SLEEP_TASK

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Description:

This wait type is a general wait indicating that a thread is waiting for some event to occur, including background task scheduling, during hash spills to tempdb, during a buffer pool free list stall, and for some query plan exchange operators where the wait isn’t tracked by either CXPACKET or EXECSYNC waits.

(Books Online description: “Occurs when a task sleeps while waiting for a generic event to occur.”)

Questions/comments on this wait type? Click here to send Paul an email, especially if you have any information to add to this topic.

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 105 in 2008 and 2008 R2, and 109 in 2012 and 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:

Although this is generally a benign wait type that is usually filtered out of the results from sys.dm_os_wait_stats, you may see this wait type while looking at sys.dm_os_waiting_tasks output (using my script). If that’s the case, look for hash spills happening and troubleshoot those.

This wait type is one that I usually filter out as a benign wait when doing wait statistics analysis.

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

1. Waiting for an exchange event during a scan
2. Waiting for an exchange event during a stream aggregate
3. Waiting for a small amount of time while trying to create a new page for the version store, and the buffer pool has no available buffers
4. Waiting to determine whether to put the ghost cleanup task to sleep or to carry on using the processor if it’s idle

And many other similar call stacks.

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

1. SOS_Task::PostWait+9e
   SOS_Task::Sleep+149
   CXPipe::GetRow+277
   CXPipeMerge::GetRow+c0
   CQScanExchangeNew::GetRowHelper+19b
   CQScanTopNew::GetRow+171
   CQueryScan::GetRow+81
   CXStmtQuery::ErsqExecuteQuery+36d
   CXStmtSelect::XretExecute+2e7
   CMsqlExecContext::ExecuteStmts<1,1>+427
   CMsqlExecContext::FExecute+a43
   CSQLSource::Execute+86c
   process_request+a57
   process_commands+4a3
   SOS_Task::Param::Execute+21e
   SOS_Scheduler::RunTask+ab
2. SOS_Task::PostWait+9e
   SOS_Task::Sleep+149
   CXPipe::GetRow+277
   CXPipeMerge::ReleaseRow+91
   CXPipeMerge::GetRow+228
   CQScanExchangeNew::GetRowHelper+19b
   CQScanStreamAggregateNew::GetRowHelper+1fc
   CQScanUpdateNew::GetRow+76
   CQScanXProducerNew::GetRowHelper+366
   CQScanXProducerNew::GetRow+15
   FnProducerOpen+57
   FnProducerThread+8c3
   SubprocEntrypoint+a7f
   SOS_Task::Param::Execute+21e
   SOS_Scheduler::RunTask+ab
3. SOS_Task::PostWait+0x9e
   SOS_Task::Sleep+0x149
   BPool::Steal+0x84e
   BPool::NewPage+0x75b
   PageRef::FormatInternal+0x87
   AppendOnlyStorageUnit::SetupPageHeaderPreAllocation+0xaf
   AllocationReq::AllocateExtent+0x285
   AllocationReq::AllocatePages+0x19fa
   AllocationReq::Allocate+0xf3
   AppendOnlyStorageUnit::AllocateNewExtent+0x2b4
   AppendOnlyVerStoreMgr::AddRecord+0xe64
   sVersionMgr::GenerateVersion+0x6b3
   IndexPageRef::Modify+0x1293
   BTreeRow::UpdateRecord+0x20ab
   IndexDataSetSession::SetDataInternal+0x9a46
   DatasetSession::SetData+0x16d
   RowsetNewSS::SetData+0x6a
   CValRow::SetDataX+0x63
   CEsExec::GeneralEval4+0xe7
   CQScanSimpleUpdateNew::GetRow+0x16b
   CQueryScan::GetRow+0x81
   CXStmtQuery::ErsqExecuteQuery+0x3dd
4. SOS_Task::PostWait+9e
   SOS_Task::Sleep+149
   CanSuspendGhostCleanup+d8
   GhostRecordCleanupTask+1c1
   CGhostCleanupTask::ProcessTskPkt+9
   TaskReqPktTimer::ExecuteTask+63
   OnDemandTaskContext::ProcessTskPkt+3e2
   SystemTaskEntryPoint+426
   OnDemandTaskContext::FuncEntryPoint+25
   SOS_Task::Param::Execute+21e
   SOS_Scheduler::RunTask+ab