G2, RAM, and Virtual Memory
Virtual memory can provide the effect of very large amounts of real memory but the benefit in space is bought with a sacrifice in time. Virtual memory is much slower than real memory, because it requires paging data between real memory and the disk.
Virtual memory systems page only when they must. If you have enough RAM to hold all executing programs in real memory, none of them will ever be paged. If G2 needs more than the amount of RAM your computer provides, or some other program competes with G2 for that memory, the operating system will page G2 as needed to provide the necessary memory virtually.
Moderate amounts of paging do not typically cause significant performance problems. However, incessant paging (thrashing) caused by lack of RAM can drastically degrade performance. Gensym strongly advises against attempting to use G2 under such circumstances.
Determining RAM Requirements
To determine how much RAM you need, measure the total G2 memory requirement as described under Measuring Memory with Operating System Commands. You should have at least that much RAM in your machine, preferably at least 16 MB more, and in no event less than 48MB. See the G2 Software Installation Guide for further information.
If you intend to run other programs on the machine that runs G2, you should provide additional RAM as needed to prevent them from competing excessively with G2. Many installations provide monitoring tools that can be used to measure how different processes use virtual memory. These can be helpful in deciding how much more RAM to provide when G2 does not run alone.
Copyright © 1997 Gensym Corporation, Inc.