PREEMPTIVE_OS_WAITFORSINGLEOBJECT

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Description:

This wait type is when a thread is calling the Windows WaitForSingleObject function for synchronization with an external client process that is communicating using that object.

(Books Online description: N/A)

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Added in SQL Server version:

2008

Removed in SQL Server version:

N/A

Extended Events wait_type value:

The map_key value in sys.dm_xe_map_values is 496 in 2008 and 2008 R2, 544 in 2012, and 560 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:

This wait type is commonly seen in conjunction with ASYNC_NETWORK_IO, depending on the network transport used to communicate with the client, so to troubleshoot, follow the same steps as for ASYNC_NETWORK_IO.

Note that when a thread calls out to Windows, the thread changes from non-preemptive (SQL Server controls the thread) to preemptive (Windows controls the thread) mode. The thread’s state will be listed as RUNNING, as SQL Server doesn’t know what Windows is doing with the thread.

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

1. Waiting for synchronization in Windows (in this case, while closing a TCP connection from a client)
2. Waiting for synchronization in Windows (in this case, while sending a TDS packet to a client)

And many more similar call stacks.

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

1. SOS_Task::PopWait+ba
   AutoSwitchPreemptive::~AutoSwitchPreemptive+6e
   Tcp::Close+374
   CryptoBase::Close+9b
   SNIClose+7d
   CNetConnection::CloseNetlibEP+96
   CNetConnection::KillConnection+1da
   CPhysicalConnection::NotifySessionLogout+f1
   CSession::LogoutSession+671
   CSession::RemoveBatch+da
   process_close_connection+be
   process_commands+674
   SOS_Task::Param::Execute+12a
   SOS_Scheduler::RunTask+96
2. SOS_Task::PopWait+ba
   AutoSwitchPreemptive::~AutoSwitchPreemptive+6e
   Np::StatusWriteNoComplPort+290
   SNIStatusWriteNoComplPort+82
   TDSSNIClient::WriteStatus+7e
   write_data+229
   flush_buffer+f0
   ‘anonymous namespace’::TSendRowClass<6,0,0>::TSendNVN+b0
   CKatmaiTds::SendRowImpl+4620
   CValOdsRow::SetDataX+3b
   CEs::GeneralEval+1d9
   CXStmtQuery::ErsqExecuteQuery+700
   CXStmtSelect::XretExecute+2dd
   CMsqlExecContext::ExecuteStmts<1,1>+375