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LCK_M_U

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Description:

This wait type is when a thread is waiting to acquire an Update lock on a resource and there is at least one other lock in an incompatible mode granted on the resource to a different thread.

General locking information:

(Books Online description: “Occurs when a task is waiting to acquire an Update lock.”)

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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 4 in all versions through 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:

General guidance around troubleshooting lock waits:

  • It is not possible to determine the lock resource from the sys.dm_os_wait_stats output. You can see the resource from sys.dm_os_waiting_tasks (using my script) or looking at the resource_description field of sys.dm_tran_locks where the request_status is ‘WAIT’.
  • You can use the blocked process report to get more detailed information on queries that are waiting for locks for a specified threshold (see here).
  • Look to see what is at the head of the blocking chain (i.e. the thread that’s holding the lock that’s blocking everyone) using a script (plenty of them available online – I don’t have a preferred one). What is that thread waiting for? Fixing that wait may help unravel the blocking. For example, a thread may be holding locks and committing a transaction, but there’s a synchronous mirror with a slow I/O subsystem so the mirror log write takes a long time, making the transaction commit take longer, and the locks take longer to be released, causing blocking.
  • Look for lock escalation, where an UPDATE transaction has escalated to a table X lock, causing widespread blocking.
  • Look for index operations causing table locks, and consider using online index operations (or if already using them, consider the WAIT_AT_LOW_PRIORITY feature in 2014+).
  • Look for code that specifies a TABLOCK (causes a table Shared lock) or TABLOCKX (causes a table Exclusive lock) hint.
  • Look for application code that will cause locks to be acquired and then waits for user input, or fails to commit a transaction for a long time.
  • Consider creating nonclustered indexes to remove row locks from the underlying heap/clustered index.
  • Consider using snapshot isolation or read committed snapshot isolation to allow readers to not take S/IS locks and reduce blocking.
  • Check the correct isolation level is being used as REPEATABLE_READ and SERIALIZABLE will hold S/IS locks until the end of a transaction.
  • Check for accidental use of the SERIALIZABLE isolation level, from using distributed transactions or incorrectly scoped .Net TransactionScope objects.

Specific guidance for LCK_M_U waits:

  • For an Update lock, the resource could be a row, page, or database.
  • Common blockers are a table X (Exclusive) lock from lock escalation occurring, or a SCH_M (Schema Modification) lock from an index build/rebuild.
  • As this lock is in preparation for conversion to an Exclusive lock, troubleshoot as if you were troubleshooting a row/page Exclusive lock, but be aware that an Update lock is compatible with a Share lock.

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

  1. Waiting for an Update lock on a row (in this case, when updating a range of rows in a single statement)
  2. Waiting for an Update lock on a database (in this case, when processing a file autogrow)

And other similar call stacks.

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

  1. SOS_Task::PostWait+9e
    EventInternal<SuspendQueueSLock>::Wait+1fb
    LockOwner::Sleep+485
    lck_lockInternal+10ed
    GetLock+1d8
    BTreeRow::AcquireLock+210
    IndexRowScanner::AcquireNextRowLock+ef
    IndexDataSetSession::GetNextRowValuesInternal+1265
    RowsetNewSS::FetchNextRow+197
    CQScanTableScanNew::GetRow+f2
    CQScanSpoolNew::LoadSpool+4f
    CQScanSpoolNew::Open+102
    CQScanNew::OpenHelper+41
    CQScanUpdateNew::Open+179
    CQueryScan::StartupQuery+23c
    CXStmtQuery::SetupQueryScanAndExpression+2e5
    CXStmtQuery::InitForExecute+34
    CXStmtQuery::ErsqExecuteQuery+36d
    CXStmtDML::XretDMLExecute+31c
    CXStmtDML::XretExecute+ad
    CMsqlExecContext::ExecuteStmts<0,1>+8bd
    CMsqlExecContext::FExecute+a58
  2. SOS_Task::PostWait+90
    EventInternal<SuspendQueueSLock>::Wait+1f9
    LockOwner::Sleep+495
    lck_lockInternal+ead
    XactWorkspaceImp::GetDBLockLocal+1c2
    XactWorkspaceImp::GetDBLock+59
    AutoDbLock::Acquire+5f
    TaskFileSizeChange::ProcessTskPkt+a8
    OnDemandTaskContext::ProcessTskPkt+285
    SystemTaskEntryPoint+431
    OnDemandTaskContext::FuncEntryPoint+25
    SOS_Task::Param::Execute+21e
    SOS_Scheduler::RunTask+a8