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Determining the Plan Cache Pressure Limit
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Cache Pressure Limit
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75 percent of visible target memory from 0 to 8 GB + 50 percent of visible target memory from 8 GB to 64 GB + 25 percent of visible target memory > 64 GB 75 percent of visible target memory from 0 to 4 GB + 10 percent of visible target memory from 4 GB to 64 GB + 5 percent of visible target memory > 64 GB 4 GB upper cap on the plan cache
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As an example, assume we are on SQL Server 2005 SP1 on a 64-bit SQL Server instance with 28 GB of target memory. The plan cache pressure limit would be 75 percent of 8 GB plus 50 percent of the target memory over 8 GB (or 50 percent of 20 GB), which is 6 GB + 10 GB, or 16 GB.
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On a 64-bit SQL Server 2008 RTM instance with 28 GB of target memory, the plan cache pressure limit would be 75 percent of 4 GB plus 10 percent of the target memory over 4 GB (or 10 percent of 24 GB), which is 3 GB + 2.4 GB, or 5.4 GB.
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Local Memory Pressure
If any single cache store grows too big, it indicates local memory pressure and SQL Server starts removing entries from only that store. This behavior prevents one store from using too much of the total system memory. If a cache store reaches 75 percent of the plan cache pressure limit, described in Table 9-3, in single-page allocations or 50 percent of the plan cache pressure limit in multipage allocations, internal memory pressure is triggered and plans are removed from cache. For example, in the situation described previously, we computed the plan cache pressure limit to be 5.4 GB. If any cache store exceeds 75 percent of that value, or 4.05 GB in single-page allocations, internal memory pressure is triggered. If adding a particular plan to cache causes the cache store to exceed the limit, the removal of other plans from cache happens on the same thread as the one adding the new plan, which can cause the response time of the new query to be increased. In addition to memory pressure occurring when the total amount of memory reaches a particular limit, SQL Server also indicates memory pressure when 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. As I mentioned previously when describing the cache stores, there are either about 10,000 or 40,000 buckets in these hash tables, for 32-bit and 64-bit systems, respectively. That means memory pressure can be triggered when either the SQL Store or the Object Store has more than 40,000 or 160,000 entries. The rst query shown here is one we saw earlier, and it can be used to determine the number of buckets in the hash tables for the Object Store and the SQL Store, and the second query returns the number of entries in each of those stores:
SELECT type as 'plan cache store', buckets_count FROM sys.dm_os_memory_cache_hash_tables WHERE type IN ('CACHESTORE_OBJCP', 'CACHESTORE_SQLCP'); GO SELECT type, count(*) total_entries FROM sys.dm_os_memory_cache_entries WHERE type IN ('CACHESTORE_SQLCP', 'CACHESTORE_OBJCP') GROUP BY type; GO
Prior to SQL Server 2008, internal memory pressure was rarely triggered due to the number of entries in the hash tables but was almost always initiated by the size of the plans in the cache store. However, in SQL Server 2008, if you have enabled Optimize for Ad Hoc Workloads, the actual entries in the SQL cache store may be quite small (each Compiled Plan Stub is about 300 bytes) so the number of entries can grow to exceed the limit before the
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