I learned the hard way that just rising it can lead to a hard performance loss :)
I looked back in the list archives to try to find your post on the underlying problem, but could only find this rather terse sentence. If you have more detailed information please post or point me at it. But...my first thought is that you have configured the shared memory region so large that the system as a whole can not fit all the working set sizes for all running processes in to physical memory. This is a common pitfall for databases with caches implemented as mapped shared user space regions (which is basically all databases). For example, if you have 1G of RAM on the box, you can't configure a cache of 900 meg and expect things to work well. This is because the OS and associated other stuff running on the box will use ~300megs. The system will page as a result. The only sure fire way I know of to find the absolute maximum cache size that can be safely configured is to experiment with larger and larger sizes until paging occurs, then back off a bit.
