Starting with the Strategy Intelligence Server 9.x there is a new Performance Counter which allows users to monitor the memory usage for Working Sets via the counter titled "Working set cache RAM usage".
In the Strategy Diagnostics and Performance Logging application, go to the Performance Configuration tab under the 'Strategy Server Users' Category as shown below:
The working set memory and its impact on user sessions is explained in the Technical note KB10138 "How are the concepts of Working Set, Working Set Result Pool, History List related to each other in Strategy Intelligence Server?".
