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EE_PMOLOCK

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Description:

This wait type is when a thread is waiting for access to a thread-safe memory object used during query execution by the Query Execution portion of the Query Processor.

(Books Online description: “Occurs during synchronization of certain types of memory allocations during statement execution.”)

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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 280 in 2008 and 2008 R2, 287 in 2012, and 294 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 point of contention.

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

  1. Waiting for access to a memory object (in this case, while allocating memory to hold details of columns while sending TDS column formats to a client)

And other similar call stacks.

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

  1. SOS_Task::PostWait+6f
    UnfairRecursiveMutexInternal<SuspendQueueSLock,0>::LongWait+179
    CPmoLock::GetAccessPmo+97
    CColMetaDataCached::PTdsColBufs+69
    CPostHydraTds::SendColMetaData+1f9
    CTdsConn<0>::SendMetaImpl+ac
    CStmtQuery::SendFormatsODS+8c
    CXStmtQuery::FSendFormatsAndExit+84
    CXStmtSelect::XretExecute+250
    CMsqlExecContext::ExecuteStmts<1,0>+15fb
    CMsqlExecContext::FExecute+aa5
    CSQLSource::Execute+981
    process_request+e61
    process_commands_internal+2df
    process_messages+253
    SOS_Task::Param::Execute+231
    SOS_Scheduler::RunTask+aa