The total memory allocated during system start-up is the sum of memory required for all the configuration needs of Adaptive Server. This value can be obtained from the read-only configuration parameter total logical memory. This value is calculated by Adaptive Server. The configuration parameter max memory must be greater than or equal to total logical memory. max memory indicates the amount of memory you will allow for Adaptive Server needs.
During server start-up, by default, Adaptive Server allocates memory based on the value of total logical memory. However, if the configuration parameter allocate max shared memory has been set, then the memory allocated will be based on the value of max memory. The configuration parameter allocate max shared memory enables a system administrator to allocate the maximum memory that is allowed to be used by Adaptive Server, during server start-up.
The key points for memory configuration are:
The system administrator should determine the size of shared memory available to Adaptive Server and set max memory to this value.
The configuration parameter allocate max shared memory can be turned on during start-up and runtime to allocate all the shared memory up to max memory with the least number of shared memory segments. A large number of shared memory segments has the disadvantage of some performance degradation on certain platforms. Check your operating system documentation to determine the optimal number of shared memory segments. Once a shared memory segment is allocated, it cannot be released until the server is restarted.
The difference between max memory and total logical memory determines the amount of memory available for the procedure and statement caches, data caches, or other configuration parameters.
The amount of memory Adaptive Server allocates during start-up is determined by either total logical memory or max memory. If you set alloc max shared memory to 1, Adaptive Server uses the value for max memory.
If either total logical memory or max memory is too high:
Adaptive Server may not start if the physical resources on your machine are not sufficient.
If it does start, the operating system page fault rates may rise significantly and the operating system may need to be reconfigured to compensate.
