A year or two back I was working on a web application which was expected to have moderate use – around 50 concurrent users. The product was generally getting thumbs up from our QA guys. It did everything we expected it to do. Then we had a go at testing under load.
We found that if we had only a few users hammering the system for any length of time, the memory usage became unacceptable. Simple maths showed that the problem was to do with the number of open sessions. Each session required 20-30MB of memory from the app server. This is a piddly small amount when we have a handful of test users. It went completely unnoticed against the background noise of a typical server’s memory use. However, once just a hundred sessions have been opened (not necessarily at the same time) we’re chewing gigabytes at a time.