Hi guys, It is difficult to describe the load: it is a Rails app with many moving parts. By "loading" I mean I am sending 10 to 20 requests at once (and repeat) just to see how it reacts. I have been able to "track" the key time consuming parts in my code. There are three of them: - one linked to DB requests - one linked to heavy computations (some optimization algos) - the third one: the thread creation above. The more I load, the more time each of these take - it is very correlated. I know how to optimize the first two, but not the last one. It doesn't seem there is anything I can do (besides switching to a different Ruby which I can't unfortunately). But Robert it seems you are right: it is just CPU intensive. I had a chance to add another box to the cluster this week-end and it makes a huge difference. I just did not realize that creating threads would be that CPU intensive. Anyway thanks a lot for your help on this. Pierre -- Posted via http://www.ruby-forum.com/.