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sys.dm_os_waiting_tasks

One can run lots of interesting queries using this view. You can even use this view to perform deadlock detection that is not resolvable by deadlock monitor, DM. For example if you have tasks waiting on external resources such as extended stored procedures and blocking others from running. This type of deadlock DM can’t detect but you can!

 

  1. Q. How many tasks are currently waiting?

select

count(*)

from

sys.dm_os_waiting_tasks

 

This query will give you an idea of how many tasks are waiting in the system. You can use this information to understand blocking characteristics of your load

 

  1. Q. How many tasks that assigned to a worker (thread/fiber) are waiting?

select

      count(*)

from

      sys.dm_os_waiting_tasks

where

      wait_type <> 'THREADPOOL’

 

This query shows how many threads are actively running in the system. Latter on I will show how to find out if number of threads can be increased

 

  1. What are the tasks waiting on?

select

      wait_type,

      count (*)

from

      sys.dm_os_waiting_tasks

group by

      wait_type

order by

      count (*) desc

One can use this query to investigate possible bottlenceks of an active load. This query groups tasks by wait type – it can’t be directly use to identify the actual bottlenecks on the system. The query gives you an idea about the wait characteristics of your load

  1. Q. Does my load have an active resource bottleneck?

You can answer this question by looking at the resource address that tasks are blocked on. Keep in mind that not all wait types have resource associated with them.

select

      resource_address,

      count (*)

from

      sys.dm_os_waiting_tasks

WHERE

      resource_address <> 0

group by

      resource_address

order by

      count (*) desc

 

  1. Q: Is my system can be possibly bottlenecked on I/O?

You can answer this question by looking at the wait type of tasks waiting on specifically you are interested in IO waits

select

      *

from

      sys.dm_os_waiting_tasks

where

      wait_duration_ms > 20 AND

      wait_type LIKE '%PAGEIOLATCH%'

 

You might want to change 20ms base on your I/O subsystem

  1. Q: Does my load have long waiting chains?

 

This information is particular interesting to understand if a single tasks, for example one that generated long I/O, blocks others. If this happens you will have a way to improve your scalability by figuring how to remove or minimize chain length.

WITH TaskChain (

waiting_task_address,

blocking_task_address,

ChainId,

Level)

AS

(

-- Anchor member definition: use self join so that we output

-- Only tasks that blocking others and remove dupliates

 SELECT DISTINCT

      A.waiting_task_address,

      A.blocking_task_address,

      A.waiting_task_address As ChainId,

    0 AS Level

FROM

      sys.dm_os_waiting_tasks as A

JOIN

      sys.dm_os_waiting_tasks as B

ON

      A.waiting_task_address = B.blocking_task_address

WHERE

      A.blocking_task_address IS NULL

UNION ALL

-- Recursive member definition: Get to the next level waiting

-- tasks

SELECT

      A.waiting_task_address,

      A.blocking_task_address,

      B.ChainId,

      Level + 1

from

      sys.dm_os_waiting_tasks AS A

JOIN

      TaskChain AS B

ON

      B.waiting_task_address = A.blocking_task_address

)

select

      waiting_task_address,

      blocking_task_address,

      ChainId,

      Level

from

      TaskChain

order by

      ChainId

If there are no chains, your load is not CPU bound and you see long waits on THREADPOOL, you might improve your throughput by increasing a number of threads in the system.

 

Keep in mind that you can extend this query to perform your own deadlock detection.

 

You can also find out more information about each individual wait_type here

https://msdn2.microsoft.com/en-us/library/ms179984.aspx

Comments

  • Anonymous
    November 19, 2006
    Slava, you provide great information not provided anywhere else, I love your blog, and I'm eager to see what else you want to share around dmv's: especially the wait stuff. What I terribly miss in SQLserver is wait information accumulated per session, and the possibility of wait information being traced.  Like this view, it shows only present waiting sessions, but there is no history, you cannot distinguish the (waitable) resource consuming sessions from 'well behaving' ones. Unless you take very frequent samples which can be a costly thing to do on busy systems. When will SqlServer take that next step and collect that waitinfo per session or batch (sqlserver seems to collect other info per batch also..)? good luck! Mario

  • Anonymous
    September 01, 2009
    Slava, I am trying to get the same current wait events but can only get the ones that have finished waiting in SQL Server 2000: DBCC SQLPERF ( WAITSTATS ) WITH TABLERESULTS , NO_INFOMSGS Do you have any idea if I can get the current wait events as with sys.dm_os_waiting_tasks and not as sys.dm_os_wait_stats? Thanks

  • Anonymous
    September 01, 2009
    Sorry for asking again, but what I meant is: is there an equivalent to sys.dm_os_waiting_tasks in SQL Server 2000?