Nginx is an open source web server written to address some of the performance and scalability issues associated with Apache. The product is open source and free, but Nginx offers support if you buy its Nginx Plus version. Apache can be configured to run in pre-forked or worker multi-process mode (MPM). Either way it creates new processes as additional users connect. The difference between the two is that pre-forked mode creates one thread per process, each of which handles one user request. Worker mode creates new processes too, but each has more than one thread, each of which handles one request per user. So one worker mode process handles more than one connection and one pre-fork mode process handles one connection only. Worker mode uses less memory than forked-mode, because processes consume more memory than threads, which are nothing more than code running inside a process. Moreover, worker mode is not thread safe. That means if you use non thread-safe modules like mod_php, to serve up php pages, you need to use pre-forked mode, thus consuming more memory. So, when choosing modules and configuration you have to confront the thread-versus-process optimization problem and constraint issues.