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sys.dm_os_waiting_tasks (Transact-SQL)

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. For more information, see sys.dm_os_wait_stats.
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 blocking the request. If this column is NULL, the request isn't blocked, or the session information of the blocking session isn't available (or can't 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 couldn't 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 being consumed. For more information, see The resource_description column.
pdw_node_id int The identifier for the node that this distribution is on.

Applies to: Azure Synapse Analytics and Analytics Platform System (PDW)

The 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>)

XACT (transaction) resource owner

Occurs when optimized locking is enabled:

  • xactlock: xactlock xdesIdLow=<xdesIdLow> xdesIdHigh=<xdesIdHigh> dbid=<dbid> id=<resource id> mode=<mode> UnderlyingResource (<keylock|ridlock>) hobtId=<hobtId> dbid=<dbid>

Permissions

SQL Server 2019 (15.x) and earlier versions require VIEW SERVER STATE permission.

SQL Server 2022 (16.x) and later versions, and SQL Server and Azure SQL Managed Instance, require VIEW SERVER PERFORMANCE STATE permission on the server.

On Azure SQL Database Basic, S0, and S1 service objectives, and for databases in elastic pools, the server admin account, the Microsoft Entra 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.

Examples

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 AS owt
     INNER JOIN sys.dm_exec_sessions AS es
         ON owt.session_id = es.session_id
     INNER JOIN sys.dm_exec_requests AS er
         ON es.session_id = er.session_id
OUTER APPLY sys.dm_exec_sql_text(er.sql_handle) AS est
OUTER APPLY sys.dm_exec_query_plan(er.plan_handle) AS eqp
WHERE es.is_user_process = 1
ORDER BY owt.session_id;
GO

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