How to Fix RESOURCE_SEMAPHORE Wait Type in SQL Server?
RESOURCE_SEMAPHORE waits occurs when a query memory request cannot be granted immediately due to other concurrent queries. High waits and wait times may indicate excessive number of concurrent queries, or excessive memory request amounts.
High waits on RESOURCE_SEMAPHORE usually result in poor response times for all database users, and need to be addressed.
It is also useful to correlate high waits on RESOURCE_SEMAPHORE with the Memory Grants Pending and Memory Grants Outstanding SQL Memory Manager performance counters. Higher values for these counters indicate a definite memory problem especially a non-zero value for Memory Grants Pending.
Resource Semaphore is a SQL Server internal mechanism that allows a query to reserve memory only if there is enough free memory available otherwise, a requesting query is forced to wait in queue. When resource semaphore receives new request, it first checks if any query is waiting or not. If any query is waiting in queue then the new request will also be added to the queue based on first come first serve allocation. All these requests in queue will show you waiting with RESOURCE_SEMAPHORE wait type. Once enough memory will be available then memory will be granted to the requests in an order they have been queued.
The root cause of this type of memory problem is when memory-intensive queries, such as those involving sorting and hashing, are queued and are unable to obtain the requested memory. The solution would be to tune the offending queries, or manage their workload so that they are executed at less busy times.
SQL Server has provided few DMVs to identify and fix such memory issues. You can have a look at DMV sys.dm_exec_query_resource_semaphore that will show the current status of resource semaphore . You can also see the output of DMV sys.dm_exec_query_memory_grants to display all queries that have memory granted and those who are waiting to be granted the required memory. All waiting queries would have null value in grant_time column in the output of DMV sys.dm_exec_query_memory_grants.
You can get the queries that are causing or eating most of the memory using above DMVs and from here you can look at the options of optimizing the memory intensive statements. You can also have a look at below article if your system is facing insufficient memory issue.
- SQL Server Error 701 and Error 802: There is Insufficient System Memory to Run this Query
- Understanding SQL Server Memory Allocation and Role of MIN/MAX Server Memory Configuration
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