The final application area we touch are agent systems were we wish to capture an agent in a Prolog goal. Such systems can be implemented using threads (see section 9) that use thread_send_message/2 and thread_get_message/1 to communicate. The main problem is that each thread is associated by an operating system thread. OS threads are, depending on the OS, relatively expensive. Scalability of this design typically ends, depending on OS and hardware, somewhere between 1,000 and 100,000 agents.
Engines provide an alternative. A detached Prolog engine currently requires approximately 20 Kbytes memory on 64 bit hardware, growing with the size of the Prolog stacks. The Prolog stacks may be minimised by calling garbage_collect/0 followed by trim_stacks/0, providing a deep sleep mode. The set of agents, each represented by an engine can be controlled by a static or dynamic pool of threads. Scheduling the execution of agents and their communication is completely open and can be optimised to satisfy the requirements of the application.
This section needs an example. Preferably something that fits on one page and would not scale using threads. Engines might work nice to implement Antrank: An ant colony algorithm for ranking web pages.167http://www.ijettcs.org/Volume3Issue2/IJETTCS-2014-04-23-113.pdf