SPACEMGR_ALLOCEXTENT_CACHE

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Description:

This latch class is when a thread is waiting for access to the per-allocation unit extent cache, a cache of extents in the allocation unit where there is at least one page available for allocation. It is more efficient to have a small, in-memory cache of such extents, rather than having to search through the relevant IAM and PFS pages to find unallocated pages. See here and here for more information on IAM and PFS pages, respectively.

(Books Online description: none)

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Added in SQL Server version:

2012

Removed in SQL Server version:

N/A

Extended Events latch_class value:

Maps to the SPACEMGR_AEC map_value in sys.dm_xe_map_values.

The map_key value in sys.dm_xe_map_values is 83 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:

I have not seen this latch be a noticeable contention point.

See the LATCH_EX wait type for more information about latches in general and how to troubleshoot them.

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

1. Adding an extent to the extent cache (in this case, while deallocating unused pages from the final extent in a bulk allocation, which preallocates all pages in each allocated extent for efficiency and then cleans up any unused ones at the end)

And many more similar call stacks.

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

1. XeSqlPkg::latch_suspend_end::Publish+138
   LatchBase::Suspend+16b8
   LatchBase::AcquireInternal+415
   AutoAllocationCaches::AddExtentToAllocCache+110
   AddExtentToAllocCache+162
   DeAllocMgr::DeAllocatePageInternal+7ea
   ExtentAllocator::DeallocatePages+2c1
   ExtentAllocatorSingleAlloc::Cleanup+ae
   CHeapBuild::CleanUp+1c3
   RowsetBulk::EndUpdate+7d
   CEndUpdate::EndUpdate+d
   CQScanUpdateNew::GetRow+84e
   CQScanXProducerNew::GetRowHelper+366
   CQScanXProducerNew::GetRow+15
   FnProducerOpen+57
   FnProducerThread+8c3
   SubprocEntrypoint+a7f
   SOS_Task::Param::Execute+21e
   SOS_Scheduler::RunTask+a8