Analysis Services divides memory into two primary categories: shrinkable memory and non-shrinkable memory
An Example with the Intraday processing
This example is at the end of the sync of the staging cube to the live cube. This will clear out the data cache of the live cube
You can see in the below diagram that the Datacache decreases to nothing at 2:52PM
Once someone runs another query on the server after a sync, analysis services needs to go to the disk to get the information.
You can see from the below diagram at 2:54 it needed to read from disk and that the datacache at the same time starts building up again in the above diagram
Each of these explained (from ssas2005 perf guide)
When the cleaner is activated, it begins evicting elements of shrinkable memory, based on a cost/benefit algorithm that takes into account a variety of factors, including how frequently the entry is used, the amount of resources required to resolve the entries, and how much space is consumed by related entries. Shrinkable memory elements include the following:
| Cached Results | Cached results include the Storage Engine data cache and Query Execution Engine calculation cache. As stated earlier in this document, the Storage Engine data cache contains measure group data and the Query Execution Calculation Engine cache contains calculation results. While both caches can help improve query response times, the data cache provides the most benefit to query performance by storing data that has been cached from disk. In situations of memory pressure, the cleaner shrinks the memory used for cached results. With this in mind, it is a good practice to monitor the usage of memory so that you can minimize the scenarios where elevated levels of memory pressure force the removal of cached results. For more information on how to monitor memory pressure, see Monitoring memory management. |
| Paged in dimension data | Dimension data is paged in from the dimension stores as needed. The paged-in data is kept in memory until the cleaner is under memory pressure to remove it. Note that this is different behavior than previous versions of Analysis Services where all dimension data was resident in memory. |
| Expired Sessions | Idle client sessions that have exceeded a longevity threshold are removed by the cleaner based on the level of memory pressure. Several server properties work together to manage the longevity of idle sessions. For more information on how to evaluate these properties, see Monitoring the timeout of idle sessions. |
Non-shrinkable memory elements are not impacted by the Analysis Services cleaner.
Non-shrinkable memory includes the following components:
| Metadata | For each Analysis Services database, metadata is initialized and loaded into memory on demand. Metadata includes the definition of all objects in the database (not the data elements). The more objects in your database (including cubes, measure groups, partitions, and dimensions) and the more databases that you have on a given server, the larger the metadata overhead in memory. Note that this overhead is generally not large for most implementations. However, you can experience significant overhead if your Analysis Services server contains hundreds of databases with tens or hundreds of objects per database, such as in hosted solutions. For more information on how to monitor metadata overheard, see Minimizing metadata overhead. |
| Active Sessions | For each active session, calculated members, named sets, connections, and other associated session information is retained as non-shrinkable memory. |
| Query Memory and Process Memory | Analysis Services reserves specific areas of memory for temporary use during querying and processing. During the execution of a query, for example, memory may be used to materialize data sets such as during the cross joining of data. During processing, memory is used to temporarily store, index, and aggregate data before it are written to disk. These memory elements are non-shrinkable because they are only needed to complete a specific server operation. As soon as the operation is over, these elements are removed from memory. |
Links
http://download.microsoft.com/download/8/5/e/85eea4fa-b3bb-4426-97d0-7f7151b2011c/ssas2005perfguide.doc
http://download.microsoft.com/download/6/5/6/6567C845-FC8D-4D62-920F-C027A349C889/SSASPerfGuide2008R2.docx
