/process-list/process') as Deadlock(Process) LEFT JOIN Victims v ON v.DeadlockID = e.DeadlockID AND v.VictimID = Deadlock.Process.value('@id', 'varchar(50)') ) X ORDER BY DeadlockID DESC
利用此脚本排查历史死锁很方便。