sys.dm_os_waiting_tasks (Transact-SQL)

Article
02/28/2023

Applies to: SQL Server Azure SQL Database Azure SQL Managed Instance Azure Synapse Analytics Analytics Platform System (PDW)

Returns information about the wait queue of tasks that are waiting on some resource. For more information about tasks, see the Thread and Task Architecture Guide.

Note

To call this from Azure Synapse Analytics or Analytics Platform System (PDW), use the name sys.dm_pdw_nodes_os_waiting_tasks. This syntax is not supported by serverless SQL pool in Azure Synapse Analytics.

Column name	Data type	Description
waiting_task_address	varbinary(8)	Address of the waiting task.
session_id	smallint	ID of the session associated with the task.
exec_context_id	int	ID of the execution context associated with the task.
wait_duration_ms	bigint	Total wait time for this wait type, in milliseconds. This time is inclusive of signal_wait_time.
wait_type	nvarchar(60)	Name of the wait type.
resource_address	varbinary(8)	Address of the resource for which the task is waiting.
blocking_task_address	varbinary(8)	Task that is currently holding this resource
blocking_session_id	smallint	ID of the session that is blocking the request. If this column is NULL, the request is not blocked, or the session information of the blocking session is not available (or cannot be identified). -2 = The blocking resource is owned by an orphaned distributed transaction. -3 = The blocking resource is owned by a deferred recovery transaction. -4 = Session ID of the blocking latch owner could not be determined due to internal latch state transitions.
blocking_exec_context_id	int	ID of the execution context of the blocking task.
resource_description	nvarchar(3072)	Description of the resource that is being consumed. For more information, see the list below.
pdw_node_id	int	Applies to: Azure Synapse Analytics, Analytics Platform System (PDW) The identifier for the node that this distribution is on.

resource_description column

The resource_description column has the following possible values.

Thread-pool resource owner:

threadpool id=scheduler<hex-address>

Parallel query resource owner:

exchangeEvent id={Port|Pipe}<hex-address> WaitType=<exchange-wait-type> nodeId=<exchange-node-id>

Exchange-wait-type:

e_waitNone
e_waitPipeNewRow
e_waitPipeGetRow
e_waitSynchronizeConsumerOpen
e_waitPortOpen
e_waitPortClose
e_waitRange

Lock resource owner:

<type-specific-description> id=lock<lock-hex-address> mode=<mode> associatedObjectId=<associated-obj-id>

<type-specific-description> can be:
- For DATABASE: databaselock subresource=<databaselock-subresource> dbid=<db-id>
- For FILE: filelock fileid=<file-id> subresource=<filelock-subresource> dbid=<db-id>
- For OBJECT: objectlock lockPartition=<lock-partition-id> objid=<obj-id> subresource=<objectlock-subresource> dbid=<db-id>
- For PAGE: pagelock fileid=<file-id> pageid=<page-id> dbid=<db-id> subresource=<pagelock-subresource>
- For Key: keylock hobtid=<hobt-id> dbid=<db-id>
- For EXTENT: extentlock fileid=<file-id> pageid=<page-id> dbid=<db-id>
- For RID: ridlock fileid=<file-id> pageid=<page-id> dbid=<db-id>
- For APPLICATION: applicationlock hash=<hash> databasePrincipalId=<role-id> dbid=<db-id>
- For METADATA: metadatalock subresource=<metadata-subresource> classid=<metadatalock-description> dbid=<db-id>
- For HOBT: hobtlock hobtid=<hobt-id> subresource=<hobt-subresource> dbid=<db-id>
- For ALLOCATION_UNIT: allocunitlock hobtid=<hobt-id> subresource=<alloc-unit-subresource> dbid=<db-id>
<mode> can be:

Sch-S, Sch-M, S, U, X, IS, IU, IX, SIU, SIX, UIX, BU, RangeS-S, RangeS-U, RangeI-N, RangeI-S, RangeI-U, RangeI-X, RangeX-, RangeX-U, RangeX-X

External resource owner:

External ExternalResource=<wait-type>

Generic resource owner:

TransactionMutex TransactionInfo Workspace=<workspace-id>
Mutex
CLRTaskJoin
CLRMonitorEvent
CLRRWLockEvent
resourceWait

Latch resource owner:

<db-id>:<file-id>:<page-in-file>
<GUID>
<latch-class> (<latch-address>)

Permissions

On SQL Server and SQL Managed Instance, requires VIEW SERVER STATE permission.

On SQL Database Basic, S0, and S1 service objectives, and for databases in elastic pools, the server admin account, the Azure Active Directory admin account, or membership in the ##MS_ServerStateReader## server role is required. On all other SQL Database service objectives, either the VIEW DATABASE STATE permission on the database, or membership in the ##MS_ServerStateReader## server role is required.

Permissions for SQL Server 2022 and later

Requires VIEW SERVER PERFORMANCE STATE permission on the server.

Example

A. Identify tasks from blocked sessions.

SELECT * FROM sys.dm_os_waiting_tasks 
WHERE blocking_session_id IS NOT NULL;

B. View waiting tasks per connection

SELECT st.text AS [SQL Text], c.connection_id, w.session_id, 
  w.wait_duration_ms, w.wait_type, w.resource_address, 
  w.blocking_session_id, w.resource_description, c.client_net_address, c.connect_time
FROM sys.dm_os_waiting_tasks AS w
INNER JOIN sys.dm_exec_connections AS c ON w.session_id = c.session_id 
CROSS APPLY (SELECT * FROM sys.dm_exec_sql_text(c.most_recent_sql_handle)) AS st 
              WHERE w.session_id > 50 AND w.wait_duration_ms > 0
ORDER BY c.connection_id, w.session_id
GO

C. View waiting tasks for all user processes with additional information

SELECT 'Waiting_tasks' AS [Information], owt.session_id,
	owt.wait_duration_ms, owt.wait_type, owt.blocking_session_id,
	owt.resource_description, es.program_name, est.text,
	est.dbid, eqp.query_plan, er.database_id, es.cpu_time,
	es.memory_usage*8 AS memory_usage_KB
FROM sys.dm_os_waiting_tasks owt
INNER JOIN sys.dm_exec_sessions es ON owt.session_id = es.session_id
INNER JOIN sys.dm_exec_requests er ON es.session_id = er.session_id
OUTER APPLY sys.dm_exec_sql_text (er.sql_handle) est
OUTER APPLY sys.dm_exec_query_plan (er.plan_handle) eqp
WHERE es.is_user_process = 1
ORDER BY owt.session_id;
GO