It is mostly custom. The load generator is written in C and using libCURL - http://curl.haxx.se/libcurl http://curl.haxx.se/libcurl 5,000 users is what one sin

It is mostly custom. The load generator is written in C and using libCURL - http://curl.haxx.se/libcurl http://curl.haxx.se/libcurl 5,000 users is what one single instance of the non-threaded load generator can simulate today. We have some optimizations we can make that should improve this though, and we are also going to implement multi-source/multi-program load generation that means we can distribute the load generation for a single test over several processes and several load generator hosts. Using only our own infrastructure I think we can scale the system quite a lot, but obviously, if we want to run really large simulations with several hundred thousand, or millions, of simulated users, then we need to buy cloud server capacity.