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Exactly as described below.
(Books Online description: “Occurs when a task is currently performing CLR execution and is waiting for a semaphore.”)
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Added in SQL Server version:
Removed in SQL Server version:
Extended Events wait_type value:
The map_key value in sys.dm_xe_map_values is 291 in 2008 and 2008 R2, 334 in 2012, and 348 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.
This wait type is one that I usually filter out as a benign wait when doing wait statistics analysis.
I have not seen this wait type be a noticeable contention point. I would imagine that if this wait type becomes prevalent on your system, you should investigate your CLR code to determine if a thread could hold on to the resource controlled by the semaphore for a long time.
Known occurrences in SQL Server (list number matches call stack list):
- Waiting for a semaphore
And other similar call stacks, depending on the CLR assemblies in use.
Abbreviated call stacks (list number matches known occurrences list):