SQLDBA- SQL Server Wait Events

 

To solve slowness of SQL Server database, you should find which wait events exists in the database.

You can find wait events of database with following query.

with waits as
(select
wait_type,
wait_time_ms / 1000.0 as waits,
(wait_time_ms - signal_wait_time_ms) / 1000.0 as resources,
signal_wait_time_ms / 1000.0 as signals,
waiting_tasks_count as waitcount,
100.0 * wait_time_ms / sum (COALESCE (wait_time_ms,1)) over() as percentage,
row_number() over(order by wait_time_ms desc) as rownum
from sys.dm_os_wait_stats
where wait_type not in (
N'CLR_SEMAPHORE', N'LAZYWRITER_SLEEP',
N'RESOURCE_QUEUE', N'SQLTRACE_BUFFER_FLUSH',
N'SLEEP_TASK', N'SLEEP_SYSTEMTASK',
N'WAITFOR', N'HADR_FILESTREAM_IOMGR_IOCOMPLETION',
N'CHECKPOINT_QUEUE', N'REQUEST_FOR_DEADLOCK_SEARCH',
N'XE_TIMER_EVENT', N'XE_DISPATCHER_JOIN',
N'LOGMGR_QUEUE', N'FT_IFTS_SCHEDULER_IDLE_WAIT',
N'BROKER_TASK_STOP', N'CLR_MANUAL_EVENT',
N'CLR_AUTO_EVENT', N'DISPATCHER_QUEUE_SEMAPHORE',
N'TRACEWRITE', N'XE_DISPATCHER_WAIT',
N'BROKER_TO_FLUSH', N'BROKER_EVENTHANDLER',
N'FT_IFTSHC_MUTEX', N'SQLTRACE_INCREMENTAL_FLUSH_SLEEP',
N'DIRTY_PAGE_POLL', N'SP_SERVER_DIAGNOSTICS_SLEEP')
)
select
w1.wait_type as waittype, 
cast (w1.waits as decimal(14, 2)) wait_s,
cast (w1.resources as decimal(14, 2)) resource_s,
cast (w1.signals as decimal(14, 2)) signal_s,
w1.waitcount wait_count,
cast (w1.percentage as decimal(4, 2)) percentage,
cast ((w1.waits / w1.waitcount) as decimal (14, 4)) avgWait_s,
cast ((w1.resources / w1.waitcount) as decimal (14, 4)) avgResource_s,
cast ((w1.signals / w1.waitcount) as decimal (14, 4)) avgSignal_s
from waits as w1
inner join waits as w2 on w2.rownum <= w1.rownum
group by w1.rownum, w1.wait_type, w1.waits, w1.resources, w1.signals, w1.waitcount, w1.percentage
having sum (w2.percentage) - w1.percentage < 95; -- percentage threshold

You can get list and status of currently waiting tasks with following script.

select wt.session_id, wt.exec_context_id, wt.wait_duration_ms, wt.wait_type, wt.blocking_session_id, wt.resource_address, wt.resource_description, s.program_name, st.text, sp.query_plan, s.cpu_time cpu_time_ms, s.memory_usage*8 memory_usage_kb
from sys.dm_os_waiting_tasks wt
join sys.dm_exec_sessions s on s.session_id=wt.session_id
join sys.dm_exec_requests r on r.session_id=s.session_id
outer apply sys.dm_exec_sql_text(r.sql_handle) st
outer apply sys.dm_exec_query_plan(r.plan_handle) sp
where s.is_user_process=1
order by wt.session_id, wt.exec_context_id

You can find wait events of database with following query.

SELECT getdate() as 'Run_Time' --script running time
    , wait_type --wait type
    ,waiting_tasks_count
, CAST(wait_time_ms / 1000. AS DECIMAL(12, 2)) AS wait_time_s --saniye cinsinden bekleme zamaný
, CAST(100. * wait_time_ms / SUM(wait_time_ms) OVER() AS DECIMAL(12, 2)) AS pct --toplam beklemeye oraný
FROM sys.dm_os_wait_stats
WHERE wait_type NOT IN ('BROKER_TASK_STOP','Total','SLEEP','BROKER_EVENTHANDLER','BROKER_RECEIVE_WAITFOR',
      'BROKER_TRANSMITTER','CHECKPOINT_QUEUE','CHKPT,CLR_AUTO_EVENT','CLR_MANUAL_EVENT','KSOURCE_WAKEUP','LAZYWRITER_SLEEP',
      'LOGMGR_QUEUE','ONDEMAND_TASK_QUEUE','REQUEST_FOR_DEADLOCK_SEARCH','RESOURCE_QUEUE','SERVER_IDLE_CHECK',
      'SLEEP_BPOOL_FLUSH','SLEEP_DBSTARTUP','SLEEP_DCOMSTARTUP','SLEEP_MSDBSTARTUP','SLEEP_SYSTEMTASK','SLEEP_TASK',
      'SLEEP_TEMPDBSTARTUP','SNI_HTTP_ACCEPT','SQLTRACE_BUFFER_FLUSH','TRACEWRITE','WAIT_FOR_RESULTS','WAITFOR_TASKSHUTDOWN',
       'XE_DISPATCHER_WAIT','XE_TIMER_EVENT','WAITFOR')
ORDER BY 4 DESC