HAProxy implements an event-driven, mono-process model which enables support
for very high number of simultaneous connections at very high speeds.
Multi-process or multi-threaded models can rarely cope with thousands of
connections because of memory limits, system scheduler limits, and lock
contention everywhere. Event-driven models do not have these problems because
implementing all the tasks in user-space allows a finer resource and time
management. The down side is that those programs generally don't scale well on
multi-processor systems. That's the reason why they must be optimized to get
the most work done from every CPU cycle.