A deep dive into SQL Server Plan Cache Management

 Decrease in throughput or an increase in query response time  Out-of-memory errors or connection timeout errors  Query executions and/or query compiles timeout errors  WAITs  CMEMTHREAD wait type  RESOURCE_SEMAPHORE_QUERY_COMPILE  High CPU usage in SQL Server 2012 or 2014 due to plan cache contention  This is a result of  Large number of memory pressure events  Memory contention  Large number of compilations

 The plan cache is used to save all the execution plans in case they can be reused  Plan caching enables SQL Server to reuse plans to avoid recompilation  The total size of memory is dynamic by default, and the space used for query plans is also very fluid Plan Cache

The content of the plan cache, cache object types and caching mechanisms

 4 stores contain query plans:  Object Plans (CACHESTORE_OBJCP)  SQL Plans (CACHESTORE_SQLCP)  Bound Trees (CACHESTORE_PHDR)  Extended Stored Procedures (CACHESTORE_XPROC)  Stores and their sizes  sys.dm_os_memory_objects  sys.dm_exec_cached_plans  sys.dm_os_memory_cache_counters 6

 Determining the Cache Store Pressure Limit value SQL Server 2005 SP1 on a 64-bit SQL Server 28 GB TM The single cache store pressure limit:(75% of 8 GB) + (50% of 20GB) = 6GB + 10GB =16 GB SQL Server 2012 RTM 64-bit with 28 GB TM The single cache store pressure limit : (75% of 4 GB) + (10% of 24 GB)= 3GB + 2.4GB = 5.4 GB

 Local  When any single cache store grows too big  A cache store reaches 62.5 percent of it’s cache store pressure limit value  Global  External global pressure (from OS)  Internal global memory pressure can occur  when virtual address space is low  when the memory broker predicts that all cache stores combined will use more than 80 percent of the plan cache pressure limit SQL Server 2012 RTM 64-bit with 28 GB TM Internal Memory Pressure Event triggers at: 62.5% of The plan cache pressure limit =62.5%*5.4 GB=3.375GB. SQL Server 2012 RTM 64-bit with 28 GB TM The single cache store pressure limit : 5.4 GB Internal memory pressure is triggered at: 62.5*5.4GB=3.375GB

 Eviction policy Algorithm  zero-cost plans are removed from cache  the cost of all other plans is reduced by half  One cycle updates the cost of at most 16 entries for every cache store  Costing of cache entries  Adhoc plans  cost is considered to be zero, increased by 1 every time the plan is reused  Non-adhoc queries  based on three factors: I/O, context switches, and memory  On plan re-use the cost is reset to the original cost

Detecting memory pressure

 The number of plans in a store exceeds four times the hash table size for that store, regardless of the actual size of the plans - 160,036 plans in a specific cache store  Heavy contention for the SOS_CACHESTORE spinlock and high CPU usage on SQL Server 2012/2014  Resolution  Apply hotfix  Fixed in SQL Server 2012 SP1 CU 14  Fixed in SQL Server 2014 CU 6  To apply the change: -T174  Optimize for AdHoc=True  Also: -T8032 (be careful) - Reverts the cache limit parameters to the SQL Server 2005 RTM setting

 SQL Server: Plan Cache/Cache Pages(_Total)  SQLServer: BufferManager/Database pages  Detecting excessive compiles and recompiles:  SQL Statistics\SQL Compilations/sec  SQL Statistics\SQL Re-Compilations/sec  Memory Manager\Target Server Memory (KB)  Memory Manager\Total Server Memory (KB)

 Always specify Optimize for AdHoc=true  Always configure MAX SERVER MEMORY Setting  Monitor the size and usage of your plan cache  Know your thresholds  Check for wasted space amount on a regular basis  Be aware of some situations that cause multiple query plans for the same procedure to be saved in cache  A difference in certain SET options  A need to go trough the implicit name resolution process

 Apply CU and fixes for SQL Server 2012/2014 if you encounter high CPU due to spinlock contention problems  Apply –T8032 only after careful analysis, monitor the buffer pool after that  Sometimes periodically cleaning the Adhoc plan cache could work better if the problem is located in SQLPlans cache store  DBCC FREESYSTEMCACHE('SQL Plans') for usecount=1 and objtype IN ('Adhoc', 'Prepared')  Change the application (Yes, I know )

Questions?   

Explore Everything PASS Has to Offer Free SQL Server and BI Web Events Free 1-day Training Events Regional Event Local User Groups Around the World Free Online Technical Training This is CommunityBusiness Analytics Training Session Recordings PASS Newsletter