DEADLOCK_ENUM_MUTEX

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Description:

From the BOL description and call stack below, my guess is that the deadlock monitor’s search may be interrupted in some way if the sys.dm_os_waiting_tasks DMV is calculating the blocking_session_id for its output, so they synchronize on this mutex so only one or the other can be running at any time.

(Books Online description: “Occurs when the deadlock monitor and sys.dm_os_waiting_tasks try to make sure that SQL Server is not running multiple deadlock searches at the same time.”)

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Added in SQL Server version:

Pre-2005/2005

Removed in SQL Server version:

N/A

Extended Events wait_type value:

The map_key value in sys.dm_xe_map_values is 261 in 2008 and 2008 R2, 268 in 2012, and 275 in 2014 RTM. After 2014 RTM, you must check the DMV to get the latest value as some map_key values have changed in later builds.

Other information:

I have not seen this wait type be a noticeable contention point.

Known occurrences in SQL Server (list number matches call stack list):

1. Running the sys.dm_os_waiting_tasks DMV

Abbreviated call stacks (list number matches known occurrences list):

1. SOS_Task::PostWait+90
   EventInternal<SuspendQueueSLock>::Wait+1f9
   DeadlockMonitor::AcquireEnumerationMutex+8a
   WaitsTable::MoveToNextRow+cd
   WaitsTable::InternalGetRow+16
   CQScanTVFStreamNew::GetRow+d5
   CQScanFilterNew::GetRowHelper+8c
   CQScanSortNew::BuildSortTable+87
   CQScanSortNew::OpenHelper+c0
   CQScanNew::OpenHelper+41
   CQScanStreamAggregateNew::Open+30
   CQueryScan::UncacheQuery+295
   CXStmtQuery::SetupQueryScanAndExpression+2d6
   CXStmtQuery::InitForExecute+34
   CXStmtQuery::ErsqExecuteQuery+36a
   CXStmtSelect::XretExecute+2f7
   CMsqlExecContext::ExecuteStmts<1,1>+400
   CMsqlExecContext::FExecute+a33
   CSQLSource::Execute+866
   process_request+73c
   process_commands+51c
   SOS_Task::Param::Execute+21e